ISSUES IN CRAWL SPACE DESIGN AND CONSTRUCTION-A SYMPOSIUM SUMMARY

William B. Rose Anton TenWolde Member ASHRAE

INTRODUCTION Washington, and Montana, observed wood decay only in a very few cases where plumbing leaks existed or where wood At the ASHRAE Winter Meeting in January 1994 in members were improperly in direct contact with the earth. New Orleans, LA, a symposium entitled “Recommended Generally, the crawl spaces inspected in this series of Practices for Controlling Moisture in Crawl Spaces” was surveys were virtually devoid of moisture-related problems. held, jointly sponsored by ASHRAE Technical Committees Moisture problems generally occur when improper TC 4.4 and TC 4.9. The aim of this summary paper is to drainage or grading around the house leads to wet soil or outline the pertinent issues in crawl space construction that even standing water in the crawl space. Evaporation of this were addressed in the eight papers presented at the sympo­ moisture then causes high humidity in the crawl space and sium. A wide variety of geographical and climatic regions often in the rest of the building and sometimes leads to high are represented: western Canada, New Zealand, Sweden, and moisture content in wood framing members in the floor and the western and eastern United States. The papers present an in the bandjoist. The traditional responses to these problems overview of the state of research in crawl space perfor­ have been to install crawl space vents and cover the soil mance. The issues presented here are important, and the with a ground cover (sometimes called a soil cover or vapor research is long overdue. We hope that the results presented retarder). But are these responses adequate or even neces­ here will be helpful to those in the fields of architecture, sary? Are there better ways of designing a crawl space? engineering, construction, code enforcement, and building How should radon entry be handled? (Radon entry was an management. issue of no concern when building code regulations for As Rose states in his paper, from their very introduction crawl spaces were formulated.) into construction practice in the northern US., crawl space homes were viewed as being trouble-prone. He cites MOISTURE RELEASE FROM CRAWL SPACES references from Johnson and Davis (1939) and the Housing and Home Finance Agency (HHFA 1948)that state strongly What is the moisture contribution to a building from the that moisture problems may be common. This sentiment is crawl space? Trethowen sought to estimate the rate of water echoed in most of the papers presented at the symposium. evaporation from a crawl space using free-water lysimetry. Flynn et al. describe the conditions in the California crawl Previous work by Abbott (1983) had revealed that, when the space condominium complex they studied thus: “Prior to the soil reaches a “critical” moisture content, its evaporation onset of this study, the conditions at this site resulted in foul rate equals that of free water. Trethowen rejected data from odors, condensation, mold growth, and wood decay (some­ soils that were below this critical moisture content. He times to the point of a loss of structural integrity or compo­ found a large variation in evaporation rates, both by site and nent failure) in all of the crawl spaces.” Fugler and Moffatt by season. The measured average rate was about 400 report that many crawl spaces in western Canada are damp, g/m2 · day (0.082 lb/ft2 · day), but Trethowen expresses experience high levels of wood moisture, and contribute to concern that this rate may be a little too high, based on the excessive house humidity and condensation. Trethowen cites amount of heat needed to evaporate that much water and the his experience in New Zealand: “Crawl space timber amount of ventilation needed to remove that much water moisture levels were in many cases very high, with floor vapor from the crawl space. Another reason (not mentioned framing often exceeding fiber saturation for lengthy periods, by Trethowen) this average is probably somewhat high when no ground cover was present.” might be that he did not include any data from soils below While moisture problems clearly do occur in some the critical moisture content where evaporation rates are crawl spaces, the majority of crawl spaces do not experience likely to be below that of free water. The rate of 400 problems. Tsongas, in his survey of crawl spaces in Oregon, g/m2 · day, however, is relatively close to the average rate

William B. Rose is a research architect at the Building Research Council at the University of Illinois at Urbana-Champaign. Anton TenWolde is a research physicist at the U.S. Department of Agriculture. Forest Service, Forest Products Laboratory, Madison, WI.

