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Moisture in Crawl Spaces

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Crawl space foundations can be designed and built to avoid moisture problems. In this article we provide a brief overview of crawl spaces with emphasis on the physics of moisture. We review trends that have been observed in the research literature and summarize cur-rent recommendations for moisture control in crawl spaces.
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WOOD DESIGN FOCUS V. 23, N. 3 11
Anton TenWolde, and Samuel V. Glass, Ph.D.
Moisture in Crawl Spaces
Abstract
Crawl space foundations can be designed and built to
avoid moisture problems. In this article we provide a
brief overview of crawl spaces with emphasis on the
physics of moisture. We review trends that have been
observed in the research literature and summarize cur-
rent recommendations for moisture control in crawl
spaces.
Introduction
What does it take to design and construct a crawl space
that is free of moisture problems? Crawl spaces are of-
ten historically and anecdotally associated with damp-
ness more so than other foundation types. This article
provides an overview of design fac-
tors that are critical for avoiding
moisture problems in crawl spaces.
A crawl space is defined by the Mer-
riam-Webster dictionary as “a shal-
low unfinished space beneath the
first floor or under the roof of a build-
ing especially for access to plumbing
or wiring.” For the purposes of this
article, we focus on foundations and
exclude under-roof spaces. Britton
(1948) defined a crawl space as
“that enclosed space (or spaces)
under the first floor of a building
where there is no basement or occu-
pancy and the first floor is some dis-
tance above the surface of the
ground.”
For the purposes of this article, we
find it useful to distinguish three dif-
ferent ways of building a crawl
space.
1. Open crawl space: pier-and-beam construction
where the perimeter is substantially open to airflow
(Figure 1)
2. Wall-vented crawl space: continuous perimeter wall
that includes vents to the outside (Figure 2)
3. Closed crawl space: continuous perimeter wall with
no vents to the outside (Figure 3)
Crawl space foundations primarily originated in the
southern United States, where homes were commonly
built on pier foundations (Rose 1994). These pier foun-
dations were typically fully open to the outside, or had
minimal skirting that allowed virtually unrestricted air
movement. During World War II “basementless” houses
Figure 1. Example of an Open Crawl Space with a Pier Foundation.
[USDA Forest Service Forest Products Laboratory]
WOOD DESIGN FOCUS V. 23, N. 3 12
began to be constructed in the north-
ern United States, and this was ac-
companied by the first requirements
for minimum vent openings in crawl
spaces, promulgated by the Federal
Housing Administration (FHA 1942,
Rose 1994). The requirements were
intended to prevent moisture problems
in crawl spaces, but there appears to
be no technical basis for these require-
ments in the literature (Rose 1994).
Recommendations to limit water evap-
oration from the ground by employing
a vapor-resistant ground cover first
begin to appear in 1949 (Britton 1949).
But as early as 1946, Diller (1946) re-
ported that ground covers significantly
lowered measured moisture content in
the wood floor members in the crawl
space, whether the vents were open
or closed. Later research affirmed the
effectiveness of ground covers (Diller
1953, Moses 1954, Amburgey and
French 1971, Dutt et al. 1988, Quarles 1989, Flynn et al.
1994, Stiles and Custer 1994). However, findings were
sometimes confounded by opening or closing of crawl
space vents at the same time that ground covers were
installed or removed (e.g. Moses and Scheffer 1962, Duff
1978). Curiously, vents were adopted as a requirement in
the building codes, while ground covers were not, even
though the technical evidence for the benefits from the
latter is much stronger.
A number of studies in various climates have shown that
closed crawl spaces (without vents to
the outside) can remain relatively dry
with a ground cover (Duff 1978, 1980;
Moody et al. 1985; Dutt et al. 1988;
Quarles 1989; Samuelson 1994; Stiles
and Custer 1994; Davis and Dastur
2004). These studies generally ob-
served more stable humidity and mois-
ture conditions in the closed crawl
spaces compared with wall-vented
crawl spaces. The reasons for this are
discussed below.
