Review of In-Service 

Moisture and Temperature 

Conditions in WoodFrame Buildings

Samuel V. Glass

Anton TenWolde

United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

General

Technical

Report

FPL–GTR–174
September 2007

Glass, Samuel V.; TenWolde, Anton. 2007. Review of in-service moisture 

and temperature conditions in wood-frame buildings. General Technical 

Report FPL-GTR-174. Madison, WI: U.S. Department of Agriculture, 

Forest Service, Forest Products Laboratory. 53 p.

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Abstract

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.

Keywords: moisture content, temperature, wood-frame 

buildings, humidity, crawlspace, foundation, floor, wall, 

roof, attic, deck

Acknowledgments

The authors are grateful to the American Forest & Paper 

Association (AF&PA) for funding this study. We also thank 

Philip Line of AF&PA; William Rose of the University 

of Illinois at Urbana-Champaign; and Charles Carll, Stan 

Lebow, and Samuel Zelinka of FPL for valuable comments 

and discussions.

Cover photo: Top left and bottom right used by permission 

of APA–The Engineered Wood Association.

Contents

Page

Executive Summary..............................................................i

1 Introduction....................................................................... 1

 1.1 Scope.......................................................................... 1

 1.2 Moisture-Related Properties of Wood........................ 1

 1.3 Role of Climate.......................................................... 2

 1.4 Instrumentation for Measuring Wood Moisture 

 Content and Temperature ................................................. 3

2 Floor Members over Basement and Crawlspace 

Foundations.......................................................................... 4

 2.1 Hot-Humid Climate ................................................... 5

 2.2 Mixed-Humid Climate............................................... 7

 2.3 Marine Climate .........................................................11

 2.4 Cold Climate .............................................................13

 2.5 Summary: Floor Members........................................15

3 Wall Framing and Sheathing............................................16

 3.1 Hot-Humid Climate ..................................................16

 3.2 Mixed-Humid Climate..............................................22

 3.3 Marine Climate .........................................................24

 3.4 Cold Climate .............................................................27

 3.5 Summary: Wall Framing and Sheathing ...................33

4 Roof Framing and Sheathing ...........................................38

 4.1 Mixed-Humid Climate..............................................40

 4.2 Hot-Dry Climate .......................................................40

 4.3 Marine Climate .........................................................41

 4.4 Cold Climate .............................................................41

 4.5 Very Cold Climate.....................................................43

 4.6 Summary: Roof Framing and Sheathing...................44

5 Exposed Wood Decks ......................................................44

6 Permanent Wood Foundations.........................................46

Conclusions.........................................................................47

Literature Cited ...................................................................48
Executive Summary

The objective of this report is to summarize available information about in-service moisture and temperature conditions in wood framing and sheathing members in floors, 

walls, and roofs of buildings. Additionally, this literature 

review documents limited information on exposed wood 

decks and permanent wood foundations. The reported 

ranges of moisture content (MC) and temperature observed 

in different types of assemblies (floors, walls, roofs, decks, 

and wood foundations), organized by climatic region and 

season, are given in the body of this manuscript. Because 

the data reported in the literature may not be statistically 

representative of all wood-frame buildings, whether a bias 

exists towards either higher or lower moisture contents cannot be determined. However, the studies include common 

construction found throughout North America, and the data 

therefore represent conditions that can occur. Overall trends 

in the different types of assemblies are as follows.

Crawlspaces

• The most extreme moisture contents in wood structural 

members above crawlspace foundations occur when the 

ground is not covered with a vapor diffusion retarder. This 

effect is magnified for sites with poor drainage.

• Two different seasonal trends have been observed for 

crawlspaces:

1. Moisture content reached a maximum in winter and 

minimum in summer. This trend was observed in 

studies prior to around 1955 in uncovered crawlspaces 

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 supplied 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 

probably warmer than the crawlspace floor (or belowgrade portions of the crawlspace walls) for most of the 

time during summer months.

2. Moisture content reached a minimum in winter and 

maximum in summer. This trend has been reported in 

hot-humid and mixed-humid climates in all studies 

conducted since around 1955 in which seasonal trends 

were investigated. These studies included various types 

of crawlspaces (both covered and uncovered, vented 

and sealed). In many of these studies, the living space 

above the crawlspace was either known to be, or was 

probably, air-conditioned during the summer. Most 

likely, the major source of crawlspace moisture in 

these studies was warm, humid outdoor air rather than 

moisture evaporating from the soil. In summer, the 

floor members can be cooler than the outdoor air 

(sometimes cooler than the outdoor dew point temperature), especially when the building is air-conditioned. 

Lower outdoor temperatures during fall and winter 

would logically lower the intensity of crawlspace moisture sources.

Walls

• High moisture contents in walls may occur in response 

to rainwater intrusion. Moisture content depends on the 

magnitude of leakage and the rate at which drying occurs.

• High moisture levels may occur in exterior wall sheathing 

in cold and marine climates during winter. Exfiltration 

of humid indoor air and diffusion of moisture through 

insulated wall cavities lacking effective air barriers and 

vapor retarders allow moisture to accumulate in the cold 

sheathing. The severity of this problem correlates with 

indoor humidity levels maintained during winter.

• In cold and marine climates, sheathing usually dries during the spring and reaches a minimum moisture content 

during the summer, unless excessive rainwater intrudes. 

However, in some cases, design or construction of the 

wall prevents effective drying.

• In cold and marine climates, framing members in insulated walls generally do not accumulate as much moisture 

as does exterior sheathing. During winter, the cold sides 

of framing members tend to be wetter than the warm sides 

for the reasons discussed above. Drying usually occurs in 

spring and summer.