1 of evaporation-published by Britton in 1949 (see paper by evaporation by at least 70% and possibly as much as 95% Rose)--of 12.1 ga1/1000 ft2 · day (490 g/m2 · day (0.10 Fugler and Moffatt describe recent changes in the draft 1995 lb/ft2 · day]). Britton also reported a maximum evaporation Canadian National Building Code: “Every heated crawl rate of 19.1 gal/1000 ft2 · day (see paper by Rose), which space must be provided with a polyethylene ground cover to corresponds with more than 770 g/m2 · day (0.16 serve as a moisture barrier; the polyethylene must be lb/ft2 · day). This is twice the average rate measured by weighted down (for example, with concrete) and sealed to Trethowen, which is already considered high. Although a the foundation wall.” Unheated crawl spaces also require a rate of 770 g/m2 · day is theoretically possible, and although ground cover, but it is permitted to use materials such as Trethowen found a wide variation around the average, it asphalt, roll roofing, or 0.100-mm (4-mil) polyethylene. For seems unlikely that Britton’s extreme rate would occur in comparison, the ASHRAE Handbook of Fundamentals practice. (ASHRAE 1993) recommends 0.15-mm (6-mil) polyethyl­ ene, 45- to 55-lb roll roofing, or 1.14-mm (45-mil) EPDM DRAINAGE membranes. In summary, the majority view is that the ground cover As with ground covers, the importance of good site is a key element in moisture control in crawl spaces. drainage is not a controversial topic. According to many of Perhaps the ground cover is a necessary (Fugler and the authors, exterior drainage is critical. Fugler and Moffatt Moffatt; Samuelson) and sufficient (Flynn et al.) means of state, “The ground outside the foundation walls must be moisture control in crawl spaces. Ground covers are un­ graded to prevent ingress of water.” Flynn et al. found that doubtedly effective. relatively level terrain around the building, downspouts that discharged adjacent to the foundation, and irrigation from VENTILATION shrubbery and lawns all contributed to the high soil moisture contents. Rose found that “much of the technical literature Whether or not to ventilate a crawl space is probably emphasizes the importance of good construction practices, the most controversial issue concerning crawl space design. including maintaining an 18-inch clearance between soil and Many building codes require installation of vents to provide floor framing, providing easy access to the crawl space, ventilation with outside air. Rose shows evidence that the removing debris from the crawl space, and providing good research that was done in order to support these crawl space drainage around the house.” ventilation regulations actually failed to support them and therefore concluded that there is no technical basis in the GROUND COVERS literature for current or past crawl space ventilation require­ ments. There is general agreement among the authors, as well Samuelson states that introducing outside air into a cool as in previous literature, that ground covers are effective in crawl space during summer, when temperatures are most reducing humidity in the crawl space and in the rest of the conducive to mold and decay, actually raises crawl space building. Rose concludes from his review of regulatory and relative humidity and can easily produce relative humidities technical literature that, although there were questions in the of 85% to 95%, even in a crawl space with dry soils or a 1940s about the durability of ground covers in contact with soil cover. His theoretical performance calculations do not the soil, these concerns were laid to rest by 1952. Building show this effect, probably because he defined moisture input regulations after 1956 responded by mandating the use of in terms of the difference in absolute humidity of the air of ground covers in crawl spaces. Flynn et al. found that a soil the crawl space and the outdoor air, but his measured data cover was effective in lowering moisture conditions suffi­ in the ventilated, uninsulated crawl space do show the ciently to prevent further wood decay and mold, even in effect. Stiles and Custer do show higher relative humidities poorly ventilated crawl spaces with wet soil. Stiles and during summer than during winter. Flynn et al. measured a Custer also present data supporting the effectiveness of smaller variation. It could therefore be argued that current ground covers. Although Tsongas found in his survey that regulations may produce the worst of all possibilities for a ground cover is not always necessary for satisfactory summer performance: enough outdoor air enters to raise the moisture conditions, he also concludes: “While some states relative humidity but not enough to raise the temperature do not require a ground cover, it appears to be an important and reduce the relative humidity in the crawl space. element.” Trethowen points out that more than 10 air changes per This view is held internationally. Samuelson reports hour (ACH) would be needed to remove the mean indicated from Sweden that “plastic sheeting on the ground is essen­ evaporation rates without exceeding 100%relative humidity tial.” Trethowen reports these findings from New Zealand: in the crawl space. A later survey by Trethowen showed “Covering subfloor ground with polyethylene film produced that likely ventilation rates are much lower than 10 ACH. almost immediately a significant drop in the mean moisture His paper goes to the heart of the moisture balance issue: if content in floor framing timber” and “Polyethylene films ventilation is intended to remove moisture down to a safe applied commercially . . . reduced the ground moisture level, can the ventilation keep up with the high anticipated