The Physics of Moisture in Crawl
Spaces
Moisture conditions in crawl spaces
are determined by the balance be-
tween moisture entering the
crawlspace, moisture removed, and
moisture stored in various hygroscopic
materials in the crawlspace, such as
wood and concrete. Although moisture
storage in materials in the crawlspace
Figure 2. Example of a Wall-Vented Crawl Space with Ground Cover.
[USDA Forest Service Forest Products Laboratory]
Figure 3. Example of a Closed Crawl Space with Ground Cover.
[Advanced Energy, Raleigh, NC]
WOOD DESIGN FOCUS V. 23, N. 3 13
can provide some moderation of wide swings in moisture
conditions, moisture storage is generally not sufficient to
affect long term conditions. Because our concern is
avoiding excessive moisture accumulation over the long-
term, we therefore focus on the remaining factors in this
equation: moisture entering and leaving the crawl space.
Moisture Sources
The main sources of water in the liquid or vapor phase
are ground water or rain water intrusion, evaporation from
the soil, and water vapor carried in with ventilation air. In
some cases, water leaks from broken water pipes have
been found as major contributors. The amount of water
entering the crawl space can be very large, dominating
the equation, and therefore limiting water entry should be
the first priority. This can be accomplished with site grad-
ing, appropriate location and drainage of downspouts,
and foundation drainage.
Evaporation from wet soil can be a significant contributor
of water vapor. TenWolde and Pilon (2007) estimate that
evaporation rates from wet soil can be as high as 0.2 kg/
(m2·h) (0.05 lb/(ft2·h)), but greatly depend on the temper-
ature of the soil, the humidity of the air in the crawlspace,
and the amount of heat available to evaporate the water.
Trethowen (1988, 1994) measured vapor release rates
from soil in crawlspaces and reported an average release
rate of 0.4 kg/(m2·da) (0.08 lb/(ft2·da)) from bare soil. This
translates into around 0.017 kg/(m2·h) (0.0034 lb/(ft2·h)),
which is less than 10% of the maximum theoretical rate
cited by TenWolde and Pilon. Trethowen found that the
evaporation rate varied greatly with soil temperature; the
rate decreased substantially as soil temperature de-
creased. He also found that sources of heat in the crawl
space, such as heating ducts or a furnace, can greatly
increase the rate of evaporation. Of course, this rate can
be drastically lowered by installing a vapor barrier
(ground cover) over the soil.
Moisture Removal
Moisture removal can occur by ventilation if outdoor air
contains less moisture than the air in the crawl space. A
simple calculation is given here for the sake of illustration.
Assuming a wall-vented crawl space with no ground cov-
er and an evaporation rate from the soil of 0.4 kg/(m2·da)
(0.08 lb/(ft2·da)), a fair amount of ventilation is needed. If
the incoming ventilation air is at 21°C (70°F) and 50%
relative humidity (RH), and the crawl space is at the same
temperature, the minimum amount of air needed to main-
tain the crawl space air below 80% RH is on the order of
100 L/s (about 200 ft3/min) for every 100 m2 (about 1100
ft2) of crawlspace floor area. Providing vents in the perim-
eter wall does not guarantee significant, reliable ventila-
tion. The actual amount of ventilation with outdoor air de-
pends on wind conditions, location of the vents, location
and surroundings of the building, obstructions in front of
the vents, and other factors.
Temperature Effects
If the dew point of the ventilation air is above the temper-
ature of the soil in the crawl space, the air is incapable of
removing moisture, and instead is a source of moisture to
the crawl space. This can become an issue during humid
weather in spring when soil temperatures remain cool.
During summer the outdoor dew point can also exceed
soil temperatures. This situation is not limited to hot-
humid climates; it also commonly occurs in northern cli-
mates during summer. Table 1 lists mean dew point tem-
peratures for the month of July in 30 U.S. locations.
An abundance of ventilation with outdoor air raises the
crawl space temperature closer to that of the outdoors.
Air exchange is typically much higher in open crawl spac-
es than in wall-vented crawl spaces. This is one reason
why the old-fashioned open pier foundation with ample
ventilation worked well in the past, and returning to that
design is another option (see Figure 1). Temperature and
absolute humidity levels in open crawl spaces generally
track outdoor levels fairly closely (Glass et al. 2010). In
contrast, temperature levels in wall-vented crawl spaces
tend to be cooler than outdoors during warm weather.
