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U.S. Department of Housing and Urban Development Office of Policy Development and Research DURABILITY BY DESIGN BY DESIGN A Guide for Residential Builders and Designers
PATH (Partnership for Advancing Technology in Housing) is a new private/public effort to develop, demonstrate, and gain widespread market acceptance for the ìNext Generationî of American housing. Through the use of new or innovative technologies, the goal of PATH is to improve the quality, durability, environmental efficiency, and affordability of tomorrowís homes. PATH is managed and supported by the U.S. Department of Housing and Urban Development (HUD). In addition, all federal agencies that engage in housing research and technology development are PATH Partners, including the Departments of Energy, Commerce, and Agriculture, as well as the Environmental Protection Agency (EPA) and the Federal Emergency Management Agency (FEMA). State and local governments and other participants from the public sector are also partners in PATH. Product manufacturers, home builders, insurance companies, and lenders represent private industry in the PATH Partnership. To learn more about PATH, please contact 451 7 th Street, SW Washington, DC 20410 202-708-4277 (phone) 202-708-5873 (fax) e-mail: pathnet@pathnet.org website: www.pathnet.org Visit PD&Rís website www.huduser.org to find this report and others sponsored by HUDís Office of Policy Development and Research (PD&R). Other services of HUD USER, PD&Rís Research Information Service, include listservs; special interest, bimonthly publications (best practices, significant studies from other sources); access to public use databases, and a hotline 1-800-245-2691 for help accessing the information you need.
U.S. Department of Housing and Urban Development HUD User P.O. Box 6091 Rockville, MD 20849 Official Business Penalty for Private use $300 Return Service Requested FIRST CLASS MAIL POSTAGE & FEES PAID HUD PERMIT NO. G-795 May 2002
Durability by Design A Guide for Residential Builders and Designers Prepared for U.S. Department of Housing and Urban Development Washington, DC Contract No. C-OPC-21289 (T-002) by NAHB Research Center, Inc. Upper Marlboro, MD May 2002
ACKNOWLEDGMENTS This guide was written by the NAHB Research Center, Inc. with support from the U.S. Department of Housing and Urban Development. The NAHB Research Center had generous contributions from many groups and individuals who have helped to develop the practices and methods that make houses stand the test of time. The primary author of this guide at the NAHB Research Center was Jay Crandell, P.E.. Contributing authors and reviewers include Michael Grothe, James Lyons, and Jeanne Leggett Sikora. Illustrations and figures were drawn by Elliott Azzam. NOTICE The work that provided the basis for this publication was supported by funding under a grant with the U.S. Department of Housing and Urban Development. The substance and findings of the work are dedicated to the public. The authors are solely responsible for the accuracy of the statements and interpretations contained in this publication. Such interpretations do not necessarily reflect the views of the Government. While the information in this document is believed to be accurate, neither the authors, nor reviewers, nor the U.S. Department of Housing and Urban Development, nor the NAHB Research Center, Inc., nor any of their employees or representatives makes any warranty, guarantee, or representation, expressed or implied, with respect to t he accuracy, effectiveness, or usefulness of any information, method, or material in this document, nor assumes any liability for the use of any information, methods, or materials disclosed herein, or for damages arising from such use. ABOUT THE NAHB RESEARCH CENTER The NAHB Research Center, Inc. is a not-for-profit subsidiary of the National Association of Home Builders (NAHB). NAHB has over 203,000 members, including 60,000 builders who build more than 80 percent of new American homes. The Research Center conducts research, analysis, and demonstration programs in all areas relating to home building and carries out extensive programs of information dissemina tion and interchange among members of the industry and between the industry and the public. I
FOREWORD Few people intentionally consider durability when designing a home, but rather rely on experience and market acceptance to make design decisions. This approach to design works best in a stable housing market where architectural preferences and material choices do not change or change very slowly. The housing market, however, tends to be dynamic rather than stable and new materials and preferences influence the market continuously, sometimes in dramatic ways. This dynamic condition also places a responsibility on designers and builders to properly apply their experiences, which are often based on older construction methods and materials, to new materials and design conditions. As a result, it is important to understand why certain practices have been effective (or ineffective) in the past so that they can be properly interpreted and considered in the design and construction of modern homes. This manual titled Durability by Design: A Guide for Residential Builders and Designers is intended to raise the awareness and understanding of building durability as a design consideration in housing. The Guide covers basic concepts of durability and presents recommended practices ó including numerous construction details and design data ó for matters such as moisture management, ultraviolet (UV) protection, insects, decay, corrosion, and natural hazards. Some attention is also given to matters that may be considered serviceability issues related to normal wear-and-tear, aesthetics, or functions not immediately associated with durability. The contents of this Guide will help to preserve and promote ìtried-and-trueî practices and concepts related to housing durability, and present them in a manner that can be used to cost-effectively design the durable homes of the future. Lawrence L. Thompson General Deputy Assistant Secretary for Policy Development and Research II
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Table of Contents ACKNOWLEDGMENTS -------------------------------------------------------------------------------------------------------------------------------------------I FOREWORD ------------------------------------------------------------------------------------------------------------------------------------------------------- I I CHAPTER 1 - INTRODUCTION -------------------------------------------------------------------------------------------------------------------------------- 1 1.1 General ------------------------------------------------------------------------------------------------------------------------------------- 1 1.2 Durability Requires Commitment -------------------------------------------------------------------------------------------------- 2 1.3 Overview ----------------------------------------------------------------------------------------------------------------------------------- 2 CHAPTER 2 - CONCEPTS OF DURABILITY --------------------------------------------------------------------------------------------------------------- 3 2.1 General ------------------------------------------------------------------------------------------------------------------------------------- 3 2.2 What is Durability? ------------------------- ------------------------------------------------------------------------------------------- 3 2.3 Building Codes and Durability ----------------------------------------------------------------------------------------------------- 5 2.4 Factors Influencing Durability ----------------------------------------------------------------------------------------------------- 5 2.5 Common Durability Issues ---------------------------------------------------------------------------------------------------------- 8 CHAPTER 3 - GROUND AND SURFACE WATER ------------------------------------------------------------------------------------------------------ 11 3.1 General ----------------------------------------------------------------------------------------------------------------------------------- 11 3.2 Recommended Practices ----------------------------------------------------------------------------------------------------------- 11 CHAPTER 4 - RAIN AND WATER VAPOR ---------------------------------------------------------------------------------------------------------------- 15 4.1 General ----------------------------------------------------------------------------------------------------------------------------------- 15 4.2 Recommended Practices ----------------------------------------------------------------------------------------------------------- 15 CHAPTER 5 - SUNLIGHT ------------------------------------------------------------------------------------------------------------------------------------- 39 5.1 General ----------------------------------------------------------------------------------------------------------------------------------- 39 5.2 Recommended Practices ----------------------------------------------------------------------------------------------------------- 40 CHAPTER 6 - INSECTS ---------------------------------------------------------------------------------------------------------------------------------------- 45 6.1 General ----------------------------------------------------------------------------------------------------------------------------------- 45 6.2 Recommended Practices ----------------------------------------------------------------------------------------------------------- 45 CHAPTER 7 - PROTECTION AGAINST DECAY AND CORROSION ------------------------------------------------------------------------------- 51 7.1 General ----------------------------------------------------------------------------------------------------------------------------------- 51 7.2 Recommended Practices ----------------------------------------------------------------------------------------------------------- 51 CHAPTER 8 - NATURAL HAZARDS ------------------------------------------------------------------------------------------------------------------------ 59 8.1 General ----------------------------------------------------------------------------------------------------------------------------------- 59 8.2 Recommended Practices ----------------------------------------------------------------------------------------------------------- 60 CHAPTER 9 - MISCELLANEOUS --------------------------------------------------------------------------------------------------------------------------- 63 9.1 General ----------------------------------------------------------------------------------------------------------------------------------- 63 9.2 Plumbing -------------------------------------------------------------------------------------------------------------------------------- 65 9.3 HVAC -------------------------------------------------------------------------------------------------------------------------------------- 65 9.4 Exterior Finishes ---------------------------------------------------------------------------------------------------------------------- 66 BIBLIOGRAPHY ------------------------------------------------------------------------------------------------------------------------------------------------- 69 GLOSSARY --- ---------------------------------------------------------------------------------------------------------------------------------------------------- 72 APPENDIX A - DURABILITY CHECKLISTS ------------------------------------------------------------------------------------------------------------- 73 APPENDIX B - ESTIMATED LIFE-EXPECTANCY OF BUILDING MATERIALS AND PRODUCTS ---------------------------------------- 74 IV
List of Tables 2.1 ñ Top Five Homeowner Warranty Claims ------------------------------------------------------------------------ 9 2.2 ñ Major Expenditures for Repairs, Maintenance, and Replacements to Owner Occupied Homes (1998) ---------------------------------------------------- 9 4.1 ñ Recommended Minimum Roof Overhang Widths for One- and Two-Story Wood Frame Buildings ----------------------------------------------------------------------------- 17 4.2 ñ Roof Pitch Factors --------------------------------------------------------------------------------------------------- 19 4.3 ñ Gutter Capacity (roof area served in square feet) Based on 1 in/hr Rainfall Intensity ----------------------------------------------------------------------------- 19 4.4 ñ Recommended Drainage Plane Characteristics for Exterior Walls in Various Climate Conditions --------------------------------------------------------- 22 4.5 ñ Drainage Plane and Vapor Retarder Material Properties ---------------------------------------------- 23 4.6 ñ Recommended Vapor Retarder Characteristics for Building Exteriors or Interiors in Various Climate Conditions ------------------------------------------------------------------ 24 4.7 ñ Roof and Crawl Space Ventilation Recommendations ------------------------------------------------- 36 4.8 ñ Caulk Characteristics and Application Recommendations -------------------------------------------- 37 5.1 ñ Solar Angle Factors ------------------------------------------------------------------------------------------------ 40 7.1 ñ Recommended Finishes for Exterior Wood ---------------------------------------------------------------- 54 7.2 ñ Recommended Preservative Retention Levels for CCA-Treated Lumber ------------------------- 55 7.3 ñ No-Rust Service Life of Nails Exposed to Normal Outdoor Environment ------------------------- 56 8.1 ñ Hurricane Damage Statistics (single-family homes) ----------------------------------------------------- 59 8.2 ñ Northridge Earthquake Damage Statistics (percent of single-family homes) -------------------- 60 V