• In hot-humid climates, the significant moisture transport 

mechanisms (besides rainwater intrusion) are infiltration 

of humid outdoor air and inward diffusion of water vapor 

through wall cavities lacking effective air barriers and 

(exterior) vapor retarders, the opposite of cold climates 

in winter. In addition, solar radiation can drive moisture 

from exterior sheathing into the wall cavity. When a vapor 

retarder is placed on the interior (cold side) of an insulated wall cavity, condensation can occur on the vapor 

retarder.

• All studies that have investigated the effect of a ventilated 

space between the sheathing and cladding have found that 

this feature reduces moisture accumulation in the wall.

• When bottom plates are in direct contact with concrete 

slab foundations (no capillary break), moisture can be absorbed by the bottom plate, resulting in moisture contents 

near 30%.

Roofs

• High moisture contents in roofs may occur in response to 

rainwater intrusion. Moisture content depends on the magnitude of the leakage and rate at which drying occurs.

• A trend of higher moisture content during winter and 

lower moisture content during summer has been observed 
in roof framing and sheathing in all studies in which seasonal trends were investigated. Higher moisture contents 

have generally been found in sheathing than in rafters or 

trusses.

• Wintertime moisture accumulation in roof members is 

worse when indoor humidity levels are high and solar 

gain is low. The dominant moisture transport mechanism 

is likely exfiltration of humid indoor air through ceilings 

lacking effective air barriers. Ventilation of the attic space 

has not been a dependable method of controlling moisture 

levels in roofs when air leakage is significant.

Wood Decks

• Untreated boards and pressure-treated boards without 

water repellent additives exhibit large fluctuations in 

moisture content with weather. Water absorption occurs 

more rapidly when the wood surface is checked.

• Treating boards with a preservative and water repellent 

additive significantly dampens these fluctuations. The 

water repellency of certain treatments improves over time. 

Water repellents also minimize checking and cupping.

Permanent Wood Foundations

• Moisture contents in permanent wood foundations are 

generally higher below grade than above grade. Factors 

such as site drainage, soil conditions, and the presence of 

a protective film may affect the moisture levels.

ii
Review of In-Service Moisture 

and Temperature Conditions 

in Wood-Frame Buildings

Samuel V. Glass, Research Physical Scientist

Anton TenWolde, Research Physicist

Forest Products Laboratory, Madison, Wisconsin

1 Introduction

The purpose of this literature review is to compile available 

information about in-service moisture and temperature conditions in wood framing and sheathing members in floors, 

walls, and roofs of buildings. In addition, limited information about conditions in exposed wood decks and permanent wood foundations is included. Moisture content and 

temperature can affect the durability of wood members and 

metal fasteners: high moisture content and high temperature 

can be conducive to fastener corrosion, especially in preservative-treated wood (Zelinka and Rammer 2005a,b). Before 

presenting information on moisture and temperature conditions, we first discuss the scope of this review, moisture-related properties of wood, the role that climate plays in building design and construction, and instrumentation typically 

used for measuring temperature and wood moisture content.

1.1 Scope

In-service moisture and temperature conditions that any 

given wood member of a building experiences depend on 

many complicated factors. In this review, we aim to respect 

this complexity while presenting the information as simply 

as possible. Results are reported from a variety of sources, 

including surveys of large samples of buildings, field experiments with a smaller group of buildings, forensic investigations of moisture damage, and controlled studies of different 

types of wood-frame construction, usually in test structures 

built specifically for research. Because the data reported in 

the literature may not be statistically representative of all 

wood-frame buildings, whether a bias exists towards either 

higher or lower moisture contents cannot be determined. 

However, the studies include common construction found 

throughout North America, and the data therefore represent 

conditions that can occur. We choose to exclude extreme 

conditions that result from catastrophic events such as hurricanes and floods. Although plumbing leaks are fairly common—one survey of a thousand houses throughout the United States found the occurrence of such leaks to be between 

10% and 30% of all houses at the time of inspection (Moses 

and Scheffer 1962)—we choose to exclude major plumbing 

leaks from this review.

1.2 Moisture-Related Properties of Wood

Moisture content (MC) is defined as the ratio of the mass of 

water in a given volume of wood to the oven-dry mass of 

the same volume of wood, usually expressed as a percentage. A number of properties of wood depend on moisture 

content, such as density, dimensional shrinkage and swelling, strength, thermal properties, and electrical properties 

(see FPL 1999). Water can exist in wood as liquid (free water) or vapor in cell lumens (cavities) or as water absorbed 

by the cell walls (bound water). Conceptually, the fiber 

saturation point is defined as the moisture content at which 

cell walls are completely saturated (all bound water) but no 

liquid water exists in the cell lumens; the fiber saturation 

point of wood averages approximately 30% MC but can 

vary by several percentage points in individual species and 

individual pieces of wood (FPL 1999).

Below the fiber saturation point, the moisture content of 

wood depends on both the relative humidity (RH) and temperature of the surrounding air. Figure 1 relates equilibrium 

moisture content (EMC) of wood to relative humidity for 

several selected temperatures, based on equation (3-3) in the 

Wood Handbook (FPL 1999). Such a plot is called a sorption 

isotherm. At RH values between 20% and 80%, the EMC 

of wood at 70°F (21°C) ranges from 4.5% to 16%. There 

is a slight temperature dependence, such that EMC 

changes by less than 1% MC for a 30°F (17°C) change 

Figure 1—Wood equilibrium moisture content as a function of relative humidity for select temperatures (based 

on FPL 1999).
2

General Technical Report FPL–GTR–174

in temperature. The Wood Handbook (FPL 1999) also lists 

calculated EMC values for wood exposed to the outdoor 

atmosphere (protected from sun and rain) in several U.S. 

locations, determined from monthly average relative humidity and temperature data. The values range from a low of 

4% (Las Vegas, Nevada, in June) to a high of 18% (Juneau, 

Alaska, from September to December, and Missoula, Montana, in December).