This means that during summer, relative humidity levels
in open crawl spaces are typically lower than in wall-
vented crawl spaces.
The majority of contemporary buildings are air-
conditioned. Indoor cooling set points are frequently close
to (sometimes below) outdoor dew point temperatures. In
air-conditioned buildings, outdoor air can thus pose a
condensation risk to subfloor sheathing or decking. In
open crawl spaces and wall-vented crawl spaces, this risk
may be mitigated by insulating the floor with foam insula-
tion of low vapor permeance (Glass et al. 2010, Lstiburek
2008), or by installing a vapor retarder at the underside of
vapor-permeable floor insulation (Verrall 1962). Air tight-
ness is key in such cases so that water vapor is not car-
ried by air leakage into the floor assembly.
Closed crawl spaces (see Figure 3) are designed with the
intent of separating the crawl space from the outdoors.
This type of construction requires a ground cover to mini-
mize entry of soil moisture, air sealing at the perimeter,
and either introduction of conditioned air into the crawl
space or direct dehumidification to control humidity levels
in the crawl space (ground covers and air sealing mini-
mize moisture entry but may not be 100% effective).
WOOD DESIGN FOCUS V. 23, N. 3 14
Measured Moisture Conditions
In a review of measured data on in-service moisture and
temperature conditions in wood-frame buildings, Glass
and TenWolde (2007) observed that high moisture con-
tent (MC) values in wood floor structural members
(joists, beams, sill plates, subfloor sheathing) have been
measured at various times of the year, in all climate
zones in the United States. Some of these readings
were well over 20% MC, which is generally recognized
as the moisture content at which we become concerned
about mold and decay. On the basis of these historical
data Glass and TenWolde (2007) make the following
specific observations:
The most extreme measured moisture contents in
wood structural members above crawlspace founda-
tions occur when the ground is not covered with a
vapor-resistant ground cover. This effect is magni-
fied for sites with poor drainage.
Two different seasonal trends have been observed
for crawlspaces:
1. Wood moisture content reached a maximum in win-
ter and minimum in summer. This trend was ob-
served in studies prior to ca. 1955 in crawlspaces
without a ground cover in both mixed-humid and
cold climates. The most likely explanation is that
when the crawlspace vents either were lacking or
were closed during winter, the uncovered soil sup-
plied moisture that condensed on the coldest wood
members in the crawlspace. During winter months,
the coldest members are the sill plates, rim joists,
and floor joists near the exterior. It should be noted
that the buildings were not air-conditioned during the
summer, and the floor framing therefore was proba-
bly warmer than the crawlspace soil (or below-grade
portions of the crawlspace walls), for most of the
time during summer months.
2. Wood moisture content peaked in summer, with a
minimum in winter. This trend has been reported in
hot-humid and mixed-humid climates in all studies
conducted since ca. 1955 in which seasonal trends
were investigated. These studies included various
types of crawlspaces (both covered/uncovered and
vented/closed). In many of these studies, the living
space above the crawlspace was either known to
be, or was probably air-conditioned during the sum-
mer. The major source of crawlspace moisture in
these studies was either warm, humid outdoor air or
moisture evaporating from the soil. In summer, the
floor members can be cooler than the outdoor air
(sometimes cooler than the outdoor dew point tem-
perature), especially when the building is air-
conditioned. Drying would have occurred during fall
and winter because outdoor air would contain less
water vapor and cooler soil would have a slower rate
of evaporation.
Recommendations
The following recommendations for moisture control in
crawl spaces are mostly based on the 2005 ASHRAE
Handbook, Chapter 24, Thermal and Moisture Control in
Insulated Assemblies—Applications (ASHRAE 2005).