List of Figures 2.1 ñ The House and the ìDuralogic Cycleî ------------------------------------------------------------------------- 7 2.2 ñ Loss of Function vs. Time for Three Hypothetical Materials or Products of Different Quality Levels (poor, acceptable, and best) -------------------------------- 8 3.1 ñ Bore Hole Used for Preliminary Site Investigation ------------------------------------------------------- 12 3.2 ñ Site Grading and Surface Drainage--------------------------------------------------------------------------- 13 3.3 ñ Basement Construction and Optional Enhancements for Wet Site Conditions ----------------- 13 3.4 ñ Typical Frost-Protected Shallow Foundation with Perimeter Drain -------------------------------- 14 4.1 ñ Frequency of Moisture Problems in Walls of Selected Buildings in a Moist, Cool Climate 16 4.2 ñ Roof Overhangs ------------------------------------------------------------------------------------------------------ 17 4.3 ñ Climate Index Map Based on Wood Decay Potential --------------------------------------------------- 17 4.4 ñ Roof Gutters and Discharge Methods ------------------------------------------------------------------------- 18 4.5 ñ Rainfall Intensity Map of the United States ------- ---------------------------------------------------------- 19 4.6 ñ Weather Barrier Construction ----------------------------------------------------------------------------------- 21 4.7 ñ Heating Degree Day (HDD) Map of the United States (65 o F basis) ----------------------------------- 25 4.8a,b ñ Basic Roof Flashing Illustrations -----------------------------------------------------------------------27, 28 4.9 ñ Eave Flashing for Preventing Ice Dams ---------------------------------------------------------------------- 28 4.10 ñ Window Flashing Illustration ---------------------------------------------------------------------------------- 29 4.11 ñ Window Sill and Jamb Flashing Detail --------------------------------------------------------------------- 30 4.12 ñ Window Flashing for Severe Weather ---------------------------------------------------------------------- 31 4.13 ñ Door and Head Trim Flashing Detail ------------------------------------------------------------------------ 31 4.14 ñ Deck Ledger Flashing Detail ----------------------------------------------------------------------------------- 32 4.15 ñ Typical Brick Veneer Flashing Details ---------------------------------------------------------------------- 33 4.16 ñ Brick Veneer Flashing at Roof Intersections -------------------------------------------------------------- 34 5.1 ñ Solar Radiation Map of the United States ------------------------------------------------------------------- 39 5.2 ñ Effect of Building Latitude on Effectiveness of Overhangs -------------------------------------------- 40 5.3 ñ Effect of Surface Coloration on Solar Heat Gain --------------------------------------------------------- 41 VI
5.4 ñ Illustration of Solarscaping -------------------------------------------------------------------------------------- 42 6.1 ñ Termite Probability (Hazard) Map ------------------------------------------------------------------------------ 46 6.2 ñ Extent of Recorded Termite Damage ------------------------------------------------------------------------- 46 6.3 ñ Use of Termite Shields --------------------------------------------------------------------------------------------- 49 6.4 ñ Use of Concrete as a Termite Barrier ------------------------------------------------------------------------- 50 7.1 ñ Details to Separate Wood from Ground Moisture -------------------------------------------------------- 52 VII
CHAPTER 1 Introduction 1.1 General Of all the issues that must be considered when building a home, durability has perhaps the broadest impact on long-term performance, the most complex set of physical interactions, and the largest potential economic consequence. Fortunately, many of the best practices intended to improve durability require little more than good judgment and a basic knowledge of the factors that affect building durability. A fundamental element of this discussion is the very meaning of durability. For this guide, durability may be defined as the ability of a material, product, or building to maintain its intended function for its intended life-expectancy with intended levels of maintenance in intended conditions of use. 1 Obvi ously this definition may take on different meanings for different groups (e.g., builders, homeowners, manufacturers), implying that communication and education are key aspects that affect durability. Addressing durability is not a pursuit of extremes, but rather a pursuit of cost-effectiveness in terms of initial and long-term (i.e., maintenance, replacement) costs. Trying to make a home too durable can add so much to the cost of a new home that it may deny access to the basic need of decent shelter in the present time. Erring in the other direction can result in an equally disastrous future in terms of homeowner complaints, unsafe or unhealthy living conditions, and excessive maintenance and repair costs. The above comparison assumes that there is a direct trade-off between durability and affordability of homes. While the saying, ìyou get what you pay forî, is generally true, there are many design and construc tion practices that have minimal construction cost impacts, and significant durability benefits. The benefits may be measured in terms of maintenance, repair, general function of the home and its compo nent parts over time, enhanced business reputation, and customer satisfaction. Moreover, many such practices are well-known and need not be re-invented, but only communicated to the builder, designer, and consumer. 1 For a standardized definition of durability, refer to ASTM E632-82 (1996) Standard Practice for Developing Accelerated Tests to Aid Prediction of the Service Life of Building Components and Materials, American Society of Testing and Materials, West Conshohocken, PA (www.astm.org) Introduction This guide strives to reinforce ìtried and trueî practices that add to the durability of homes, shed some light on areas of confusion, and identify important trade-offs between cost and durability that should be carefully considered by the designer, builder, and homeowner. The guide focuses on practical solutions in key areas that are known to create significant and reoccurring durability problems. The guide also identifies timeless design concepts and principles that, once understood, can be applied to a variety of conditions and applications in modern housing design, construction, and maintenance. Finally, an attempt is made to draw attention to innovative materials and techniques that hold promise for improved durability in houses of the future. WHY IS DURABILITY IMPORTANT? Avoidance of short-term durability or performance problems (i.e., callbacks) is important to the builderís and designerís reputation and business profitability. The long-term condition of a home is important to retaining its investment value as well as its continued function as a safe, healthy, and aesthetic living environment. Poor durability adds to the operating and maintenance cost of home ownership. Failure to meet reasonable expectations for durability increases liability exposure. People donít like maintenance (i.e., high durability and low maintenance are important sales and purchasing factors). New products designed without adequately considering durability can prematurely fail, leading to both customer dissatisfaction and manufacturer losses. 1
Chapter 1 DURABILITY CHECKLISTS To assist in using this guide and in applying selected recommended practices, a durability checklist is provided in Appendix A. It lists various actions or considerations that should occur during the course of designing and constructing a house. Also included are action items appropriate for homeowners. Feel free to use and modify the checklist to suit your needs and level of interest. 1.2 Durability Requires Commitment Building and designing a durable home does not require a building scientist or durability specialist, but it does require commitment. Achieving durable construction not only includes the basicsómaterial selection, verification of manufacturer warranties, and passing minimum code-required inspectionsóbut it also involves a reasonable consideration of key details in the production of a home and understanding of the interactions between different materials and trades. Furthermore, durability also requires the appropriate use and installation of specified materials and, equally important, the functional integration of various materials and products such that the house performs as intended. In tandem, durability design criteria should inc orporate concepts such as ease-of-repair or replacement where appropriate. Building a durable home is relatively simple if the right information and guidance is available. In fact, including durability as a design criterion (though often subjective in nature) can add marketable features to homes at very little additional cost or design effort. Some features may already be incorporated into existing designs while others can be added through a simple modification of plans and specifications. Admittedly, although some aspects of designing for durability are rather straight forwardó such as the building code requirement of keeping untreated wood from contacting the groundó other tasks may involve somewhat greater effort. Achieving cost-effective and durable construction requires a reasonable commit ment in the planning, design, and construction of houses. 1.3 Overview This guide is arranged in the most practical and user-friendly way possible. However, there are many interrelated topics, which make any arrange ment of information on durability somewhat challenging. To the degree possible, redundancy in content is minimized and interrelated topics or discussions are appropriately cross-referenced so that the reader can seek the depth of information needed with relative ease. A glossary is provided at the end of this guide to aid in the proper under standing of this writing. The chapters of this guide are organized mainly by the factors that affect durability, i.e., ground and surface water, rain and water vapor, sunlight, etc. Within each chapter, the first section is always directed toward a general understanding of the concepts and issues related to the specific topic(s) of the section. An effort has been made to include geographically-based data and other technical information that allows the reader to quickly determine the relevance of a particular durability issue to local conditions or requirements. Chapter 2 introduces the topic of durability and presents some important over-arching concepts and issues that create a foundation of understanding upon which the remainder of the guide builds. Chapter 3 addresses concerns related to ground and surface water, primarily affecting site and foundation design. Chapter 4 addresses rain and water vapor and their effect on the above-ground structure. Combined, Chapters 3 and 4 cover some of the most prevalent housing durability issues related to wateróthe most formidable durability factor known to man. Chapter 5 deals with sunlight and methods to mitigate the effects of ultraviolet (UV) radiation on building materials. In Chapter 6, methods to prevent insect infestation and damage are presented. Chapter 7 addresses the issue of wood decay and corrosion of metal fasteners, both associated with the effects of moisture. Practices to improve the durability of homes that are subject to natural hazards, such as hurricanes and earth quakes, are presented in Chapter 8. Finally, Chapter 9 covers several miscellaneous and ìserviceabilityî issues related to durability, including items such as wear-and-tear, nuisances, plumbing/ mechanical/electrical systems, and exterior appurtenances. 2
CHAPTER 2 Concepts of Durability 2.1 General In this chapter, some fundamental concepts of durability related to the design of residential buildings are addressed. This background information is intended to establish a baseline of understanding and to introduce concepts and information important to developing a balanced perspective regarding durability. Before discussing the concept of durability, some discussion on unrealistic notions surrounding the topic of durability is in order. Despite the best efforts of the most knowledgeable and capable people, unforeseen problems will continue to occur in homes (e.g., premature failures of building products, components, and systems) . This undesirable outcome is often a consequence of taking calculated risks in moving toward more resource efficient, affordable, functional, and appealing homes. Further, it is impractical to think that the durability of all building components and systems can be exactly designed and crafted such that they all last just as long as intended. (This point is a matter of poetic parody, see inset of ìThe Wonderful One-Hoss Shayî by Oliver Wendell Holmes on the following page). In fact, the service life of building materials and products varies substantially (see Appendix B ñ Estimated Life- Expectancy of Building Materials and Products). Thus, it can be expected that some components of a home will require some vigilant attention along the way (i.e., maintenance, repair, and eventual replace ment of ìworn-outî components). Note that many changes have occurred in home building over the past several decades that will likely affect the durability of houses in the short and long termñsome good and some bad. Examples of material changes include the increased use of engineered wood products, adhesives, and plastics, among many others. At the same time, housing designs have tended to grow in complexity and size, thereby increasing exposure to the elements and vulnerability. Also, newer materials and technologies have changed both the susceptibility and exposures of building materials in modern homes. New homes are also increasingly complex to operate and Concepts of Durability maintain. In short, there are more durability issues to deal with and more material choices than ever before. 2.2 What is Durability? Durability is the ability of a material, product, or building to maintain its intended function for its intended life-expectancy with intended levels of maintenance in intended conditions of use. However, we all know that the road to success is not just paved with good intentions. Ultimately, what is built must work as expected, or as nearly so as practicable. What is a reasonable expectation or goal for durability? It depends. It depends on how much it costs. It depends on the expectations of the end user and the long term investment value of the product. It depends on the local climate. It also depends on expected norms when the end user is not intimately involved with or knowledgeable of various design decisions and their implications. It also depends, of course, on the material itself. For example, a house is expected (at least in theory) to last for 75 years or more with normal maintenance and replacement of various components (see Appendix B ñ Estimated Life-Expectancy of Building Materials and Products). But then again, what one person considers normal maintenance may be perceived differently by another. Durability is, therefore, an exercise in the management of expecta tions as well as an application of technology. For this reason, some builders and designers make significant efforts to inform their clients and trade contractors about reasonable expectations for the durability, performance, maintenance, and operation of a home. Some references to help in this matter include: Caring For Your Home: A Guide to Maintaining Your Investment (NAHB/Home Builder Press, 1998); 3