2 moisture loads via evaporation? Indications are that it can­ ventilation seems to distract attention from more effective not, especially in damp crawl spaces without ground covers. techniques for moisture control. Fugler/Moffatt’s and Samuelson’s papers both distin­ guish between heated and unheated crawl spaces and are in RADON complete agreement that a heated, well-insulated crawl space should not be ventilated with outdoor.air. Samuelson shows There is legitimate concern for radon entry from the measured relative humidities of more than 80% during the soil into the living space via the crawl space. Soil covers entire year in a ventilated, insulated crawl space. The draft and effective separation between the living space and the 1995 Canadian National Building Code (CNBC) defines a crawl space all contribute to the solution in unheated crawl heated crawl space as any enclosed crawl space that is either spaces. Unvented crawl spaces may pose more of a chal­ connected to heated spaces elsewhere in the building or lenge, and the requirement in the draft Canadian National contains unsealed and uninsulated heating ducts. The draft Building Code for a carefully installed, heavy, 0.15-mm CNBC does require unheated crawl spaces to be vented to polyethylene ground cover for heated crawl spaces was the outdoors. Samuelson presents an innovative approach in originally intended for radon control (Fugler and Moffatt). which the crawl space is ventilated mechanically with The integrated approach presented by Samuelson effectively indoor air exhausted to the crawl space, and crawl space air addresses this concern as well by exhausting air from the is exhausted to the outdoors, with heat exchange for energy crawl space directly to the outside and providing a slight efficiency. By ensuring a slight negative pressure in the negative air pressure in the crawl space. crawl space, air in the crawl space should not enter the living space. His integrated approach to crawl space and CONCLUSIONS AND RECOMMENDATIONS whole-house ventilation not only promises an attractive and energy-efficient solution to crawl space moisture problems, The following conclusions can be derived from the it also should alleviate any concerns of radon entry into the eight papers in this symposium. living space. Tsongas found no evidence in his survey that crawl There should be proper drainage, clearance, and access. spaces with a ground cover need to be ventilated and Crawl spaces should have ground covers for moisture strongly recommends that ventilation requirements be elimi­ control. These should be installed carefully to limit nated from the building codes. Stiles and Custer also evaporation from the soil as well as the entry of radon propose that crawl spaces should have an effective moisture into the crawl space. barrier and should be totally sealed, with no operable vents, There is no convincing technical basis for current code while some conditioning should be included to ensure that requirements for ventilation. the crawl space is dried. Finally, DeWitt and Bunn do not Heated crawl spaces should not be vented with outdoor directly address the need for ventilation but point out that air. the rated net free area of a vent is not likely to correlate Unheated crawl spaces may be vented, but there is no with its ability to enhance or reduce ventilation and that overriding need to do so for reasons of moisture control equivalent leakage area measurements for crawl spaces may if an effective ground cover is present. give less misleading estimates of the actual air exchange There is at least one innovative, integrated, energy- rates in crawl spaces. efficient approach to crawl space design that combines In summary, this symposium presented no research the need for whole-house ventilation, control of mois­ findings that point to a need for ventilation of crawl spaces ture, and radon entry. This approach is presented by with outdoor air. Two crawl space designs with differing Samuelson in this publication. ventilation requirements seem to emerge from this analysis: Several authors make specific recommendationsfor fur­ 1. If the crawl space is heated (Samuelson, Fugler and ther work. Following is a compilation of recommendations: Moffatt), it may be ventilated with indoor or heated air, provided this is done in an energy efficient manner. Tsongas recommends that a side-by-side comparison of 2. If the crawl space is unheated and the envelope pro­ two sets of identical houses or apartments be under­ vides effective thermal and moisture separation between taken. Other researchers have noted the difficulty of the crawl space and the living space, the crawl space providing a side-by-side comparison when issues such may be vented with outdoor air (Fugler and Moffatt). as drainage or local soil structure may so greatly affect the outcome. Of course, many crawl spaces have no effective • Stiles and Custer point to the need to address not only moisture separation from the living space, but it seems clear the moisture control issues but also the energy issues that no problems can be effectively resolved by require­ inherent in crawl space design and construction. The ments for venting. In short, ventilation is not an effective second phase of their work is pointed in that direc­ means of moisture control in crawl spaces. Emphasis on tion.