Accessibility
Location °F °C
Salt Lake City, UT 49.8 9.9
Denver, CO 52.4 11.3
Seattle, WA 53.8 12.1
San Francisco, CA 54.0 12.2
Portland, OR 55.6 13.1
Phoenix, AZ 58.7 14.8
Los Angeles, CA 61.4 16.3
Minneapolis, MN 62.3 16.8
Boston, MA 62.7 17.1
Chicago, IL 63.4 17.4
New York, NY 65.6 18.7
Philadelphia, PA 66.4 19.1
Washington, DC 66.7 19.3
Baltimore, MD 66.8 19.3
Louisville, KY 67.9 19.9
St. Louis, MO 68.1 20.1
Kansas City, MO 68.2 20.1
Atlanta, GA 69.3 20.7
Dallas, TX 69.8 21.0
Norfolk, VA 70.4 21.3
Memphis, TN 71.4 21.9
Wilmington, NC 72.8 22.7
Savannah, GA 72.9 22.7
Tallahassee, FL 73.0 22.8
Orlando, FL 73.5 23.1
Charleston, SC 73.6 23.1
Houston, TX 73.7 23.2
Miami, FL 74.3 23.5
New Orleans, LA 74.4 23.6
Corpus Christi, TX 74.9 23.8
Table 1. July Mean Dew Point Temperatures for 30 U.S.
Locations From 1984 to 2012 (NCDC 2012)
WOOD DESIGN FOCUS V. 23, N. 3 15
One of the principle reasons that problems occur in crawl
spaces is that owners or occupants do not regularly in-
spect the crawl space. By inspecting regularly, problems
with standing water or plumbing leaks are discovered
and corrected sooner, hopefully before major damage
occurs. Inspection can also uncover problems with water
entry from outside, allowing timely corrective action. The
crawlspace therefore needs to be easily accessible, well
illuminated, and clean. Although a minimum clearance of
18 inches (0.46 m) between the soil and the bottom of
the floor joists is often recommended, it is advisable to
increase this to 40 inches (1 m) for easier access.
Water Entry
The soil in the crawl space should be kept as dry as pos-
sible, and therefore water entry into the crawl space
should be prevented. It is recommended that the crawl
space floor level not be below the exterior grade. Proper
site drainage is also critical. Gutters and downspouts
should carry rain water away from the foundation, and
the site should be sloped away from the foundation to
allow water to drain away. If this is not possible, berms,
retaining walls, and other means may be used to guide
the water around and away from the building. In case of
high ground water levels, installing sump pumps may be
useful.
If a building is to be constructed on a site with poor grad-
ing and drainage or where the water table is close to the
surface, an open pier foundation with substantial grade
clearance would be the most viable option. With wet
soils, capillary rise through stem walls may be an issue.
This issue is largely side-stepped with open-pier founda-
tions.
Ground cover
Measurements have consistently shown that ground co-
vers can significantly lower moisture conditions in the
crawl space. Recommendations usually call for ground
cover material with a water vapor permeance of no more
than 1 perm, and the material must be strong enough to
withstand foot and knee traffic. Polyethylene with a mini-
mum thickness of 6 mil (0.006 in, 0.15 mm) is commonly
used. A concrete slab may be poured over the ground
cover to keep out rodents. Debris must be removed and
the soil leveled before installing the ground cover. The
seams of the ground cover should be lapped 4 to 6 in
(100 to 150 mm), and no sealing is required.
Open pier-and-beam construction generally does not
require a ground cover because the amount of air flow
under the floor is sufficient to carry away excess mois-
ture (Glass et al. 2010).
Vents
The 2006 International Residential Code (IRC) (ICC
2006) contains a standard requirement for minimum vent
openings of 1 ft2 per 150 ft2 of crawlspace floor area (1
m2/150 m2). As noted earlier, there is no known technical
basis for these requirements, and providing vents does
not guarantee actual airflow. Research has also shown
that with warm humid outdoor conditions, providing 1/150
vents can be counterproductive. However, the 2006 IRC
does allow omitting the vents in a crawl space with pe-
rimeter insulation if a) a ground cover is installed (sealed
and taped), with the cover extending 6 inches (150 mm)
up the side walls; and b) the crawlspace has a continu-
ously operated exhaust fan, or conditioned air is supplied
to the crawl space, or the crawl space is used as a ple-
num.
If local codes require vents or vents are desired, one
should consider going well beyond the minimum require-
ment of 1/150 to ensure that there is enough air move-
ment to raise the crawl space temperature above the
dew point of the outside air during summer.