Chapter 2 THE DEACON’S MASTERPIECE: OR THE WONDERFUL “ONE-HOSS SHAY” Oliver Wendell Holmes Have you heard of the wonderful one-hoss shay, That was built in such a logical way It ran a hundred years to a day, And then, of a sudden, itñah, but stay, Iíll tell you what happened without delay, Scaring the parson into fits, Frightening people out of their wits,ñ Have you ever heard of that, I say? Seventeen hundred and fifty-five. Georgius Secundus was then alive,ñ Snuffy old drone from the German hive. That was the year when Lisbon-town Saw the earth open and gulp her down, A nd Bradockís army was done so brown, Left without a scalp to its crown. It was on the terrible Earthquake-day That the Deacon finished the one-hoss shay. Now in building of chaises, I tell you what, There is always somewhere a weakest spot, ñ In hub, tire, felloe, in spring or thill. In panel, or crossbar, or floor, or sill, In screw, bolt, thoroughbrace, ñlurking, still, Find it somewhere you must and will, ñ Above or below, or within or without, ñ And thatís the reason, beyond a doubt, A chaise breaks down, but doesnít wear out. But the Deacon swore (as Deacons do, With an ìI dew vum,î or an ìI tell yeouî) He would build one shay to beat the taown ëNí the keounty ëní all the kentry raouní; It should be so built that it couldnít break daown. ñîfur,î said the Deacon, ìëtís mighty plain Thut the weakesí place musí staní the strain; ëní the way tí fix it, uz I maintain, is only jest Tí make that place uz strong uz the rest.î So the Deacon inquired of the village folk Where he could find the strongest oak, That couldnít be split nor bent nor broke, ñ That was for spokes and floor and sills; He sent for lancewood to make the thills; The crossbars were ash, from the straightest trees, The panels of white-wood, that cuts like cheese, But lasts like iron for things like these; The hubs of logs from the ìSettlerís ellum,î ñ Last of its timber, ñthey couldnít sell ëem, Never an axe had seen their chips, And the wedges flew from between their lips, Their blunt ends frizzled like celery-tips; Step and prop-iron, bolt and screw, Spring, tire, axle, and linchpin too, Steel of the finest, bright and blue; Thoroughbrace, bison-skin, thick and wide; Boot, top, dasher, from tough old hide Found in the pit when the tanner died. That was the way he ìput her through.î ñ ìThere!î said the Deacon, ìnaow sheíll dew.î Do! I tell you, I rather guess She was a wonder, and nothing less! Colts grew horses, beards turned gray, Deacon and deaconess dropped away, Children and grandchildrenñwhere were they? But there stood the stout old one-hoss shay As fresh as on Lisbon-earthquake-day! EIGHTEEN HUNDRED; ñit came and found The Deaconís masterpiece strong and sound. Eighteen hundred increased by ten; ñ ìHahnsum kerridgeî they called it then. Eighteen hundred and twenty came; ñ running as usual; much the same. Thirty and forty at last arrive, And then came fifty, and FIFTY-FIVE. Little of all we value here Wakes on the morn of its hundredth year Without both feeling and looking queer. In fact, thereís nothing that keeps its youth, So far as I know, but a tree and truth. (This is a moral that runs at large; Take it. ñYouíre welcome. ñNo extra charge.) FIRST OF NOVEMBER, ñthe Earthquake-day. ñ There are traces of age in the one-hoss shay, A general flavor of mild decay, But nothing local, as one may say. There couldnít beñfor the Deaconís art Had made it so like in ever part That there wasnít a chance for one to start. For the wheels were just as strong as the thills, And the floor was just as strong as the sills, And the panels just as strong as the floor, And the whippletree neither less nor more, And the back-crossbar as strong as the fore, And spring and axle and hub encore. And yet, as a whole, it is past a doubt In another hour it will be worn out! First of November, fifty-five! This morning the parson takes a drive. Now, small boys, get out of the way! Here come the wonderful on e-hoss shay, Drawn by a rat-tailed, ewe-necked bay. ìHuddup!î said the parson.ñOff went they. The parson was working his Sundayís text, ñ Had got to fifthly, and stopped perplexed At what theñMosesñwas coming next. All at once the horse stood still, Close by the meetíní-house on the hill. First a shiver, and then a thrill, Then something decidedly like a spill, ñ And the parson was sitting upon a rock, At half-past nine by the meetíní-house clockñ Just the hour of the Earthquake shock! What do you think the parson found, When he got up and stared around? The poor old chaise in a heap or mound, As if it had been to the mill and ground. You see, of course, if youíre not a dunce, How it went to pieces all at once, ñ All at once, and nothing first, ñ Just as bubbles do when they burst. End of the wonderful one-hoss shay, Logic is logic. Thatís all I say. 4
Your New Home and How to Take Care of It (NAHB/Home Builder Press, 2001); and A Builderís Guide to Marketable, Affordable, Durable, Entry-Level Homes to Last (HUD, 1999). 2.3 Building Codes and Durability Numerous requirements found in building codes imply a minimum level of durability performance or expectation. Building codes specify the minimum type and nature of various materials, including certain installation requirements that may vary according to local or regional climatic, geologic, or biologic conditions. Despite the extensive framework of require ments found in building codes, there are still gaps in the details or in the reliability of the information for any specific application or local condition. In some instances, the requirements are clear, e.g., ìa metal connector with minimum G60 galvanic coating shall be usedî and in other cases the guidance is quite vague, e.g., ìuse corrosion resistant fasteners.î Likewise, standardized durability tests for materials are rarely calibrated to performance in actual conditions of use. Further, building codes and standards are often driven by various opinions and data or experiences expressed in the code development process. Sometimes the evidence is contradictory or incomplete. Nonetheless, it is legally required that a builder and designer adhere to code prescribed requirements related to durability and, when deemed appropriate, seek approval of alternate means and methods of design or construction that are at least equivalent to that required or implied by the locally approved building code. The major U.S. model building codes currently available are listed in the sidebar to the right. However, the reader should be informed that earlier versions may be in use locally since these codes do not become law until they are legislatively adopted at the local level. In addition, these national model codes are often amended to address local issues and concerns. Concepts of Durability 2.4 Factors Influencing Durability The manner in which materials and buildings degrade over time depends on their physical make up, how they were installed, and the environmental conditions to which they are subjected. It is for this reason that environmental conditions, such as humidity and temperature, are carefully controlled in museums to mitigate the process of degradation. Even then, artifacts still require periodic care and maintenance. Houses, depending on where they are located with respect to geology and climate, are more or less subjected to various types of durability ìfactors.î Each of the ìfactorsî listed below, which can be managed but not externally controlled, is addressed in this guide: Moisture Sunlight (UV radiation) Temperature Chemicals Insects Fungi Natural Hazards Wear and Tear In essence, a house is part of an environmental cycle as depicted in Figure 2.1 and is subject to the same powerful forces of nature that create and then erode mountains, cause organic matter to decom pose, and change the face of the earth. MODEL U.S. BUILDING CODES One- and Two-Family Dwelling Code (OTFDC), Council of American Building Officials (CABO), Falls Church, VA, 1995. International Residential Code (IRC), International Code Council, Inc., Falls Church, VA, 2000. International Building Code (IBC), International Code Council, Inc., Falls Church, VA, 2000. Uniform Building Code (UBC), International Conference of Building Officials (ICBO), Whittier, CA, 1997. Standard Building Code (SBC), Southern Building Code Conference International (SBCCI), Birmingham, AL, 1999. National Building Code (NBC), Building Officials and Code Administrators International, Inc., Country Club Hills, IL, 1999. 5
Chapter 2 Over the course of time, the greatest concerns (and impacts) regarding durability are those pro cesses that occur constantly over the life of a home. Most notable of these factors is moisture. Moisture comes in many forms (i.e., rain, snow, ice, vapor) and is linked to other durability factors. For instance, moisture must be present in sufficient quantity to promote corrosion (e.g., chemical degradation), insect habitation (e.g., subterranean termites), and rot (e.g., wood decomposition). By controlling exposure to moisture, many other durability problems are also solved. Other problems, such as mold and indoor air quality, are also related to moisture. It is for this reason that there is a major emphasis on moisture in this guide. In fact, the effects of moisture on building durability have been associated with enormous economic impact in the United States for wood construction alone. The UV radiation from sunlight also has a tremendous impact on the exterior finishes of homes. For example, sunlight causes coatings to chalk-up or fade in color, plastics to degrade, wood to weather, and asphalt roof shingles to become brittle. Sunlight can also fade carpets, drapes, and furnishings inside homes. In relation to moisture, sunlight can heat surfaces and drive moisture into or out of materials and buildings; intermittent sunlight can also cause temperature cycling. Temperature causes materials to expand and contract. Temperature cycling, particularly in the presence of water, can cause some materials to weaken or fatigue. Thermal expansion and contraction can also cause materials to buckle and warp and, therefore, become less effective in their intended function (e.g., buckling of improperly installed siding which may allow increased rain water penetration). When temperature cycles above and below the freezing temperature of water, even more damaging effects can occur to materials with high moisture content. Chemical reactions, most often occurring in the presence of water, are responsible for a variety of durability problems and can dramatically accelerate otherwise normal rates of degradation. For example, a galvanic reaction between dissimilar metals can cause one metal to degrade relatively rapidly. This effect is evidenced by more rapid corrosion of galvanized fasteners in preservative-treated wood (i.e., chromated copper arsenate or CCA) relative to untreated wood. Another example is the pitting of copper piping due to the presence of certain salts and minerals in water or soil. Certain insects are particularly fond of wood and, in fact, depend on wood for food. In the presence of wood-consuming insects such as termites and carpenter ants, an unprotected wood-frame home is nothing more than a free food source. Natural hazards form a special class of durability concerns that are generally associated with localized climatic or geologic conditions. These conditio ns are generally considered from a life-safety perspective, but they are considered here in the broader sense of durability. For example, a life-safety provision in a building code may require that an extreme wind or earthquake event be considered in the structural design of a home. However, durability impacts may be realized in even moderate or mild natural events. Even a mild hurricane can cause significant water penetra tion and salt deposition resulting in immediate (e.g., flooding) and long-term (corrosion, mold growth) damage. Natural hazards that affect durability include hurricanes, earthquakes, floods, wildfires, hail, snow, thunderstorms, and tornadoes. Wear and tear is simply the result of abrasion, physical damage, staining and other symptoms of continued use. Homeowner habits and lifestyles are particularly important for this durability factor. In summary, all houses are under attack by a mighty and unstoppable foe, namely the forces of nature, along with kids, pets, and other ìuse condi tions.î Recognizing this issue is not intended to signal retreat or resignation, but rather to draw attention to the need for action. Of course, actions must be practical in that the benefits of improved durability should be reasonably balanced with the costs and efforts of doing so. Appropriate actions to consider include selecting high quality material, using appropriate design detailing, following proper installation procedures, and perform ing judicious maintenance. The concept of durability as a function of material quality is illustrated in Figure 2.2. Note the different levels of maintenance required to retain acceptable function of the three hypothetical materials in Figure 2.2. In many cases, however, installation quality may actually be more important than material quality. In other cases design decisions can have a profound effect on making poor quality materials or installations perform satisfactorily. Proper maintenance and repair are critical factors in some instances. Usually, all of these factors are important considerations. 6
Concepts of Durability Figure 2.1 - The House and the ìDuralogic Cycleî 7