Wood in service, however, is rarely in moisture equilibrium, 

being exposed to both long-term (seasonal) and short-term 

(daily) changes in relative humidity and temperature of the 

surrounding air and is thus continually undergoing slight 

changes in moisture content. Short-term fluctuations in air 

temperature and relative humidity tend to influence only 

the wood surface. On the other hand, contact with liquid 

water can rapidly increase the moisture content above fiber 

saturation, and it can take a long time for wet wood to dry 

to its EMC. Nevertheless, the Wood Handbook (FPL 1999) 

recommends that exterior sheathing and siding be installed 

with an average MC of 12% (allowing a range of 7–14%) 

for most areas of the United States. Softwood lumber intended for framing may be air-dried or kiln-dried to a MC of 

19% or less (FPL 1999).

A number of the studies cited in this review refer to mold 

and decay. The growth of decay fungi in wood requires 

several conditions: favorable temperature (~50–95°F 

(10–35°C)); a supply of oxygen; adequate moisture (neither 

too little nor too much); and a suitable food supply, such as 

wood cell walls (FPL 1999). The wood moisture content 

needs to exceed the fiber saturation point for decay fungi 

to propagate, and at MCs below 20% their development is 

completely inhibited; traditionally, the guideline for protection of wood and wood products from decay has been to 

keep the moisture content below 20% (Carll and Highley 

1999). Scheffer (1971) developed an index based on monthly climatic data (mean temperature and number of days with 

precipitation) to estimate the relative potential at different 

locales for decay propagation in above-ground wood assemblies that are fully or semi-exposed to weather. While molds 

require a similar temperature range to that of decay fungi, 

they can propagate on surfaces without free water, provided 

the surface RH remains near 80% (International Energy 

Agency 1991).

1.3 Role of Climate

In this review, we attempt to summarize in-service moisture 

and temperature conditions in a simple, rational format. 

We therefore draw attention here to major factors that affect the flow of heat and moisture in buildings (Straube and 

Burnett 2005). Exterior climate is the first major factor to 

consider. Figure 2 depicts various ways in which moisture 

can be transported between the exterior environment and 

the building and among various parts of the building. The 

various spaces within a building (living space, crawlspace or 

basement, and attic) are not isolated from each other or from 

the building envelope components—walls, roof, and foundation. Moisture migration can occur through three distinct 

pathways:

1. Intrusion of liquid water into buildings has the potential 

to carry the largest quantity of moisture (Carll 2000, 

Christian 1994, Verrall and Amburgey 1980). This could 

occur through leaks in roofs; leaks in walls due to wetting by wind-driven rain, seepage at and around roof 

edges, or splash from the ground; poor detailing around 

windows and doors; leaks into the basement or crawlspace due to poor site drainage; or capillary transport 

(wicking) in porous material such as brick, concrete, 

and wood.

2. Water vapor can be transported by the flow of air (convection) through vents or unintended leaks in the building. For example, venting of crawlspaces and attics is 

commonly used as a strategy to remove excess moisture; 

however, under certain conditions, venting can have the 

opposite effect and actually introduce moisture into the 

building. Examples of unintentional air flow include 

leakage around penetrations in walls or ceilings, such 

as electrical outlets, windows, doors, light fixtures, and 

pipes.

3. Water vapor can migrate by diffusion through materials from regions of high to low concentration. Although 

vapor diffusion usually is negligible in comparison to 

liquid water intrusion and air leakage, it can be significant when interior and exterior conditions differ greatly, 

such as during the winter in cold climates and during the 

summer in hot-humid climates.

Figure 2—Depiction of the interactions of various parts 

of a building and the exterior environment. Pathways for 

moisture transport include liquid water flow by gravity, 

wind pressure, diffusion, and capillary suction and water 

vapor migration by air flow and vapor diffusion.
 3

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

Wetting can occur through any of these three mechanisms. 

Drying can occur by air flow and vapor diffusion. Liquid 

transport can contribute to drying in the sense of redistribution of moisture from a wet region over a larger volume.

Major factors that affect flow of heat and moisture in buildings include the following:

•	 Exterior environmental conditions

o	 Temperature

o	 Humidity

o	 Precipitation

o	 Wind

o	 Solar radiation

o	 Cloud cover

•	 Building orientation and exposure to exterior conditions

•	 Building envelope and material characteristics

o	 Susceptibility to wetting by intrusion of liquid 

water, air leakage, and vapor diffusion

o	 Ability to store and dissipate intruding water

o	 Air exchange rate

o	 Water vapor permeance

o	 Drying potential

o	 Thermal resistance

o	 Solar absorptance and surface emissivity of roof

•	 Building operation and interior environmental loads

o	 Heating/cooling

o	 Humidification/dehumidification

o	 Ventilation

o	 Other interior moisture sources

Given the importance of climate to thermal and moisture 

transport in buildings, we organize this review according 

to a recent classification of the climatic regions of North 

America (Lstiburek 2006a) (Figure 3, Table 1). A number 

of other similar classifications are given by Straube and 

Burnett (2005). Measurements of moisture and temperature 

conditions are reported for most climatic regions; however, 

due to lack of data, we have omitted the Subarctic/Arctic, 

Mixed-Dry, and Hot-Dry climates.

1.4 Instrumentation for Measuring Wood 

Moisture Content and Temperature

Techniques for measuring moisture in buildings have been 

reviewed by TenWolde and Courville (1985), James (1988), 

Derome et al. (2001), Straube et al. (2002), and Healy 

(2003). The most commonly used techniques are based on 

electrical resistance, electrical capacitance, and gravimetric 

analysis.

Electrical resistance moisture meters are the most commonly used method in the studies reported here. This technique is based on the principle that the resistivity of wood 

decreases with increasing moisture content. A simple direct 

current (DC) circuit is established when two pins or probes 

are inserted into a wood specimen. The probes can be insulated with only the tips exposed so that moisture content 

can be measured at various depths. This technique can be 

implemented with hand-held meters or with numerous pins 

wired to a central data acquisition system. Accuracy is typically within a few percentage points in the range 6–30% 

MC, with better accuracy at lower moisture content. When 

the wood moisture content is above 30%, “only approximate qualitative readings may be obtained” (James 1988). 