Other Considerations
From the perspective of energy use, it is best not to lo-
cate ducts for heating and cooling in unconditioned spac-
es. Locating ducts in a crawl space with vents in the pe-
rimeter walls will also complicate air sealing and insulat-
ing of the floor over the crawlspace. If it is necessary to
locate ducts in a crawl space that is vented with outdoor
air, air sealing and insulating those ducts is very im-
portant. Poorly sealed supply ducts often fail to deliver
adequate conditioned air to locations where it is desired.
Poorly sealed return ducts may introduce crawlspace air
into the living space. If the crawlspace air is humid or
contains contaminants (soil gases, mold spores, mold
metabolites, or volatile chemicals), the humidity or the
contaminants (or both) will be introduced into the living
space. Properly insulating the ducts limits energy losses
and reduces the chance of condensation on the ducts
when the air-conditioning is running. If it is necessary to
locate ducts in a crawl space, it may be viable to con-
struct a closed crawlspace and to insulate the walls
(Davis and Dastur 2004). This, of course, assumes that
water entry into the crawlspace is controlled, that there is
a functioning soil cover, and that volatile substances
(e.g., gasoline or gasoline-powered tools) are not stored
in the crawlspace.
It is of paramount importance to vent clothes dryers out-
WOOD DESIGN FOCUS V. 23, N. 3 16
doors (not into the crawlspace), and to repair any leak-
ing water pipes.
Acknowledgements
The authors thank William Rose of the University of
Illinois, C. R. Boardman of the Forest Products Labora-
tory (FPL), and the late Charles Carll, formerly of FPL for
critical comments that improved this manuscript.
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Anton TenWolde is Research Physicist (retired), USDA
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Samuel V. Glass is Research Physical Scientist, USDA
Forest Service, Forest Products Laboratory.
svglass@fs.fed.us
... The ground and perimeter walls are covered with a vapor retarder, and the crawlspace may be provided with conditioned supply air. A number of studies in various climates have shown that this type of crawlspace can remain safely dry (Advanced Energy 2005b; Dastur et al. 2009;Davis and Dastur 2004;Dutt et al. 1988;Duff 1980;Moody et al. 1985;Quarles 1989;Samuelson 1994;Stiles and Custer 1994). ...
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In flood-prone areas, elevating a building's floor system above the anticipated flood level can significantly limit the extent of property damage associated with flooding. In hot and humid climates, such as the Gulf Coast region, raised floor systems may, however, be at risk for seasonal moisture accumulation, as the majority of residential buildings in such climates are now mechanically cooled. Conditions were monitored over a one-year period in a sample of 12 houses with insulated raised floors, eight in New Orleans and four in Baton Rouge, Louisiana. Eleven of the 12 houses were located in flood hazard areas and were constructed with open pier foundations. Several types of insulation were compared across the sample of houses. In all houses, crawlspace vapor pressure was essentially the same as outdoor vapor pressure and exceeded indoor vapor pressure from May to October. Moisture conditions within plywood or solid wood subfloors were found to depend on several variables: season, indoor temperature during summer, type of interior floor finish, and type of under-floor insulation. In most cases subfloor moisture levels were higher in summer than in winter. For a given type of insulation and interior floor finish, subfloor moisture content generally increased with decreasing indoor temperature during summer. For a given indoor temperature and type of insulation, subfloor moisture content was generally higher under an impermeable finish, such as vinyl tile, than under a more permeable finish, such as carpet. Floors with foil-faced polyisocyanurate foam board installed below floor joists displayed no discernible seasonal trend in moisture content and little difference between various interior floor finishes. Subfloor moisture readings were consistently in the 10–14% moisture content (MC) range. Floors with closed cell sprayed polyurethane foam showed only a slight seasonal trend and subtle differences between interior floor finishes. Subfloor moisture contents with closed cell foam in all cases were less than 16% MC in plywood and less than 18% MC in solid wood. Clear seasonal trends were observed in floors with open cell foams and with kraft-faced glass fiber batt insulation. Subfloor moisture content readings above 20%, particularly under impermeable interior floor finishes and with low indoor temperatures during the cooling season, suggest that these insulation types do not reliably protect subfloors from seasonal moisture accumulation. For carpeted floors, where subfloor moisture contents were relatively low, application of vapor retarder paint to open cell foam had no discernible effect. In contrast, for floors finished with vinyl, vapor retarder paint applied over open cell foam appeared to result in lower summertime subfloor moisture content, as compared to that for floors insulated with open cell foam without the paint.