Chapter 2 Figure 2.2 - Loss of Function vs. Time for Three Hypothetical Materials or Products of Different Quality Levels (poor, acceptable, and best) Enough said on the concepts, theory, and philosophy of designing for durability. The next section reviews some of the most common durability or performance issues experienced in modern homes, many of which are addressed in the remaining parts of this guide. 2.5 Common Durability Issues The type and frequency of durability related problems and general performance problems experienced in modern homes can be gathered from various information sources, such as trade organiza tions, industry surveys, warranty claims, popular literature, and others. These problems may be related to design, materials, methods, maintenance, or a WHAT’S THE COST OF MAINTENANCE? Most people donít consider long-term repair and maintenance costs as an issue in making a home purchase. However, a typical annual, out-of pocket (i.e., not including do-it-yourself tasks) maintenance and repair expenditure is about $300 to $600. (Source: NAHB Housing Economics, Nov 1997. Based on data from 1995 American Housing Survey). This amount may actually reflect a tendency to defer maintenance. Items like replacing appliances or HVAC equipment will create even greater costs as a house becomes older. combination of these factors. For this reason, this guide focuses primarily on design issues, but also has significant content on installation, materials selection, and maintenance topics as well. The following summaries, including Tables 2.1 and 2.2, illustrate some commonly reported durability issues: Problem Areas in New Construction Paints/Caulks/Fi nishes Flooring Windows and Skylights Doors Foundations and Basements Siding and Trim Structural Sheathing Wallboard Foundation Insulation and Waterproofing Framing Source: Survey of builders conducted by NAHB Research Center, Upper Marlboro, MD, January 1992. Most Frequent House Problems Improper Surface Grading/Drainage Improper Electrical Wiring Roof Damage Heating System Poor Overall Maintenance Structurally-Related Problems Plumbing Exteriors Poor Ventilation Source: ASHI NEWS Press Release, American Society of Home Inspectors, Des Plaines, IL, 2000. 8
TABLE 2.1 - TOP FIVE HOMEOWNER WARRANTY CLAIMS Based on Frequency of Claim Based on Cost of Claim Gypsum wall board finish Foundation wall Foundation wall Garage slab Window/door/skylight Ceramic tiles Trim and moldings Septic drain field Window/door/skylight frames Window/door/skylight & other Mortgage Housing Corporation, Ottawa, Ontario, Canada, November 1994. Concepts of Durability action lawsuits in the United States have given builders and designers some reason to think twice about specifying new products. Past examples include: Source: Defect Prevention Research Project for Part 9 Houses, Ontario Home Warranty Program, Canada Home Builder and Housing Consumer Product Problems 1. Foundations and basements ñ Leaks, construction cost is higher than the perceived value, difficult to insulate; 2. Paints, caulks, finishes ñ Caulk shrinkage, premature discoloration and fading, peeling and blistering, mildew growth, imperfections of surface, poor coverage; 3. Windows and skylights ñ Air and water leakage, glass fogs and frosts; 4. Doors ñ Warping, poor weather stripping, checking and splitting of panels, swelling; 5. Finish flooring ñ Seams visible, damages easily, inconsistent color, coming up at edges, poor adhesion; 6. Structural sheathing ñ Excessive swelling, delamination of sheets; 7. Roofing ñ Leaks, does not seal properly, wind damage, inconsistent coloration; 8. Siding and trim ñ Siding buckles, nails bleed, algae grows on it, paint peels, seams are noticeable, moisture induced swelling; 9. Wallboard, interior coverings ñ Nail pops, finish shows seams and/or nail heads; 10. Framing ñ Warped/twisted lumber, checking/splitting, too many large knots; 11. HVAC Equipment ñ Wrong sizing, insufficient warm air. Source: Product Failure Early Warning Program, prepared for NAHB by the NAHB Research Center, Inc., Upper Marlboro, MD, 1996. All of these summaries of housing durability issues point to the previously mentioned problem areas of installation and material quality, design, and proper maintenance. And while these perfor mance problems are not necessarily related to any specific building product, itís worth mentioning that builders are generally averse to a certain class of products ñ those that are ìtoo new.î Major product and installation failures that have resulted in class Exterior Insulated Finish Systems (EIFS); Fire-Retardant Treated (FRT) Plywood Roof Sheathing; Certain Composite Sidings and Roofing Products; and Polybutylene Water Piping. It should be noted, however, that many of these problems have been resolved by subsequent product improvements. For example, EIFS systems are now almost exclusively used with a ìdrainage planeî system such that any moisture that enters the wall can escape without harm. In other cases, products such as polybutylene piping have been entirely removed from the market. Although costly examples, these experiences demonstrate the risk and complexity in the develop ment and application of new materials and methods of home construction. TABLE 2.2 - MAJOR EXPENDITURES FOR REPAIRS, MAINTENANCE, AND REPLACEMENTS TO OWNER OCCUPIED HOMES (1998) Category 1998 Value ($ Millions) Roofing 8,740 Painting and Papering 8,641 HVAC 5,872 Windows and Doors 5,769 Plumbing 3,368 Siding 1,853 Driveways and walkways 1,138 Flooring 826 Electrical 493 Others (including materials on hand) 10,814 TOTAL 47,514 Source: U.S. Department of Housing and Urban Development. 9
Chapter 2 From a recent pilot study 2 of homes of two different age groups (1970ís and 1990ís), some important trends and observations regarding durability of housing in one locality (Anne Arundel County, MD) have been identified: 1. The size of roof overhangs decreased between the 1970s and 1990s. 2. Use of vinyl siding and window frames have increased dramatically. 3. When present, signs of poor site grading (i.e., surface depressions next to house) were associated with an increased tendency for foundation cracks. 4. The occurence of wood rot (predominantly associated with exterior trim) in newer and older homes was 22 percent and 31 percent, respectively. 5. Masonry foundations tended to evidence cracks more frequently than concrete foundations. 2 Assessing Housing Durability: A Pilot Study, U.S. Department of Housing and Urban Development, Washington, D.C., November 2001 10
CHAPTER 3 Ground and Surface Water 3.1 General Nearly all building sites have some potential to experience problems with ground moisture, particu larly when the water table is high or drainage is poor. Poor site drainage and difficult site conditions, such as ìlooseî soils or fills, can contribute to eventual building settlement, foundation wall cracking, and aggravated moisture problems. Years ago, it was generally much easier to select a suitable building location on a larger site or to seek alternate sites that provide better drainage and bearing support characteristics. However, such a luxury is not easily afforded in todayís market. Thus, this section gives recommendations that recognize the need to be resourceful with the land that is available. The objective of a foundation is to separate the building materials and the indoor environment from the earth while also providing adequate structural support. The following rules of thumb and recom mended practices of Section 3.2 should serve to minimize the potential for durability and performance problems related to foundations (refer to Section 2.5, Common Durability Issues). RULES OF THUMB Most damp foundations are caused by improper surface drainage. Wet site ñ ìwaterproofî basement walls per code and use a sump pump; damp/dry site ñ ìmoisture proofî basement walls. Do not build below-ground space below highest seasonal water table level. Using only typical construction practices, as many as 1 out of 3 basements experience some form of water problem within the first two years. When in doubt, seek advice from a qualified geotechnical engineer. Moisture entering a house through the foundation will contribute to potential moisture problems in the above-ground portions of the building, even the attic through added water vapor loading. Ground and Sur face Water 3.2 Recommended Practices 3.2.1 Recommendation #1: Preliminary Site Investigation The following actions may help to identify potential site problems that can be accounted for in planning and design. An illustration of a typical bore- hole used to explore subsurface conditions is shown in Figure 3.1. Survey the surface conditions and local plant species for signs of seasonal or constant high ground water levels. Consider the lay of the land and surface water flow onto and off of the site to ensure that proper surface water drainage can be achieved around the building site. Check soil maps from USDAís Natural Resources Conservation Service or use a hand auger to bore one or more test holes at the proposed building location; and determine general soil type/characteristics and ascertain the water table level (be sure to factor in any seasonal or recent climate conditions such as the amount of precipitation over the previous month or so) (see Figure 3.1). At least one hole should be at the building location and extend at least a couple of feet below the proposed footing elevation. If deeper subsurface problems are expected (as by local experience), then a geotechnical engineer may need to use special drilling equipment to explore deeper below grade to ensure that adequate support and stability exists. 11
Chapter 3 Figure 3.1 - Bore Hole Used for Preliminary Site Investigation If possible, test the soil for bearing capacity at the depth and location of proposed footings. A simple hand-held penetrometer (e.g., a standardized metal rod and drop weight) used in accordance with the manufacturerís instructions serves this purpose. If fill or questionable soil conditions are suspected (as on a steep slope), the services of a design professional and knowledgeable foundation contractor may be needed to appropriately prepare the site (e.g., compaction) or design a suitable foundation system. Do not use basement foundations on sites with high ground water table. Avoid silt, heavy clay, or expansive clay backfill, particularly for basement walls. Granular soils are preferable. Use minimum 3,000 psi concrete in slabs and foundation walls with welded wire fabric in slabs and light reinforcement (#3 rebar) in foundation walls to control cracking, improve concrete resistance to moisture and weathering, and improve concrete finishing. 3.2.2 Recommendation #2: Site Grading and Surface Water Drainage Site grading plans should consider the existing natural water flow and change the water flow to direct water away from the building foundation, particularly if the building is located down-slope from a hill or similar land formation that may produce significant rainfall runoff. Use of grassy swales is a common and cost-effective practice when the potential water volume is not large, wetting is not constant, and the swale is not sloped steeply enough to produce high water velocities (see Figure 3.2). The range of acceptable swale slope depends on many factors, but slope should not be less than about 1% to prevent ponding, nor more than about 15% unless rip-rap (4 to 8 inch stone) is used to line the swale with a filter cloth underlay. The grading immediately adjacent to the building should be sloped a minimum of about 4% (or 1/2 inch in 12 inches) for at least 6 feet outward from a building foundation or as far as practical. If concrete flatwork (i.e., patio slabs, driveways, and walks) are adjacent to the building, they should be sloped not less than 2% (about 1/4 inch in 12 inches) away from the building. Backfill should be tamped firmly to prevent excessive settlement or the grade should be adjusted to allow for future backfill settlement. In addition, gutters and gutter drains should be used to further remove roof run-off from the foundation area (See Section 4.2.2). 12
Figure 3.2 - Site Grading and Surface Drainage Ground and Sur face Water 3.2.3 Recommendation #3: Foundation Construction Foundation options generally include base ment, slab-on-grade, crawl space, or a mix of these foundation types (e.g., split level construc tion). One thing is common in all foundation construction: ground moisture will find its way ìinî unless appropriate measures are taken. An important measure to include is a ground vapor barrier under all basement, slab-on-grade, or crawl space construction. This will eliminate (or suitably minimize) a large potential water vapor source to a house that can result in or aggravate above-ground moisture vapor problems (see Chapter 4). The ground vapor barrier should be placed directly below and immediately prior to pouring the concrete slab to avoid damage during construction. Second, some method of removing ground water from around the foundation is recommended in all but the driest and most well-drained site conditions. Typical basement construction practice and optional enhancements (i.e., polyethylene sheeting) for particularly wet sites are illustrated in Figure 3.3. However, ìwater proofingî is not meant to resist Figure 3.3 - Basement Construction and Optional Enhancements for Wet Site Conditions 13