The measured resistance also depends on temperature and 

wood species, so these need to be factored in to determine 

moisture content. For an increase of 20°F (11°C), resistance 

decreases such that the moisture content reading should be 

corrected to roughly 1% MC lower (James 1988).

A variation on moisture measurement by DC resistance 

is the “matchstick” sensor (or probe) developed by Duff 

(1966). By virtue of their compact size, these sensors can 

be individually calibrated with relative ease, thereby permitting accuracies of roughly ±1% MC. The sensors’ small 

size, combined with the fact that they are composed largely 

of wood, means that they are, relative to other types of sensors, non-disruptive of conditions within wood members or 

within assemblies of interest. This probe can alternatively 

be used to monitor relative humidity at specific locations 

within assemblies where wood may not be present, such as 

within a layer of glass fiber insulation or against a polyethylene vapor retarder (Sherwood 1985, TenWolde and Mei 

1986), or where the measurement location is at the surface 

of a material whose sorption isotherm may differ from that 

of wood (TenWolde et al. 1995). When used to monitor relative humidity, the sensor readings are influenced by sorption 

hysteresis (Carll and TenWolde 1996).

Figure 3—Climatic regions of North America (Lstiburek 

2006a). Used with permission of Building Science Corporation, Westford, Massachusetts.
4

General Technical Report FPL–GTR–174

Another technique based on the electrical properties of 

wood involves measuring the capacitance, which increases 

with moisture content. Hand-held meters based on this principle typically consist of two pads that are placed against a 

surface. Because the accuracy of these sensors varies and 

the moisture content is averaged over a large area, this technique is most useful for detecting high moisture contents 

and water leaks.

Gravimetric analysis requires a comparison of the weight 

of a specimen before and after ovendrying. In practice, this 

technique is executed either by cutting out a specimen from 

the wood member for analysis or by inserting a plug that 

will later be removed. Although this method is very accurate, it has several drawbacks. First, it is inherently destructive; the wood member of interest is necessarily damaged. 

This method cannot be used for wood members that are 

enclosed within an assembly unless the assembly is opened. 

Second, it is labor intensive, which limits the frequency and 

extent of data collection. Third, when a plug is used, there is 

uncertainty about whether the plug represents the moisture 

content of the surrounding material due to the lack of continuity and possible changes in the moisture behavior of the 

wood member due to the plug.

Temperature measurement is fairly straightforward. Simple 

sensors such as thermocouples, thermistors, and resistance 

temperature devices can be incorporated into hand-held digital devices or data acquisition systems to give readings with 

accuracies as good as ±0.5°F (±0.3°C). Hand-held non-contact infrared thermometers may be accurate to ±2°F (±1°C). 

Although temperatures are reported in some studies cited 

below, most of the studies use the measured temperatures 

for correcting moisture content readings but unfortunately 

do not report the actual measured temperatures. In addition, 

the air temperature is reported in some cases rather than the 

actual wood member temperature.

2 Floor Members over Basement 

and Crawlspace Foundations

In this section we discuss measurements of wood moisture 

content and temperature in floor structural members such 

as sill plates, joists, beams, and subfloor sheathing. Several 

factors influence these conditions: climate, quality of site 

drainage, interior conditions (such as heating and cooling), 

and the presence and location of insulation. For crawlspaces 

in particular, the presence of a ground cover (vapor diffusion retarder) and whether the space is vented with outdoor 

air or sealed from the exterior may have a large effect. In 

addition, moisture and temperature conditions may depend 

on the proximity of the floor members to the perimeter of 

the foundation. Rose and TenWolde (1994) summarized the 1

Table 1—Climatic regions with defining temperature and moisture criteria 

(Lstiburek 2006a)a

Designation Temperature criteria Moisture criteria 

Hot-humid Winter mean monthly temperature 

remains above 45°F (7°C) 

Annual precipitation > 20 in. (0.5 m) 

Mixed-humid (1) HDDFb < 5400 (HDDCc < 3000); and

(2) winter mean monthly temp. 

drops below 45°F (7°C) 

Annual precipitation > 20 in. (0.5 m) 

Hot-dry Winter mean monthly temp. remains 

above 45°F (7°C) 

Annual precipitation < 20 in. (0.5 m) 

Mixed-dry (1) HDDF < 5400 (HDDC < 3000); and 

(2) winter mean monthly temp. 

drops below 45°F (7°C) 

Annual precipitation < 20 in. (0.5 m) 

Marine (1) Coldest month mean temp. between 

27°F (–3°C) and 65°F (18°C); 

(2) warmest month mean temp. 

below 72°F (22°C); and 

(3) at least 4 months with mean temp. 

over 50°F (10°C) 

Dry season in summer; the month 

with heaviest precipitation in the cold 

season has at least three times as 

much precipitation as the month with 

least precipitation 

Cold 5400  HDDF < 9000 

(3000  HDDC < 5000) 

Very cold 9000  HDDF < 12,600 

(5000  HDDC < 7000) 

Subarctic/arctic HDDF  12,600 (HDDC  7000) 

a

 The climatic definitions given in Table 1 are the same as those adopted by the U.S. Department of Energy’s 

 Building America Program (http://www.eere.energy.gov/buildings/building_america/climate_zones.html). b

 Heating degree days, 65°F basis. The number of heating degree days is equivalent to the difference between 

 the baseline temperature (65°F) and the mean daily temperature summed over all the days of the year for 

 which the mean daily temperature is less than the baseline temperature. c

 Heating degree days, 18°C basis. 
 5

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

main issues in crawlspace design and construction. As noted 

by Rose (1993, 1994, 2001) from a historical perspective, 

moisture problems have been associated with crawlspaces 

since the 1940s (Britton 1948). A substantial body of work 

has since been published toward understanding and solving 

these problems. More of the literature in this section therefore deals with crawlspaces than with basements.