Article
Purpose: Millions of properties have suspended timber ground floors globally, with around 10 million in the UK alone. However, it is unknown what the floor void conditions are, nor the effect of insulating such floors. Upgrading floors changes the void conditions, which might increase or decrease moisture build-up and mould and fungal growth. This paper provides a review of the current global evidence and presents the results of in-situ monitoring of 15 UK floor voids. Research method: An extensive literature review on the moisture behaviour in both uninsulated and insulated suspended timber crawl spaces is supplemented with primary data of a monitoring campaign during different periods between 2012 to 2015. Air temperature and relative humidity sensors were placed in different floor void locations. Where possible, crawl spaces were visually inspected. Implications: Comparison of void conditions to mould growth thresholds highlights that a large number of the monitored floor voids might exceed the critical ranges for mould growth, leading to potential occupant health impacts if mould spores transfer into living spaces above. A direct comparison could not be made between insulated and uninsulated floors in the sample due to non-random sampling and because the insulated floors included historically damp floors. The study also highlighted that long-term monitoring over all seasons and high-resolution monitoring and inspection are required; conditions in one location are not representative of conditions in other locations. Value: This study presents the largest UK sample of monitored floors, evaluated using a review of current evidence and comparison with literature thresholds.https://www.emeraldinsight.com/doi/abs/10.1108/IJBPA-05-2018-0041
Conference Paper
Full-text available
In flood-prone areas, elevating a building's floor system above the anticipated flood level can significantly limit the extent of property damage associated with flooding. In hot and humid climates, such as the Gulf Coast region, raised floor systems may, however, be at risk for seasonal moisture accumulation, as the majority of residential buildings in such climates are now mechanically cooled. Conditions were monitored over a one-year period in a sample of 12 houses with insulated raised floors, eight in New Orleans and four in Baton Rouge, Louisiana. Eleven of the 12 houses were located in flood hazard areas and were constructed with open pier foundations. Several types of insulation were compared across the sample of houses. In all houses, crawlspace vapor pressure was essentially the same as outdoor vapor pressure and exceeded indoor vapor pressure from May to October. Moisture conditions within plywood or solid wood subfloors were found to depend on several variables: season, indoor temperature during summer, type of interior floor finish, and type of under-floor insulation. In most cases subfloor moisture levels were higher in summer than in winter. For a given type of insulation and interior floor finish, subfloor moisture content generally increased with decreasing indoor temperature during summer. For a given indoor temperature and type of insulation, subfloor moisture content was generally higher under an impermeable finish, such as vinyl tile, than under a more permeable finish, such as carpet. Floors with foil-faced polyisocyanurate foam board installed below floor joists displayed no discernible seasonal trend in moisture content and little difference between various interior floor finishes. Subfloor moisture readings were consistently in the 10–14% moisture content (MC) range. Floors with closed cell sprayed polyurethane foam showed only a slight seasonal trend and subtle differences between interior floor finishes. Subfloor moisture contents with closed cell foam in all cases were less than 16% MC in plywood and less than 18% MC in solid wood. Clear seasonal trends were observed in floors with open cell foams and with kraft-faced glass fiber batt insulation. Subfloor moisture content readings above 20%, particularly under impermeable interior floor finishes and with low indoor temperatures during the cooling season, suggest that these insulation types do not reliably protect subfloors from seasonal moisture accumulation. For carpeted floors, where subfloor moisture contents were relatively low, application of vapor retarder paint to open cell foam had no discernible effect. In contrast, for floors finished with vinyl, vapor retarder paint applied over open cell foam appeared to result in lower summertime subfloor moisture content, as compared to that for floors insulated with open cell foam without the paint.