Chapter 3 water from flooding or a high water table. It should be noted that concrete has a considerably lower vapor permeability (i.e., can stop water vapor better) than masonry. However, available data seems to suggest no significant difference between concrete and masonry relative to the potential for basement water problems in actual practice. For slab-on-grade and crawlspace foundations, moisture protection usually involves placing the building on a slight ìmoundî relative to the surround ing site. The use of a gravel layer under the slab or on the crawlspace floor is considered optional for mounded foundations, however, a vapor barrier should always be used. If the site is properly graded, a perimeter drain system is unnecessary in mounded foundation systems. 3.2.4 Recommendation #4: Frost Protection Foundations are conventionally protected from frost (i.e., heave), by placing footings below a locally prescribed frost depth. An alternative in northern climates is the Frost Protected Shallow Foundation technology which offers the benefits of frost protec tion, energy efficiency, warmer slab edge tempera tures (reduced condensation potential and improved comfort), and material savings. This technology uses the heat generated within a building and stored in the ground to raise the frost depth around the structure, allowing for reduced-depth footings. A typical frost protected shallow foundation detail is shown in Figure 3.4. The technology and concept can be used to protect a variety of foundation types and site structures from frost heave. Refer to Design Guide for Frost-Protected Shallow Foundations (NAHB Research Center, 1996) for additional design and construction guidance. It should be noted that current building codes prohibit the use of foundation insulation in areas with ìheavyî termite infestation probability (i.e., southeastern United States). The foam can create a ìhidden pathwayî for termite access to wood building materials. Refer to Chapter 6 for methods to deter termite infestation. Figure 3.4 - Typical Frost-Protected Shallow Foundation with Perimeter Drain 14
CHAPTER 4 - Rain and Water Vapor 4.1 General The most common and disastrous durability problems are frequently related to bulk moisture or rain penetrating a buildingís exterior envelope without any opportunity to drain or dry out rapidly. If rain penetration occurs repetitively and continues undetected or uncorrected, it can cause wood framing to rot, mold to grow, and steel to corrode. In fact, particularly bad cases of this type of problem have resulted in severely rotted wood frame homes within the period of a couple of years. However, most rain penetration problems can be isolated to inadequate detailing around windows and door openings and similar penetrations through the building envelope. The objective of designing a weather barrier system is pure and simpleñkeep rain water away from vulnerable structural materials and interior finishes. Keeping these components dry will maintain a buildingís structural integrity and help prevent moisture-related problems like mold. W ithin this guide, ìweather barrierî is a general term for a combination of materials used as a system that protects the building from external sources of moisture. Important related issues are water vapor diffusion and drying potential. These issues are considered in tandem since they are practically inseparable design issues, creating the need to have an integrated design approach (i.e., one that adequately considers all factors and their potential impact on durability). Some of the information presented in this chapter is generic in nature and will apply to most house designs (e.g., overhangs), while other recommendations are geared more towards specific configurations like vinyl or wood siding installed over wood sheathing. The Rules of Thumb listed in the sidebar to the right and the recommendations in this chapter should help to address the durability and performance issues related to liquid moisture (rain), perhaps the most significant durability factor. Rain and Water Vapor 4.2 Recommended Practices Building walls are subject to water penetration and repeated wetting depending on their exposure, the climate, and the integrity of the siding system. While you canít change the climate in which you build, it is possible to improve the shielding of walls and to design walls that are appropriate for ìimper fectî (i.e., leaky) siding systems. 4.2.1 Recommendation #1: Roof Overhangs Figure 4.1 illustrates the frequency of building walls having moisture penetration problems in a particularly moist, cool climate (British Columbia) as a function of roof overhang length. The shielding effect of roof overhangs is illustrated in Figure 4.2. Note that a roof overhangís impact will depend on the climate (Figure 4.3) and type of construction protected. The potential for wind-driven rain should also be considered. The climate index map of Figure 4.3 does not directly account for wind-driven rainó RULES OF THUMB Liquid water or rain obeys the following rules with respect to movement: Gravity - water runs downhill Capillary - water is attracted into small cracks due to capillary action or surface tension Wind - wind can drive rain into places it would not otherwise go and create building interior and exterior pressure differentials that move it uphill, breaking the first rule (gravity) NO wall or roof covering is perfectly waterproof, especially considering that there will be wall openings, roof penetrations, and other materials that compromise even the ìwaterproofî materialsóparticularly in view of the effects of time. Avoid depending on caulk as a primary barrier to moisture penetration (i.e., use flashing). 15
Chapter 4 a condition that varies with local climate or site exposure. Some important considerations regarding roof overhangs include: Roof overhangs protect exterior walls and foundations from excessive wetting by rain wateróthe culprit in many moisture problems in residential buildings. Just as the safety factor is important to providing for a reasonable structural design that accounts for foreseen events and unexpected extremes, so is the roof overhang to those interested in durable wood-frame building construction. The width of roof overhang to use depends on a variety of factors, including construction cost, wall type below, amount of windows and doors exposed, and the height of the wall. Recommended overhang widths are provided in Table 4.1 for typical conditions. Greater flexibility in architectural design with respect to the use (or non-use) of overhangs for rain water protection is afforded in more arid climate conditions; in other areas there are significant durability trade-offs (see Figure 4.1). In moist climates with significant rainfall, liberal use of overhangs is recommended. Roof overhangs also provide durability and energy benefits in terms of solar radiation (see Section 5.2). In Table 4.1, the recommended overhang widths are given with the assumptions that: all walls have a properly constructed weather barrier, roofs are adequately guttered, and normal maintenance of exterior will occur. For overhangs protecting more than two-story walls with exposed windows and doors, larger overhangs should be considered. Rake (gable end) overhangs also deserve special consid eration because more costly ìoutriggerî framing methods will be required for overhangs exceeding about 12 inches in width and the appearance may not be acceptable to some home buyers. Also, for sites subject to frequent wind-driven rain, larger overhangs and drainage plane techniques that include an air space behind the siding should be considered (see Section 4.2.3). For non decay- resistant wood sidings and trim (as for windows and door casings), greater overhangs and porch roofs are recommended. 4.2.2 Recommendation #2: Roof Gutters and Down-spouts Properly designed roof gutters reduce the amount and frequency of roof run-off water that wets above-grade walls or the foundation. A list of recommendations and a rule-of-thumb design approach are presented below to help in the proper use of gutters. Figure 4.4 illustrates a typical gutter installation and components. Figure 4.1 - Frequency of Moisture Problems in Walls of Selected Buildings in a Moist, Cool Climate (Climate Index of approximately 70 based on Figure 4.3) Source: Morrison Hershfield Limited, Survey of Building Envelope Failures in the Coastal Climate of British Columbia, Canada Mortgage and Housing Corporation, Burnaby, BC, Canada, 1996. Figure is based on a selection of 46 buildings of up to eight years old, three to four stories, wood-frame, with various wall claddings. Fifty percent of walls with problems used direct-applied stucco cladding over building paper and oriented strand board (OSB) wood panels. 16
Climate Index Rain and Water Vapor Source: Modification of Prevention and Control of Decay in Homes by Arthur F. Verrall and Terry L. Amburgey, prepared for the U.S. Department of Agriculture and U.S. Department of Housing and Urban Development, Washington, DC, 1978. 1 Table based on typical 2-story home with vinyl or similar lap siding. Larger overhangs should be considered for taller buildings or wall systems susceptible to water penetration and rot. TABLE 4.1 - RECOMMENDED MINIMUM ROOF OVERHANG WIDTHS FOR ONE- AND TWO-STORY WOOD FRAME BUILDINGS 1 Climate Index (Figure 4.3) Eave Overhang (Inches) Rake Overhang (Inches) Less than 20 N/A N/A 21 to 40 12 12 41 to 70 18 12 More than 70 24 or more 12 or more Figure 4.2 - Roof Overhangs Figure 4.3 - Climate Index Map Based on Wood Decay Potential Prepared by the U.S. Weather Bureau. Source: Theodore C. Scheffer, ìA climate index for estimating potential for decay in wood structures above ground,î Forest Products Journal, Vol. 21, No. 13, October 1971. Site specific indices may be determined using the following formula, where T is the monthly mean temperature ( o F), D is the mean number of days in the month with 0.01 inch or more of precipitation, and Σ is the summation of products (T-35)(D-3) for respective months of the year. Climate Index NOTE: Roof overhangs also provide protection from sunlight; refer to Chapter 5 for advice on using overhangs to minimize the impact of UV radiation. Roof overhangs in hurricane-prone locales may require additional anchorage of the roof. 17
Chapter 4 Downspouts that discharge to the surface should do so at least two feet outward from the building. Splash blocks or plastic corru gated pipe are recommended to prevent erosion and to give further extension of discharge water away from the foundation, particularly for downspouts located at inside corners of buildings. Downspouts that discharge water below grade should do so into non-perforated corrugated or smooth plastic pipe. The pipe should be run underground to a suitable outfall. Do not connect the gutter drain pipe to the perforated foundation drain pipe, this practice will soak the foundation. Gutters and downspouts should be resistant to corrosion and abrasion from flowing water; material choices include aluminum (most popular), vinyl or plastic, copper, and coated metal (baked enamel or galvanized). Use a gutter splash shield at inside corners (i.e., valleys) where fast moving water in a roof valley may ìovershootî the gutter. Gutters, downspouts, and splash blocks must be cleaned and properly maintained by the homeowner. Sizing of Gutters and Downspouts Generally, a standard 5-inch deep gutter and 2 inch by 3-inch downspouts are adequate for most homes in most climate conditions in the United States. However, the following simplified sizing method may help to avoid problems when unique situations are encountered. An example is provided on page 20. Step 1: Determine the horizontal projected roof area to be served by the gutter and multiply by the roof pitch factor from Table 4.2. Step 2: Estimate the design rainfall intensity (see map in Figure 4.5). Step 3: Divide selected gutter capacity (Table 4.3) by the rainfall intensity estimated in Step 2 to determine the maximum roof area served. Step 4: Size downspouts and space along gutter in accordance with factored roof area calculated in Step 1 for the selected gutter size and type. As a rule of thumb, one square inch of down-spout cross section can serve 100 square feet of roof area (i.e., 2îx3îdownspout for 600 ft 2 ; 3îx4î downspout for 1,200 ft 2 ). (Source: ìAll About Guttersî by Andy Engel, Fine Homebuilding, August/September 1999). Figure 4.4 - Roof Gutters and Discharge Methods 18
TABLE 4.2 - ROOF PITCH FACTORS Roof Pitch Factor Flat to 3:12 1 4:12 to 5:12 1.05 6:12 to 8:12 1.1 9:12 to 11:12 1.2 12:12 1.3 Rain and Water Vapor TABLE 4.3 - GUTTER CAPACITY (ROOF AREA SERVED IN SQUARE FEET) BASED ON 1 IN/HR RAINFALL INTENSITY 1 Gutter Shape Gutter Size 5-inch depth 6-inch depth K-style 5,520 ft 2 7,960 ft 2 Half-round 2,500 ft 2 3,840 ft 2 Note: 1. Values based on a nearly level gutter. Increasing gutter to a slope of 1/16 inch per foot, multiply values by 1.1 or by 1.3 for 1/8 inch per foot slope. Figure 4.5 - Rainfall Intensity Map of the United States 4.2.3 Recommendation #3: Weather Barrier Construction Weather barrier is a broad term for a combina tion of materials including siding, roofing, flashing, sheathing, finishes, drainage plane, and vapor retarders that, as a system, exhibit water retarding and vapor retarding characteristics and may also possess thermal insulation and air infiltration barrier characteristics. Drainage Planes The primary goal in protecting a building wall is to shield the wall from bulk moisture through the use of overhangs, gutters, siding, and opening protection (i.e., flashing or overhangs). As a second line of defense, a drainage plane provides a way out to drain any moisture that penetrates the wallís primary line of defenses (i.e., rain water that gets behind cladding). In less severe climates (low climate index - see Figure 4.3) or when a wall is otherwise protected from rain, the use of a specially detailed DRAINAGE, VAPOR, AND AIR Drainage planes do just what their name impliesó they drain away liquid water that gets past siding or exterior cladding. But thatís not all they do. Drainage planes made from building paper or housewrap can affect how water vapor passes (or tries to pass) through a wall. Table 4.4 gives recommendations on this. Drainage planes like housewrap may a lso serve as air barriers, a boundary around the house that reduces air infiltration. Even if housewrap is only used as an air barrier to cut down air infiltration, itís crucial to understand that it will also collect and channel liquid water that gets past the wallís claddingólike it or not. Housewrap Recommendations (page 25) gives guidance on this issue. 19