3 • Fugler and Moffatt provide an explicit list of further duction of moisture from the outside air. However, research research required: (1) mechanical ventilation require­ can only provide technical information on the ineffective­ ments in crawl spaces, (2) standards for interface ness of ventilation. The responsibility of refining the airtightness in unheated crawl spaces, (3) customizing building codes so that they better reflect the research moisture control strategies for the peculiaritiesof site or findings is the task not only of researchers but of the larger local hydrogeology, and (4) how to build good moisture professional design and construction community. The active barriers. participation of researchers in such refinement, as demon­ • DeWitt and Bunn call for improved methods of rating strated in British Columbia, can be very constructive and vents. They hope to better characterize and quantify the helpful. movement of air into and out of a crawl space. • Trethowen notes that measurements ofevaporation rates REFERENCES in crawl spaces are prone to imprecision. Given the importance of accurate moisture load values, his effort Abbott, J.E. 1983. Subfloor evaporation rates. Paper 31, to refine techniques for estimating evaporation rates New Zealand Annual Conference. Hamilton, NZ: may prove to be very important. However, his current Institution of Professional Engineers. techniques are not applicable to dry soils or to crawl ASHRAE. 1993. 1993 ASHRAE handbook-Fundamentals, spaces with soil covers. p. 21.11. Atlanta: American Society of Heating, Refrig­ erating and Air-conditioning Engineers, Inc. It is increasingly clear that crawl space ventilation may HHFA. 1948. Basement vs. no basement. HHFA Technical not be necessary. In crawl spaces with high loads, ventila­ Bulletin No. 3-5. Washington, DC: Housing and Home tion is not adequate to the task of diluting the moisture. Finance Agency. Crawl spaces with ground covers generally have low loads. Johnson, R.P.A., and E.M. Davis. 1939. Use and abuse of These loads are low enough that ventilation is not necessary; wood in house construction. Forest Products Laboratory indeed, ventilation may be counterproductive from the Miscellaneous Publication No. 358. Washington, DC: perspective of energy efficiency, pipe freezing, and intro- U.S. Department of Agriculture.

American Society of Heating. Refrigerating and Air-conditioning Engineers, Inc. 1791 Tullie Circle, N.E., Atlanta, GA 30329 American Society of Heating ASHRAE Refrigerating and Air-conditioning Technical Data Bulletin Engineers,Inc. Volume 10, Number 3

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Recommended Frank M. Coda Publisher W. Stephen Comstock Practices for Director, Communications/Publications Mildred Geshwiler Editor, Special Publications Controlling Michelle Moran Associate Editor Lynn Montgomery Moisture in Associate Editor Stefanie Frick Crawl Spaces Editorial Secretary PRODUCTION STAFF

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A Collection of Papers These papers are also published in from the ASHRAE Meeting ASHRAE Transactions, Volume 100. Part 1 at New Orleans, Louisiana January 1994

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