Technical Report
Full-text available
This literature review reports in-service moisture and temperature conditions of floor, wall, and roof members of wood-frame buildings and exposed wood decks and permanent wood foundations. A wide variation exists in reported wood moisture content, spanning a range from as low as 2% to well above 30%. Relevant studies are summarized, and measured values of wood moisture content and temperature are tabulated. Trends are discussed that relate moisture conditions to climate and season, moisture sources and transport mechanisms, and building design and construction.
Article
The combined effect of ground cover and ventilation levels on the moisture content (MC) of wood framing members in crawl spaces was investigated. The experimental design included three ventilation and four ground cover levels. Temperature-compensated, resistance-type moisture probes were used to monitor MC. Results showed that, with venting reduced to 1 ft.2 per 1,500 ft.2 of floor area, adequate protection against high MCs could be obtained with as little as 90 percent ground cover. Results also showed that wood MCs could vary significantly within a relatively small crawl space.
Article
In order to avoid rotted, icky, and stinky crawlspaces, crawlspaces must have plenty cross ventilation and good drainage. A code-ventilation, continuous impermeable ground cover gives an excellent drainage, however it still a mess. Addressing these issues, there are solutions offered including old crawlspaces, old crawlspace temperatures, insulated crawlspace temperatures, moisture dynamics, warming the wood, and vapor barrier.
Article
This study compared the performance of closed crawlspaces, which had sealed foundation wall vents, a sealed polyethylene film liner, and 1.0 ft 3 /min (0.5 L/s) of HVAC supply air for each 30 ft 2 (2.8 m 2 ) of crawlspace ground surface, to traditional vented crawlspaces with wall vents and polyethylene film covering 100% of the ground surface. The study was conducted at 12 owner- occupied, all-electric, single-family detached houses with the same floor plan located on one cul-de-sac in the southeastern United States. Using the matched pairs approach, the houses were divided into three study groups of four houses each. Comparative mois- ture measurements for these crawlspaces and submetered heat pump kWh use were recorded. Findings supported that for the humid conditions of the southeastern United States, properly closed crawlspaces were a robust measure that produced substan- tially drier crawlspaces and significantly reduced occupied space conditioning energy use on an annual basis.
Article
With the introduction of moisture engineering and new design approaches for moisture control in buildings, it has become important to formulate a realistic design value for indoor humidity. The design value for indoor humidity is one of the most impor­ tant parameters when determining the need for vapor retarders and other building envelope design features, especially in colder climates. Seasonal indoor humidity is primarily determined by a balance between moisture production rates and removal rates (by ventilation or dehumidification). However, experience has shown that a simple mass balance calculation tends to produce indoor humidity results that are too high for humid cool (coastal) climates and too low for dry climates. In these calculations, moisture sources are assumed to be constant and not a function of the ambient indoor humidity. In this paper we examine the most common sources of water vapor in homes and how they might vary with indoor humidity. Our review indicates that most of the sources, such as contributions from inhabitants and their activities, are virtually independent of humidity. However, moisture contributions from potted plants and from a wet foundation vary with indoor humidity levels. Both types of sources contribute less when the humidity is high and more when the humidity is low. This behavior is especially important because moisture from wet foundations overwhelms all other contributions. We show in this paper how taking the variability with humidity into account can lead to substantially lower estimates of indoor humidity, especially in airtight homes with low ventilation rates. Given the importance of moisture from foundations, we believe much more measured data are needed, both on the quantity of water vapor contributed by foundations as well as on its variability with indoor humidity and temperatures, including the temperature of the foundation itself.
Plastic soil covers reduce the moisture content in basementless homes
  • T L Amburgey
  • D W French
Amburgey, T.L.; French, D.W. 1971. Plastic soil covers reduce the moisture content in basementless homes. Forest Products Journal. 21(8):43-44.
Thermal and Moisture Control in Insulated Assemblies—Applications
ASHRAE. 2005. Thermal and Moisture Control in Insulated Assemblies—Applications. In: 2005 ASHRAE Handbook—Fundamentals. American Society of Heating, Refrigerating and Air-conditioning Engineers, Inc., Atlanta. Chapter 24.