Chapter 4 Step 1 Divide by rainfall intensity as follows: (5,520 ft 2 * in/hr)/(7 Horizontal projected roof area = (14í x 12í) + (14í x 34í) = in/hr) = 788 ft 2 > 708 ft 2 OK 644 ft 2 Therefore, the gutter is capable of serving this area. Factored area = (1.1)(644 ft 2 ) = 708 ft 2 Step 4 Step 2 A single 2î x 3î downspout is not large enough (i.e., 600 ft 2 From rainfall intensity map, Figure 4.5, the estimated < 708 ft 2 ). Therefore, use one 3îx 4î downspout (at one of rainfall intensity is 7 in/hr. the outside corners) or two 2î x 3î downspouts (one at each outside corner). Be sure the gutter is sloped evenly Step 3 from near its midpoint toward each downspout so that a Select a K-style gutter with a 5-inch-depth and a 5,520 ft 2 - nearly equal roof area is served by each. in/hr rating from Table 4.3. barrier may have little durability benefit. However, for wall systems that are not extremely well-protected from bulk moisture, that are in wind-driven rain climates, or that are sensitive to wetting, the use of a secondary drainage plane should be employed. Figure 4.6 shows a typical wall system with siding. Itís safe to assume that all types of wall coverings (siding, brick, masonry) are imperfect and will leak at some pointó some more than others. Therefore, it is important to consider the use of a drainage plane behind the siding material. In some climates, like arid regions with infrequent rain events, a drainage plane may be unnecessary or of very little use. Rain water that does penetrate wood-framed wall systems in these regions can take advantage of woodís capacity to temporarily store moisture, and the wall can dry out via air movement and vapor diffusion once arid outdoor conditions resume (see below for more about Drying Potential). It may be advisable to use an air space between siding and a drainage plane if: A house is in a particularly severe climate (frequent rainfall or wind-driven rain) such as coastal regions subject to hurricanes; and Moisture-sensitive siding materials (e.g., wood) are used. This air space (e.g., use of furring in Figure 4.6), in conjunction with vents (and general air leaks) that allow air to move behind the exterior siding or cladding, provides pressure equalization and creates 20
a capillary break between the back of the siding and the drainage plane. These features will help to reduce the amount of rain water that penetrates behind the exterior cladding and promote better drying potential for the siding and the inner wall. However, creating this space using furring strips applied on top of the drainage plane material must account for the effect on details for flashing and finishing around wall openings such as windows and doors. Depending on the wall design approach and the climate, a drainage plane needs to exhibit certain characteristics for allowing or retarding the transmis sion of water vapor, while still rejecting the passage of liquid water like rain. Table 4.4 provides guidance in selecting appropriate wall drainage plane charac teristics for various climates. The table considers both how well certain materials reject liquid water and how readily they allow water vapor to pass through them. This is an important issue that affects the drying potential of walls. Rain and Water Vapor The properties of materials that can be used for drainage planes are found in Table 4.5. In all applicatio ns, any material used as a drainage plane should have high resistance to liquid water penetration. Vapor Retarders While itís obvious that the drainage plane of a wall must be located on the outer face of a wall or just behind the siding, it is just as important to remember one rule of thumb related to moisture vapor transport in walls. Namely, any vapor retarder must be located on the warm-in-winter side of the wall (i.e., inside) in all climates except hot/humid climate where it should be placed on the warm-in summer side of the wall (i.e., outside) if one is used at all. Water vapor in the air is transported by vapor diffusion and bulk air movement. Vapor retarders are intended to restrict the transmission of water vapor via diffusion. A common application of a vapor Figure 4.6 - Weather Barrier Construction 21
Chapter 4 retarder would be the use of a polyethylene sheet or kraft paper between drywall and framing of exterior walls in cold climates. However, bulk air movement (i.e., air leakage containing water vapor) is far more significant in terms of the amount of water vapor that can be transmitted, moving roughly 10 to 100 times more moisture than diffusion. This being said, the vapor retarder can still play an important role in controlling the movement of water vapor in walls, particularly in very cold climates. Table 4.6 provides guidance on appropriate locations and characteristics of vapor retarders for various climates. When using a vapor retarder, it must be installed on the correct side of the wall or ceiling. Otherwise, condensation will form and cause sudden or eventual damage. Also, some older codes established minimum perm ratios for the inner and outer faces of a wall (e.g., a minimum outer face to inner face perm ratio of 5:1 in cold climates to facilitate drying to the outside). Design rules like this one point out that many materials can and will affect vapor diffusion even if they are not classified as vapor retarders. This point, and the fact that air movement can also move large amounts of water vapor, are equally important to designing a wall to handle water vapor. Building Paper vs. Housewrap The question ìshould I use building paper or housewrapî is often asked. And for certain climates in Table 4.4, the question remains. This leads to a discussion of the two product categories and their relative performance characteristics. Any discussion of this sort should be prefaced by recognizing that neither product will work effectively if not installed correctly ñ and could even do serious harm to a buildingís durability if used incorrectly. TABLE 4.4 - RECOMMENDED DRAINAGE PLANE CHARACTERISTICS FOR EXTERIOR WALLS IN VARIOUS CLIMATE CONDITIONS Climate Condition 1 Drainage Plane Characteristic Recommended Product Type Liquid Water Resistance Water Vapor Permeability (low = little vapor passes; high = vapor passes easily) Hot & Humid High Moderate to Low 2 15# tarred felt Climate Index >70 HDD < 2,500 Mixed High High to Moderate 15# tarred felt or Climate Index >20 housewrap 2,500 < HDD < 6,000 Cold High High 3 15# tarred felt or HDD > 6,000 housewrap Dry N/A N/A 4 optional Climate Index < 20 Notes: 1 HDD refers to Heating Degree Days relative to 65F (see Figure 4.7). See Figure 4.3 for Climate Index. 2 HOT/HUMID CLIMATE CONCERNS: The drying potential of hot/humid climates is through the interior wall, and the layer of lowest vapor permeability (i.e., vapor retarder) must be located to the outside of the wall. If a drainage plane material is used with a low permeability (i.e., polyethylene sheet or foam panel insulation) then it is imperative that a high permeability is achieved on the inside face of the wall (which may affect interior finish selection such as paint type and limit use of materials such as wall paper ñ see Table 4.5 below). In addition, it be comes more important in hot/humid climates to carefully size HVAC systems so that they operate without ìshort cycling.î Again, moisture entry to the building and condensation potential can be significantly reduced by use of a foundation/ground vapor barrier (Chapter 3). 3 COLD CLIMATE ALTERNATIVES AND CONCERNS: In this case, energy efficiency can be a conflicting objective to the tableís recommendation. For instance, interest in energy efficiency (or code mandated minimum R-values) often leads builders in cold climates to place an impervious layer of insulation (i.e., polystyrene or foil-faced poly isocyanurate) on the outer surface of the wall. These materials generally have a low permeability to water vapor (see Table 4.5). Since vapor barriers are often required on interior (warm-in-winter side) of walls in cold climates, this can create a situation where a wall has low drying potential. Therefore, this approach should be used with caution in areas that are cold but are also subject to substantial rainfall which may penetrate an improperly installed weather barrier or one that fails to maintain its resistance to liquid water penetration over time. In addition, it becomes critical to seal key leakage areas judiciously to prevent leakage of moist, warm indoor air into the wall cavity where it may condense. Condensation in the wall cavity can also be prevented by controlling indoor air humidity. At a minimum, interior moisture sources should be addressed by using bathroom and kitchen exhaust fans to remove the significant moisture that is produced in these areas of the building. Finally, moisture entering the building/walls from the ground should be minimized by the use of foundation and ground vapor barriers (see Chapter 3). 4 No drainage plane is required for durability purposes in a dry climate, although care should be taken to seal major air-leakage points for sake of keeping infiltration air out of wall assemblies. 22
Housewrap products are sometimes viewed solely as air barriers ñ a product that will reduce air infiltration and do nothing else. Wrong. As discussed in Table 4.4, housewrap products also block liquid water that gets past siding, making this type of product useful for a drainage plane. And in fact, housewraps will act to collect and channel liquid water whether the installer intends for them to do so or not. This can lead to trouble if housewrap is installed in a manner (e.g., not lapped correctly, drains water behind windows) that doesnít allow for channeling water out of a wall system. So the lesson is: housewraps are not just air barrier products, they can ñ and should be ñ used as drainage planes as well. Their vapor diffusion characteristics arenít sufficient to allow quick drying should misinstallation result in bulk water penetra tion. Rain and Water Vapor PLUG UP THE LEAKS In all cases, major air leakage points through the building envelope should be sealed to limit the flow of air, heat, and moisture. Places to air seal include areas around door and window frames, attic hatches, kneewalls, HVAC chases, and electrical and plumbing penetrations into attics. TABLE 4.5 - DRAINAGE PLANE AND VAPOR RETARDER MATERIAL PROPERTIES 1,2 Material Weight or Thickness Permeance, Perms 3 Liquid Water Loss 5 (vapor retarder = 1 perm or less) 15# asphalt felt 14 lb/100 sf Dry-cup Method Wet-cup Method Other 30% 1.0 5.6 ó 15# tar felt 14 lb/100 sf 4.0 18.2 4 ó ó Building wraps (6 brands) ó 5.0 - 200.0 5.0 - 200.0 ó 0 to 80% 6 Blanket Insul., asphalt coated paper 6.2lb/100 sf 0.4 0.6 - 4.2 ó ó 6mil polyethylene 0.006 in 0.06 ó ó ó Aluminum foil 0.001 in 0.0 ó ó ó Gypsum board 3/8 in ó 50.0 ó ó Plywood (interior glue) 1/4 in ó 1.9 ó ó Block 8 in ó 2.4 ó ó Bri ck 4 in ó 0.8 ó ó Concrete 4 in ó 0.8 ó ó Polystyrene, expanded board 1 in 2.0 - 5.8 ó ó Polystyrene, extruded board 1 in 1.2 ó ó Vapor retarder paint 0.0031 in ó 0.5 ó ó Primer sealer paint 0.0012 in ó 6.3 ó ó Exterior acrylic house and trim paint 0.0017 in ó 5.5 ó ó Notes: 1 These values only relate to performance in standardized and constant test conditions and do not necessarily represent actual behavior under actual conditions of use. Leakage as a result of discontinuities and other conditions experienced in construction of buildings may easily alter, by a factor of 2 or more, the overall or localized performance of a vapor retarder in comparison of these standardized values. Therefore, these values can be used for indexing purposes only. 2 Differences in perm ratings between dry-cup, wet-cup, and other test methods are substantial and any cross comparison should be made on the bases of similar test methods and conditions. Manufacturer data should be consulted when available. 3 Usually tested according to ASTM E 96. 4 Value can vary to more than 60 perm in 95% relative humidity test conditions. 5 Tested using AATCC 127 test method modified to a 3.5 inch head for 2-hour duration (University of Massachusetts, Building Materials and Wood Technology, Paul Fisette, as reported on www.umass.edu/bmatwt/weather_barriers.html, October 1999). 6 Of six brands tested, R-Wrap and Tyvek received the best possible rating of 0 water loss (liquid water transmission). However, when these products were subjected to soapy water and a cedar extractives water solution, the loss rates increased slightly. 23
6,000 Outer side of wall Foundation (slab, crawl, or basement) Attic & Cathedral Roof Inner side of wall Foundation (slab, crawl, or basement) Attic & Cathedral Roof (ceiling side) 4 Inner side of wall Foundation (slab, crawl, or basement) Attic & Cathedral Roof (ceiling side) 4 Low to moderate (see Table 4.4, Drainage Plane) 3 Low High Moderate (2,500 HDD) to Low (6,000 HDD) Low High (2,500 HDD) to Moderate (6,000 HDD) Low Low Moderate (6,000 HDD) to Low (9,000 HDD) 15# tarred felt 6 mil polyethylene plastic sheet on ground None Kraft paper on batts or vapor retarder paint on interior 6 mil polyethylene plastic sheet on ground None to Kraft paper on batts (6,000 HDD) 3 mil polyethylene or vapor retarder paint on interior 6 mil polyethylene on ground Kraft paper on batts to 3 mil polyethylene or vapor retarder paint on interior Notes: 1 HDD refers to Heating Degree Days relative to 65F (see Figure 4.7). 2 These recommendations are based on both the material properties (perm s) and how they are used. A product that is not applied continuously over a surface (e.g., kraft faced batts in a ceiling) will allow more vapor to pass than a continuous layer. 3 Because it is equally important to ensure that the interior surface of a wall has a high permeability finish, select paint with high permeability and avoid finishes such as vinyl wall paper that will act as a vapor barrier. Prevention and Control of Decay in Homes, USDA/HUD, 1978, recommends that ìIn warm climates, walls and ceilings without vapor barriers are safer.î 4 Attic vapor barriers for hip and gable roofs, if used in mixed and cold climates, should be placed on the warm-in-winter side of the attic insulation. The same applies to cathedral ceilings. 24]]>