2.1 Hot-Humid Climate

Diller (1953) investigated the effects of soil cover and ventilation on crawlspace moisture in groups of four houses 

in seven locations throughout the United States, including 

North Charleston, South Carolina. The studies were conducted from 1948 to 1951. In two of the four houses in 

each location, the crawlspaces were covered with 55-lb 

(2.5-kg/m2) roll roofing;1 one was vented with 10% of the 

“2 + 1/3” formula,2 and the other was without ventilation. In 

the two houses with uncovered crawlspaces, one was vented 

with 100% of this formula, and the other was without ventilation. In many cases, however, the occupants altered the 

vents, typically closing them in the winter. Diller remarked 

that in about 90% of all houses in the study, the fundamental causes of crawlspace moisture were “improper grading 

around the house, the absence of gutters and downspouts, 

and ineffective splash boards, all leading to the accumulation of water in the crawl spaces.” The houses in North 

Charleston were built on continuous cinder block foundations, with an average of 27 in. (0.69 m) of clearance in the 

crawlspace. The soil was sandy with a high water table.

Wood moisture contents were measured periodically with a 

resistance-type meter in sill plates at each of the four corners 

and in a central girder. The covered crawlspaces had average MCs between 15% and 20%, with ventilation having no 

significant effect. In the uncovered crawlspaces (both vented 

and nonvented), moisture contents ranged from 20% during 

summer to peaks of over ~40%3 during the winter.

Moses and Scheffer (1962) measured wood moisture contents in a thousand houses in various U.S. climate regions 

with the purpose of locating conditions that would make 

wood members susceptible to decay. Given the large number 

of measurements, conditions were monitored only once for 

each house, and therefore meaningful seasonal trends could 

not be inferred from the data. A resistance-type moisture 

meter was used; readings were not corrected for temperature 

because only a rough estimate of moisture content was desired to detect critical wetness.

In houses with crawlspaces in the southeastern United 

States, Moses and Scheffer (1962) found that up to 20% of 

The weights of roll roofing cited here and elsewhere in this report are 

based on an area of 108 ft2 (10 m2). The recommendation at the time by the Housing and Home Finance 

Agency was that crawlspace vents should have a net unobstructed area 

equal to 2 ft2 for each 100 ft of wall perimeter, plus 1/3 ft2 for each 100 ft2

of crawlspace area.

MC readings above 30% by electrical resistance are approximate (see 

Section 1.4).

the houses monitored between August and October had 

average wood moisture contents in the range of 20–30%. 

The majority of houses had average MC values between 

12% and 20%. These averages represent measurements in 

floor joists, rim joists, edge joists, and sill plates. The results were grouped by crawlspaces with dry soil, those with 

covered damp soil and good ventilation, and those with 

uncovered damp soil and poor ventilation. The corresponding percentages of moisture content readings in the 20–30% 

range for crawlspaces in each category were 8%, 17%, and 

20%, respectively. Unfortunately, because the presence of 

a ground vapor retarder and ventilation were lumped in 

this study, their separate effects cannot be ascertained. In 

contrast, measurements in the Gulf Coast region between 

October and December found no crawlspaces with average 

wood moisture content above 20%. This difference between 

the Southeast and the Gulf Coast regions likely has less to 

do with geographical location than with the time of year in 

which readings were taken. Measurements in houses with 

basements were also reported for wood members that were 

close to the basement walls. All the houses with basements 

in the southeastern United States (August–October) had an 

average MC below 20%.

Verrall (1962) conducted a survey of air-conditioned buildings throughout the hot-humid region including residences, 

office buildings, and commercial buildings. Locations included Corpus Christi, Kingsville, and Orange, Texas; New 

Orleans, Louisiana; Gulfport, Mississippi; Pensacola, Jacksonville, and Key West, Florida; Brunswick, Georgia; and 

Charleston, South Carolina. Wood decay in floor members 

was noted in many instances and was usually linked to 

wet (uncovered) soil in crawlspaces, continuous airconditioning, or low indoor temperatures (≤70°F (21°C)).

Moisture contents were measured mainly in subflooring 

with a resistance-type meter. Readings were reported for a 

New Orleans clubhouse built with wood-frame construction 

over a wet crawlspace with poor ventilation. Large areas of 

the floor needed replacement. The crawlspace soil was then 

covered with 55-lb (2.5-kg/m2) roll roofing and blowers 

were installed to hasten drying, but subfloor MCs remained 

relatively high at 21–22% (values prior to these were not 

reported). The air in the crawlspace was at 80–87°F (27–

31°C) and 70–80% RH. Eventually the crawlspace vents 

were closed and dehumidifiers were installed to keep the 

RH below 70% and the dew point temperature below 70°F 

(21°C).

Verrall (1962) also reported results from an investigation of 

an office building in Saucier, Mississippi. This wood-frame 

building had an open pier foundation with a solid brick wall 

on one side. The soil was dry and dusty. A window air-conditioning unit was installed in a room with dimensions 7.5 

by 13 by 8 ft (2.3 by 4.0 by 2.4 m). Pins for resistance moisture content measurements were installed in the subflooring. 

One section of the floor (between joists) was insulated with 
6

General Technical Report FPL–GTR–174

glass-wool blanket insulation with an aluminum foil vapor 

retarder on the under side. The subfloor MC was 11% prior 

to installation of the vapor retarder. Readings were taken periodically between mid-June and mid-September. Although 

conditions were extreme—the indoor temperature was decreased to 60°F (16°C), the outdoor dew point was 70–75°F 

(21–24°C) for most of the time, and rain fell on 57% of the 

days during the experiment—the subfloor moisture content 

in the section with the insulation and vapor retarder remained at 10–12% MC next to the joists and 12–14% MC 

midway between the joists. However, in the floor section 

without the vapor retarder, the subfloor moisture content 

gradually rose to a peak of 30% between the joists and 24% 

adjacent to the joists.