vapor transmission through the wall (in either direction). This characteristic is a benefit in hot and humid regions and in designs where some resis tance to vapor movement from outside to inside is desired (e.g. behind brick veneer or unsealed wood siding). While building paper is not usually viewed as an air barrier product, it can still be used in conjunc tion with other measures (e.g. caulk and foam sealants) to produce a wall system with reduced air infiltration. So both products can shed liquid water. Housewrap tends to be more vapor permeable than building paper (check the perm rating for specific brands though), allowing water vapor to diffuse more easily; but neither product would be considered a vapor retarder even though both slow the movement of vapor to some degree. Housewrap can be used as an air barrier, whereas building paper would likely be used in tandem with other air sealing measures. These differences, as well as price, should be the basis for a choice when a decision needs to be made. But once more, keep in mind that neither type of product will perform the way itís supposed to if itís not properly installed and integrated with flashing of windows and doors (see Section 4.2.4 on flashing and housewrap installation). Housewrap Recommendations Housewraps are relatively new materials that serve a dual role as a secondary ìweather resistantî barrier and an air barrier. However, this dual role of Rain and Water Vapor building materials has been known for some time for materials such as building paper or ìtar paperî (USDHEW, 1931). Even lath and plaster has been classified as an effective air barrierña finding that also stands for its modern day counterpart, gypsum wallboard. Of course, an air barrier is not a substitute for proper sealing of penetrations in the building envelope around windows, doors, utilities, and other leakage points. Therefore, as with the application of building paper, housewraps should be viewed and installed with the main goal of serving as a secondary weather-resistant barrier (i.e. drainage plane). Like tar paper, the edges of housewrap should be lapped to provide a drainage pathway for water out of the wall. It is only necessary to tape lapped edges if some improvement in air-barrier performance is desired. However, building wraps are not all created equal in terms of their ìbreathabilityî and this additional sealing can affect the drying time of the wall system should it become inadvertently wetted by condensation or, more importantly, rain water (See Table 4.5). At wall penetrations, the housewrap should be properly detailed or flashed (See Section 4.2.4). In some cases, housewraps are installed after window and door installation (Figure 4.13), and manufacturer-recommended tapes must be used to seal the joints. While this practice is not uncommon, a preferred method is to install the building wrap prior to window and door installation and to additionally flash window and door heads as shown in Figure 4.12. Figure 4.7 - Heating Degree Day (HDD) Map of the United States (65 o F basis) 25
Chapte r 4 Drying Potential Drying potential, the ability of a wall system to dry out after it is wetted, is important because it can compensate for conditions when water gets where itís not supposed to be. High drying potential will allow walls that are moist to dry out in a reasonable amount of time and limit the consequences. An ideal wall would be one that doesnít let any moisture in from interior vapor, exterior vapor, rain, snow, or ice. This would require a hermetically-sealed wall, which is not practical in residential construction. If this design approach of a ìperfectî sealed wall is pursued and water does get into the wall, it will be trapped there and the results can be disastrous. Therefore, it is imperative to make less than ideal materials work satisfactorily through careful design, careful construction, and an expectation that water will get into walls. Appropriate solutions will depend on climate conditions, the building use conditions, and common sense. An ideal wall material acts as a storage medium, safely absorbing excess moisture and expelling it when the relative humidity decreases during periods of drying. Heavy masonry walls do this. To some degree, natural wood materials also exhibit this characteristic and create a beneficial ìbuffering effectî to counter periods where moisture would otherwise accumulate to unacceptable levels. This effect is part and parcel of the ìbreathing buildingî design approach and it serves as a safety factor against moisture problems, just like a roof overhang. Materials such as concrete, masonry, and brick also exhibit a moisture storage or buffering capacity as do many contents of a home. This creates a lag effect that should be considered in building design and operation. For example, moisture levels in building materials tend to increase during warm summer months. As the weather cools in the fall, a moisture surplus exists because the expulsion of excess moisture lags in comparison to the rate of change in season temperatures. Bear in mind that most building moisture problems are related to exterior moisture or rain. Moisture vapor and condensation is usually only a problem in extremely cold climates (upper Midwest and Alaska) or in extremely hot and humid climates, particularly when significant moisture sources exist within a home. For instance, a small house in a cold climate with high internal moisture loads (people, bathing, cooking), little natural or mechanical ventilation, and the lack of a suitable interior vapor retarder (i.e., between drywall and external wall framing) will likely experience moisture problems. 4.2.4 Recommendation #4: Proper Flashing Flashing is perhaps one of the disappearing crafts in the world of modern construction and modern materials that seem to suggest simple installation, ìno-worryî performance, and low maintenance. An emphasis on quick installations often comes at the expense of flashing. Good flashing installations take time. But itís time well invested. So, if flashing is to be installed, it is best to invest the effort to make sure itís done right. In Figures 4.8 - 4.16 some typical but important flashing details are provided as models for correct installation techniques. RULES OF THUMB AND TIPS Flashing is necessary for proper drainage plane performance in walls and for roofing systems. Most leakage problems are related to improper or insufficient flashing details or the absence of flashing. All openings in exterior walls and roof penetrations must be flashed. Caulks and sealants are generally not a suitable substitute for flashing. Water runs downhill, so make sure flashing is appropriately layered with other flashings or the drainage plane material (i.e., tar, felt, or housewrap). Water can be forced uphill by wind, so make sure that flashings have recommended width overlap. Sometimes capillary action will draw water into joints between stepped flashing that is not sufficiently lapped or that is placed on a low- pitch roof ñ take extra precaution in these situations. Avoid joint details that trap moisture and are hard to flash. Treat end joints of exterior wood trim, railings, posts, etc. prior to painting; paint end joint prior to assembly of joints; if pre-treating, be sure the preservative treatment is approved for use with the type of paint or stain being used. Minimize roof penetrations by use of ventless plumbing techniques, such as air admittance valves, side wall vents, and direct vented appliances (check with local code authority for approval). Use large roof overhangs and porches, particularly above walls with numerous penetrations or complex window details. 26
Rain and Water Vapor BASIC FLASHING MATERIALS AND TOOLS Flashing stock (coated aluminum, copper, lead, rubber, etc.) 15# felt paper Bituminous adhesive tape Utility knife Aviator snips or shears Metal brake (for accurate bending of custom metal flashing) Figure 4.8a - Basic Roof Flashing Illustrations 27
Chapter 4 Figure 4.8b - Basic Roof Flashing Illustrations (continued) NOTES for Figure 4.9: ï Extend eave flashing 18 to 24 inches inside the plane of the exterior wall. ï Overhang eave flashing 1/4 - inch beyond drip edge flashing. ï Apply mastic continuously to joints in eave flashing. ï If joints in the eave flashing are not avoidable, locate them over the soffit rather than the interior area of the building. Figure 4.9 - Eave Flashing for Preventing Ice Dams ï While eave flashing is generally recommended for areas with an average January temperature less than 25F, ice dams can be prevented by (1) adequate sealing of ceilings and tops of interior and exterior walls to prevent warm indoor air from leaking into the attic space, (2) adequate attic/roof insulation (usually local code requirements are sufficient) all the way out to the plane of the exterior walls and (3) proper ventilation through the eave and attic space. 28
Rain and Water Vapor Figure 4.10 - Window Flashing Illustration (building wrap installed prior to window; typical nail flange installation) 29
Chapter 4 Figure 4.11 - Window Sill and Jamb Flashing Detail (building wrap installed after window) 30
Rain and Water Vapor Figure 4.12 - Window Flashing for Severe Weather (areas subject to frequent wind-driven rain) Figure 4.13 - Door and Head Trim Flashing Detail 31
Chapter 4 Figure 4.14 - Deck Ledger Flashing Detail 32
Rain and Water Vapor Figure 4.15 - Typical Brick Veneer Flashing Details 33
Chapter 4 Figure 4.16 - Brick Veneer Flashing at Roof Intersections 34
4.2.5 Recommendation #5: Sealants and Caulking In general, do not depend on sealants and caulking for long-term service. Using normal quality caulks and sealants with typical surface preparation, combined with shrinkage and swelling of building components, usually results in failure of a water tight seal within 2 to 3 years or less, particularly on southern exterior exposures. Nonetheless, there will be joints and seams that will benefit from appropriate use and maintenance of caulks and sealants. Optimally, joints in exterior wood trim or framing should be simple enough not to trap water and allow quick drying. With reasonable adherence to manufacturer instructions (particularly with respect to surface preparation and conditions during installation), high quality caulks and sealants can be made to endure for a reasonable time (i.e., up to 5 years or consider ably more when not severely exposed). Some recomm endations regarding selection of quality caulks and sealants are provided in Table 4.7. In addition, caulks and sealants should be stored in a warm environment and should not be stored for more than a couple of years before use. Finally, the need for homeowner maintenance and replacement of caulking must be strongly emphasized. 4.2.6 Recommendation #6: Roof and Crawl Spaces To Ventilate or Not to Ventilate The use of ventilation has been a topic of confusion for some time. Until recently there has been little convincing research to confirm traditional practices or to suggest better ones. To aid in decisions regarding roof and crawlspace ventilation, recommendations are provided in Table 4.8 based on the best information available on the topic. Prior to use, the reader should consult local building code requirements and roofing manufacturer warranties to identify potential conflicts. Roofs vents (when required) must be installed in accordance with the local building code or accepted practice. Plastic vent louvers commonly used on gable ends must contain UV inhibitors. Vents must be adequately screened to prevent vermin or insect entry. In addition, ridge vents (if used) should be installed and attached to the roof in accordance with manufacturer recommendations ñ numerous incidents of improper installation have resulted in damage during wind events or rain/snow entry to the roof. Vent area ratios, such as 1 square foot of vent Rain and Water Vapor opening for every 300 square feet of attic area refer to the net vent area, not gross area; so the sizing of vents must account for obstructions to vents from louvers and screens. The roof ventilation recommendations in Table 4.8 are based primarily on durability concerns. These recommendations are further based on the assump tion that the following good practices have been employed: All bath and kitchen exhaust fans exhaust moist indoor air directly to outdoors. Indoor relative humidity is kept within reasonable limits (i.e., 40-60%) and significant point sources of moisture (e.g. hot tubs) are controlled with ventilation. Ceiling vapor barriers are used in accordance with Table 4.6. Proper attic insulation levels are installed for the given climate and location. While non-vented roof assemblies are a viable alternative (especially in hot/humid climates), performance data on such designs over time is still lacking. Further, the required detailing that goes along with such a design (e.g., insulation detailing, controlling surface temperatures in the assembly to prevent condensation) may be less forgiving than a traditional ventilation approach in terms of durability. If a non-vented design is employed, some critical items to consider include: Local building department approval; Implications for roofing material warranty; All major air leakage points between the living space and the attic (wire penetrations, recessed light cans, plumbing lines, HVAC boots and chases, attic hatches) have been sealed to limit air leakage; and Perimeter wall insulation detailing to satisfy local fire and insect design requirements. 35
36 T ABLE 4.7- CAULK CHARACTERISTICS AND APPLICA TION RECOMMENDA TIONS 1 Chapter 4 Caulk Oil-base Acrylic- latex Butyl rubber Polysulfide rubber Silicone rubber Urethane W eatherstrip/ caulking cord Life (yrs) 1-7 2-10 7-10 20+ 20+ 20+ to 20 Best Uses not desirable indoors, protected, or painted narrow openings in wood, metal, glass, masonry anywhere outdoor metal, heat ducts, shallow joints anywhere temporary draft sealing and hole plugging Adhesion fair-good excellent, except metal very good excellent good, excellent with primer excellent none Shrink-free poor fair fair excellent excellent excellent excellent Primer Use 2 porous surfaces porous surfaces for best results none needed special primer on all but metal porous surfaces none needed none needed Joint T ype non-moving to 1/4îw , 3/4îd non-moving to 1/4î w non-moving up to 1/4îx1/4î all up to 1/2îx1/2î all from 1/4î d all to 1/4îx1/2î non-moving T ack-free (hrs) 2-24 1/4-1/2 1/2-1 1/2 24-72 2-5 4-14 - Cure (days) to 365 3 7 7 2-5 4-14 none Clean up with 3 paint thinner water paint thinner , naphtha TCE, toluene, MEK paint thinner , naphtha, toluol, xylol MEK, acetone, lacquer thinner not sticky Paint must best best if desired read label if desired no A vailable Colors white, natural, gray white, black, gray , bronze white, clear , gray , black, brown, redwood, beige, bronze, sandstone white, black, gray limestone, bronze, brown white, black, clear , gray white, gray , black, limestone, bronze; special colors clear , gray Source: Structures and Environment Handbook , Eleventh Edition (Midwest Plan Service, 1983) NOTES: 1 Based on advancement in caulk formulation and materials, this table may be in need of revision and may not include newer materi als. 2 ìPorousî includes wood, wood products, concrete, and brick. 3 MEK = methyl-ethyl-ketone, TCE = trichloroethylene.