Choong and Cassens (1985) studied the effects of soil cover 

and ventilation in three adjacent houses with pier-and-beam 

foundations in the (levied) flood plain of the Mississippi 

River in Baton Rouge, Louisiana. The houses were 3 ft 

(0.9 m) off the ground, and only the front of the crawlspace 

of each house was enclosed (partially so) with mortared 

brick, leaving the under-floor area mostly open. House 

A had open vents in the brick stem wall and 100% of the 

crawlspace covered with 6-mil (0.15-mm) polyethylene. The 

vents in House B were partially blocked with hardboard and 

shrubbery, and the ground was 50% covered. House C had 

open vents and no soil cover. No insulation was present between floor joists in any of the houses. Moisture content was 

measured periodically over 18 months with a resistance-type 

meter in 10 samples of southern pine blocking, 1 by 3 by 

6 in. (25 by 75 by 150 mm), attached to the subflooring at 

various places under each house. The report did not mention 

whether the houses were air-conditioned during the summer 

months.

Average moisture contents among the three houses were 

generally not significantly different. Average values ranged 

from minima of 8–10% MC to maxima of 16–20% MC. 

Many fluctuations (probably due to periods of wet and dry 

weather) were evident, with no discernable seasonal trend. 

The lack of differences in average moisture contents implies 

that evaporation of water from the soil was not a major 

source of moisture in these crawlspaces, likely because they 

were well ventilated, being enclosed only on the front side. 

However, when all individual data points were considered 

(as opposed to averages), maximum values for the three 

houses were 20% (A), 24% (B), and 25% (C). These differences in maximum moisture contents imply that localized 

extremes may result when a ground cover is absent.4 The 

times of year when these maximum moisture contents occurred were not reported.

Recently, Advanced Energy (2005a,b) conducted a survey of 

45 houses with wall-vented crawlspaces in North Carolina, 

If individual maximum moisture contents occurred during summer months 

and if the houses were air-conditioned, then the differences in moisture 

content may have been an artifact of different thermostat settings rather 

than a result of the presence or absence of a ground cover.

including 22 houses in coastal New Hanover County in the 

hot-humid climatic region. The remaining houses were in 

Durham, Wayne, and Wilson counties in the piedmont region (mixed-humid climate), but the results generally were 

pooled and not classified by county. The houses represented 

a variety of construction types and ranged in age from 2 to 

60 years. Almost all had heating, ventilating, and air-conditioning (HVAC) ducts located in the crawlspace. Although 

about 75% of the crawlspaces had a polyethylene vapor 

retarder covering the ground, in most cases it covered less 

than 80% of the floor area. Vents in the crawlspace foundation walls were fully open in 67% of the houses, partially 

open in 27%, and closed in 7%.

Crawlspace inspections and wood moisture content and 

temperature measurements were made between July and 

December 2004. Moisture contents and wood surface temperatures were recorded at 10–12 locations within each 

crawlspace using a resistance-type meter and a non-contact 

thermometer. These locations included the sill plate and 

rim joist next to the crawlspace access; the floor joist above 

and below insulation next to the access; the sill plate, rim 

joist, and floor joist above and below insulation at what 

was judged to be potentially the worst location;5 the center 

floor joist above and below insulation; the center beam; the 

subflooring in the middle of the floor; and any other locations where moisture or mold appeared to be significant. In 

addition to single-point temperature and relative humidity 

measurements at each site visit, data loggers were attached 

to floor framing in a central location in each crawlspace for 

long-term monitoring.

Specific wood moisture contents were not given; instead, 

proportions of the readings that exceeded specified moisture levels were reported. Values exceeding the range of the 

moisture meter were found (>30% MC). Furthermore, 36% 

of all houses had at least one reading of 25% MC or higher, 

and 67% of all houses had at least one reading of 19% MC 

or higher. Table 2 provides further details of the moisture 

content readings for the various wood members. Several 

other observations relating to moisture were reported. Sixtytwo percent of the crawlspaces had visible mold growth, 

and 47% had visible wood decay in floor members. About 

half the houses with decayed wood also had plumbing leaks. 

Other signs of moisture damage included discoloration and 

efflorescence on foundation walls. Water puddles on top of 

the polyethylene were found in 42% of the crawlspaces that 

had ground covers. In 16% of the crawlspaces, a clothes 

dryer was vented into the crawlspace rather than ducted to 

the outside.

Air temperature and relative humidity measurements acquired over a period of 11 months showed that during the 

summer months, outdoor air contained more moisture than 

This was often the location of least clearance between the ground and the 

floor framing, though other locations were selected, such as near vents or 

near the center beam.
 7

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

crawlspace air. Furthermore, crawlspace air temperatures 

(dry-bulb) often were below outdoor dew point temperatures. This means that when humid outdoor air entered the 

vented crawlspace, moisture could condense on surfaces 

with temperatures at or below the dew point. Measured 

crawlspace RH values ranged from 40% to 100%. Nearly 

all the crawlspace RH readings exceeded 70% during June, 

July, and August (except in a few crawlspaces with dehumidifiers).

The study also measured pressure differentials, airflows, 

and effective leakage areas (ELAs) between the crawlspace 

and living space. The ELAs were on the order of 0.5 ft2

(0.05 m2), and for the majority of houses sampled, crawlspace-to-house air leakage represented 11–30% of the total 

house air leakage. Natural stack effect pressures and mechanical HVAC systems caused airflow through the holes in 

the floor.