For crawlspaces, a non-ventilated crawlspace design can be employed in all of the climate regions shown in Table 4.8. A non-ventilated crawlspace offers benefits in terms of both moisture control and energy performance. Ventilated crawlspaces, especially in humid and mixed regions, often introduce moist outdoor air into a cooler crawlspace environment. The result is condensation and the resulting problems like mold and degradation of building materials. In terms of energy, an unventilated crawlspace also provides an area for HVAC equipment and ducts that doesnít present the temperature swings (and energy penalties) found in ventilated crawlspaces. Rain and Water Vapor Thereís more to it than just taking out the vents however. The following steps must also be followed when building a unventilated crawlspace: Careful attention to exterior grading (4% slope minimum); Air sealing between outdoors and the crawlspace area to prevent humid air from getting into the crawlspace; Insulating at the crawlspace perimeter wallsñ not the floor; 6 mil polyethylene groundcover in crawlspace with joints lapped; and Damp-proof foundation wall. TABLE 4.8 - ROOF AND CRAWL SPACE VENTILATION RECOMMENDATIONS Climate 3 Attic 1,5 Cathedral Roof 4 Crawl Space 2 Hot/Humid Yes Yes No Mixed Yes Yes Not Preferred Cold Yes Yes Optional Arid (dry) Yes Yes Optional NOTES: 1 All roof ventilation recommendations are based on the ceiling being sealed at all major air leakage points (i.e., chases, electric and mechanical penetrations, etc.) and bath and kitchen vent ducts adequately routed to expel air out-of-doors. In some climates (see Table 4.6), a ceiling vapor retarder (i.e., vapor retarder paint, polyethylene sheet, or asphalt coated paper) is required in addition to adequate attic/roof insulation. 2 All recommendations are based on properly graded sites and the use of a continuous ground vapor retarder applied to the foundation area. 3 Climates are defined as in Table 4.4. 4 Cathedral roof ventilation must be continuous along soffit/eave and ridge. 5 Net attic vent area should be 1/300 of attic area and vents shall be continuous along soffit/eave and also located at the ridge and/or gable ends. 37
Chapter 4 38
CHAPTER 5 Sunlight 5.1 General Sunlight is made up of visible light and non- visible radiation such as ultraviolet (UV) and infrared (IR). Depending on the color and surface character istics of an object, various wavelengths of solar radiation may be absorbed, reflected, and emitted (i.e., ìreleasedî). The more light absorbed and the less heat capacity (i.e., thermal mass ), the greater the objectís ability to be heated by sunlight. For example, a dark driveway becomes much hotter on a sunny day than a light colored concrete sidewalk. Thus, the sun produces two significant effects that attack materials and shorten their life-expectancy: (1)chemical reaction (i.e., breakdown) from ultraviolet radiation and heat (2)physical reaction (i.e., expansion and contraction) from daily temperature cycles caused by objects absorbing and emitting heat gained from sunlight. The chemical and physical reactions caused by sunlight can cause colors to fade and materials to become brittle, warp, or crack. Deterioration can happen relatively quickly (a year or less) or over longer periods of time depending on the characteris tics of a material and its chemical composition. In some cases, materials like plastics that are vulner able to UV degradation can be made resistant by adding UV inhibitors to the chemical formulation. Sunlight A prime example is vinyl siding. As an alternative approach, materials can be protected from sunlight by matter of design (e.g., providing shading or using reflective coatings). UV light from the sun is not all bad. For example, it is UV light that causes a chemical reaction on special paper that forms the blue lines on blue prints. However, most everyone has witnessed or experi enced the painful effects of UV radiation on skin, which causes sunburn. Consider that the exterior of a house is like its skin. Therefore, the proper selection of materials determines to what degree the building exterior will be able to withstand the damaging effects of UV radiation. The amount of solar radiation also varies by geography (see Figure 5.1); the number of cloudless days affects the dose of UV radiation over the lifetime of a product. The following section presents a few measures that can help to counter the effects of solar radiation on building materials and systems. For homes, some of the primary problems associated with solar radiation are color fading, premature asphalt roof shingle failure, and vinyl siding warping. Excessive exposure to sunlight will also cause caulk joints to fail quickly. In addition, when shining through windows, sunlight can cause interior colors to fade. Figure 5.1 - Solar Radiation Map of the United States Source: National Renewable Energy Laboratory 39
Chapter 5 5.2 Recommended Practices 5.2.1 Recommendation #1: Overhangs As with rain on the building envelope, properly sized roof overhangs can minimize the exposure to solar radiation and, hence, minimize radiation-related problems. The width of a roof overhang that will protect walls from excessive solar exposure in the summer while allowing heat gain through windows from winter sunshine depends on where the building is located with respect to the equator. The sun is higher overhead in the summer than in the winter. In addition, for any day of the year, at higher latitudes the sun is lower in the sky than at lower latitudes. Therefore, buildings situated farther south receive greater protection from the summer sun by roof overhangs, as shown in Figure 5.2. The solar angle factors of Table 5.1 can be used to help determine overhang width to achieve the desired shading effect on south-facing surfaces. An example calculation shows how the solar angle factor is used. TABLE 5.1 - SOLAR ANGLE FACTORS 1 Date To prevent winter shading: Dec 21 Jan 21 and Nov 21 Feb 21 and Oct 21 Mar 21 and Sept 21 To produce summer shading: April 21 and Aug 21 May 21 and July 21 June 21 24 1.5 1.2 0.8 0.4 0.2 0.1 0.0 Latitude (degrees North) 32 40 2.0 3.0 1.7 2.4 1.0 1.4 0.6 0.8 0.4 0.5 0.2 0.4 0.1 0.3 48 5.4 3.8 1.9 1.1 0.7 0.5 0.5 Source: Structures and Environment Handbook, Eleventh Edition, Midwest Plan Service, Iowa State University, Ames, Iowa, 1983. 1 Factors apply for t imes between 9:00am and 3:00pm for winter shading and at noon for summer shading. Direct south facing orientation is assumed. Figure 5.2 - Effect of Building Latitude on Effectiveness of Overhangs 40
EXAMPLE: DETERMINE ROOF OVERHANG WIDTH TO PROTECT WALL AGAINST SUMMER SUN Find the overhang length (OL) to shade 6 feet of wall below the roof overhang for June through July. Building is located at latitude of 32 degrees North (consult Atlas for latitude). It is desired to provide shade for 6 feet of wall below the overhang at mid-day (i.e., to bottom edge of windows). Solar Angle Factor (SAF) = 0.2 (for July 21) from Table 5.1 Wall distance below overhang to shade (WD) = 6 feet OL = SAF x WD = (0.2)(6 feet) = 1.2 feet Use a 16-inch (1.33 feet) overhang which will provide roughly 6 feet 8 inches of shading below the overhang. Determine degree of shading in the winter (using Feb 21) as follows: WD = OL/SAF = 1.33 feet / 1.0 = 1.33 feet or 16 inches. The selected overhang width will provide no more than about 16 inches of shading to the wall during the main winter months of November through February. However, some shading to the top few inches of windows will occur in the early and late winter months when maximum solar heat gain may be desirable. But, in this case, the overhang width should not be decreased in the interest of maintaining weather protection of the wall. Sunlight 5.2.2 Recommendation #2: Light Colored Exterior Finishes As a second line of defense against damage from solar radiation, light colored materials and finishes can be selected. White is excellent and aluminum, reflective-type coatings are even better. Light colors can also reduce summertime cooling load and should reduce energy bills especially in cooling dominated climates by lowering the solar heat gain into a building. If properly accounted for in cooling load calculations, lighter colored roofing may allow for the use of smaller capacity air conditioning units. In addition, light colored roof shingles reduce shingle temperature and, therefore, increase shingle life. The effect of building exterior color on solar heat gain is illustrated in Figure 5.3. It is very important, however, to keep light colored finishes like roofs relatively clean to take full advantage of their reflectivity. 5.2.3 Recommendation #3: UV Protective Glazing Windows that receive direct sunlight and that are not treated to block UV radiation will allow sunlight to enter and fade susceptible materials such as furniture coverings, carpeting, and drapes. One solution is to specify interior materials that have UV inhibitors or that are not susceptible to UV radiation. Another solution is to specify colors that will not show fading. However, if these options are not desired or considered sufficient, there are glazing options for windows and doors that block UV radiation. These relatively expensive treatments need only be specified for south-facing windows. Figure 5.3 - Effect of Surface Coloration on Solar Heat Gain 41
Chapter 5 5.2.4 Recommendation #4: UV Resistant Materials Some materials are naturally UV-resistant, while others require the addition of UV inhibitors in the make-up of the material. For example, concrete or clay tile roofing and Portland Cement stucco or brick siding are naturally resistant to UV radiation and are also resistant to temperature effects compared to other exterior building materials. On the other hand, plastics are prone to ìdry rotî (embrittlement from excessive UV exposure) unless UV inhibitors are provided. Plastics are also prone to significant expansion and contraction from temperature swings. Be sure that UV inhibitors are used in materials that require protection. Many low budget compo nents, such as some plastic gable end vents, may also lack UV resistance. All other factors being equal, choose the material with the best UV resis­