2.2 Mixed-Humid Climate

Diller (1946, 1950) measured moisture contents in crawlspaces of four government-built houses in Washington, 

D.C., between 1942 and 1950. The houses were built in 

1941 on concrete piers averaging 30 in. (0.8 m) in height. 

There was no foundation wall, but 1/4-in. (6-mm) mineral 

board skirting was placed against the outside of the building, extending down into the soil. Vents were cut at regular 

intervals, but the vent area was less than recommended at 

the time. Occupants usually closed the vents during winter, 

and in some instances did not reopen them in spring. The 

combination of clay soil and poor drainage resulted in standing water in the crawlspaces after rain.

Moisture contents were measured in sill plates and floor 

joists at 25–45 locations under each house with a resistancetype meter at intervals of 1 to 4 months. After the first year 

of measurements, the soil under the two wettest houses, 

designated FP 5 and FP 15, was covered with 55- and 90-lb 

(2.5- and 4.1-kg/m2) roll roofing, respectively. The wood 

moisture contents in these two houses dropped significantly 

over a period of less than 6 months. For the subsequent 7 

years, the two houses with covered soil remained below 

20% MC, with average values between 12% and 18% MC, 

whereas in the year prior to application of the soil covers 

they had averaged around 25% MC. Condensation on framing members was never observed with the soil covers in 

place. In contrast, the two houses without soil covers had 

wood MCs ranging from ~20% in summer to ~35%6 in 

winter, with the MCs exceeding 30% each winter. In these 

two houses, heavy condensation was evident on the sill 

plates and floor joists near exterior walls from November 

through May. This was the first observation of a seasonal 

trend where moisture levels reached a maximum in winter 

and minimum in summer. Differences in ventilation of the 

crawlspaces were usually negligible; opening and closing 

the vents had relatively little influence on the wood moisture contents. No significant difference in effectiveness was 

found between the two weights of roll roofing.

Diller (1953) also reported similar measurements with 

groups of four houses in seven U.S. locations, including 

Washington, D.C., Philadelphia, Pennsylvania, and Oak 

Ridge, Tennessee, in the mixed-humid climatic region. The 

studies were conducted as described in Section 2.1, examining the various combinations of soil covers and ventilation.

The houses in Washington, D.C., (different from those discussed above) were built on cement block piers with mineral 

board skirting placed against the outside. The floors were 

insulated, and the average crawlspace clearance was 20 in. 

(0.5 m). In the houses with soil covers, the average moisture 

contents were in the 10–15% range, with no significant difference between the vented and unvented crawlspaces. The 

uncovered, unvented crawlspace had winter peaks between 

20% and 30% MC, but the uncovered, vented crawlspace 

remained in the 15–20% MC range.

The Philadelphia houses were built on untreated oak piers, 

with mineral board skirting. The floors were insulated, and 

the average crawlspace clearance was 16 in. (0.4 m). Covered crawlspaces had average MCs ranging from ~10% to 

~15%, similar to those in Washington. The uncovered, unvented crawlspace had winter peaks over 30% MC and one 

peak as high ~50% MC.6 In the uncovered crawlspace 

with full ventilation, the MC stayed just below 20% until 

MC readings above 30% by electrical resistance are approximate (see 

Section 1.4). 2

Table 2—Proportion of moisture content measurements 

in wood floor members in North Carolina crawlspaces 

above 19% and 25% MC (Advanced Energy 2005a)

Percentage of MC readings 

Wood member/location MC  19% MC  25% 

Sill plate (access) 16 0 

Rim joist (access) 11 0 

Floor joist (access, below insulation) 24 0 

Floor joist (access, above insulation) 8 0 

Center joist (below insulation) 40 9 

Center joist (above insulation) 12 6 

Sill plate (worst) 32 3 

Rim joist (worst) 30 9 

Floor joist (worst, below insulation) 47 16 

Floor joist (worst, above insulation) 29 9 

Subflooring 30 16 

Other (worst)a

 59 28 

Total of all locations 28 8 a

 This was often the location of least clearance between the ground and the 

floor framing, though other locations were selected, such as near vents or 

near the center beam. 

Table 3—Summary of reported moisture contents of wood floor 

members in New Jersey crawlspaces (Dutt and others 1988) 

Group Ground

cover

Wall 

insulation Vents

Range of 

maximum 

 MC (%) 

Average 

maximum 

MC (%) 

Average 

minimum 

MC (%) 

1 Yes Yes Closed 12–17 14 10 

2 Yes Yes Open 12–17 15 11 

3 No No Open 13–25 19 15 

Table 4—Summary of reported average moisture contents of floor joists and rim 

joists and air temperatures in southern New Jersey crawlspaces (Stiles 

and Custer 1994)

 Moisture content (%) Air temperature (°F (°C)) 

Group Houses Fall Winter Spring Summer Fall Winter Spring Summer

Control 7 15–19 8.5–15 12–14 15–19 68–74

(20–23)

54–59

(12–15)

58

(14)

72

(22)

Moisture

barrier 5 14–20 8–12 9–11 12–16 69–71

(21–22)

60–62

(16–17)

62

(17)

71–72

(22)

Full

treatment 5 15–21 9–13 10–13 12–17 68–71(20-22)

56–61

(13-16)

60

(16)

68–71

(20–22)
8

General Technical Report FPL–GTR–174

February, when the occupants closed the vents; the MC then 

rose to ~25%, but dried out in the spring. Summer moisture 

contents for the uncovered crawlspaces were similar to those 

for the covered crawlspaces, with values near ~15%.

The houses in Oak Ridge, Tennessee, had a continuous cement block foundation, with an average of 23 in. (0.6 m) 

of crawlspace clearance. The floors were not insulated. The 

building site was sloped, allowing water to seep through the 

foundation walls and wet the soil under the houses. Results 

were similar to those for the Washington and Philadelphia 

houses. Covered crawlspaces had average MCs in the 

10–16% range. The uncovered crawlspace without ventilation had winter peaks around 30% MC, but the uncovered, 

vented crawlspace remained in the 15–20% MC range.

Moses and Scheffer (1962) surveyed houses in Baltimore, 

Maryland (August), and Memphis, Tennessee (January). 

For houses with crawlspaces, none of those in Baltimore 

had average moisture contents above 20%. In Memphis, the 

percentage of readings in the 20–30% MC range were as 

follows: crawlspaces with dry soil, 9%; those with covered 

damp soil and good ventilation, 8%; and those with uncovered damp soil and poor ventilation, 17%. For Baltimore 

houses with basements, the percentages in this range were 

as follows: floor joists, 5%; rim joists, edge joists, or sill 

plates on basement walls, 5%.

Amburgey and French (1970) monitored conditions in two 

adjacent houses with crawlspaces in Raleigh, North Carolina, to determine the extent to which a soil cover could aid 

in reducing wood moisture contents. Both were single-story, 

air-conditioned houses on well-drained lots. The crawlspace in house A was 2 to 2.5 ft (0.6 to 0.8 m) high with 

vents blocked by dense shrubbery. Insulation was installed 

between floor joists, many of which were covered with 

fungus. A heating/air-conditioning unit was located in the 

crawlspace. House B had a 3- to 4-ft (0.9- to 1.2-m) high 

crawlspace with unblocked vents and without insulation. 

There was no obvious mold growing on the joists or subflooring. The heating/air-conditioning unit was not located 

in the crawlspace. The vents in both houses remained open 

year-round.

Moisture content was measured periodically with a resistance-type meter in the subflooring, floor joists, and sill 

plates. In addition, wooden blocks were placed near some 

sampling points in house A; moisture meter readings for 

these blocks were checked gravimetrically. House A was 

monitored at 22 sampling points from November 1965 to 

January 1967; a 6-mil (0.15-mm) polyethylene ground cover 

was installed over 70% of the crawlspace in November 

1966. House B was monitored at 38 sampling points from 

June 1966 to January 1967, and a soil cover was installed 

over 100% of the crawlspace in August 1966.

Measurements revealed a seasonal trend, where moisture 

levels reached a maximum in summer and a minimum in 

winter. As is seen in many other studies that follow, this 

trend has been widely observed. The average moisture 

content of the floor joists and sill plates of house A varied 

between 14% during winter and 19% during summer, prior 

to installation of the soil cover. Individual sampling points 

ranged from 11% to 22%. After the soil cover was applied, 

average MC dropped to 10% during the second winter. Most 

sampling points were in the 9–11% range, with a maximum 

of 14%. Thus, on average the soil cover reduced moisture 

content of the wood members in the crawlspace by 3–4% 

(assuming the drop was not caused by other factors such as 

differences in weather). In house B, summer average MC of 

the subflooring, floor joists, and sill plates was between 14% 

and 17% without the soil cover. Following the installation 

of the soil cover, average MC decreased steadily through the 

fall and reached a minimum of 8.5% in winter. Because no 

measurements were taken the previous winter (before the 

soil cover was applied), it could not be determined whether 

the soil cover had any effect on moisture conditions in 

house B.

In two separate studies using a test house near Athens, 

Georgia, Duff (1978, 1980) monitored moisture conditions 

in floor assemblies over a crawlspace. Moisture content was 

measured with sensors designed by Duff (1966) in plywood 

subflooring and floor joists in an 8- by 24-ft (2.4- by 7.3-m) 

section of the house with the crawlspace. The height of the 

crawlspace ranged from 29 in. to 33 in. (0.74 to 0.84 m) 

from the soil to the bottom of the floor joists. Temperature 

inside the house was kept at 75°F ± 5°F (24°C ± 3°C). Interior relative humidity was maintained at 30% ± 5% during the winter and was not controlled during the summer, 

though dehumidification was provided by a window heat 

pump, which typically kept the RH below 70%.

In the initial 2-year study, Duff (1978) investigated the effects of several different types of construction on wood 

moisture content, relative humidity, and temperature within 

the crawlspace. Direct comparisons were drawn between 

cavities with and without insulation, with and without a vapor retarder, and with a vapor retarder placed above insulation and below the insulation. For the first year of the study, 

the soil was covered with 6-mil (0.15-mm) polyethylene and 

the vents were closed; during the second year, the soil cover 

was removed and the vents were opened.

We designate the baseline case to be the cavity without insulation and without a vapor retarder during the first year 

(vents closed, soil covered). The floor joist MC peaked at 

~20% in August but stayed around 12–15% during fall, 

winter, and spring. The plywood subflooring also peaked in 

August at ~18% MC, with values between 10% and 15% 

the rest of the year. Removing the ground cover and opening the vents actually made little difference, aside from the 

initial release of moisture that had been contained under the 

ground cover. The MC of floor joists and subflooring peaked 

at ~20% in July; for the rest of the second year, the floor 

joists varied between 13% and 17% and the subflooring between 10% and 14%.
 9

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

A polyethylene vapor retarder placed under the uninsulated 

plywood subflooring had a significant effect on reducing 

the flow of moisture into the subflooring. The MC stayed at 

~9–12% through both years of the study (with vents 

closed and soil covered and with vents open and soil uncovered). Insulating the floor of the crawlspace with 4-in.- 

(100-mm-) thick fiberglass batts between the joists (without 

a vapor retarder) did not have a significant effect on wood 

moisture contents: in both years of the study, the floor joists 

and plywood subflooring peaked at ~18–20% MC in late 

summer, similar to the baseline case. However, the subflooring temperature stayed about 10°F (6°C) warmer in winter 

when it was insulated. When the floor was insulated and a 

vapor retarder was placed directly under the joists, the moisture content of the joists remained at 10–12% throughout 

both years of the study. Similarly, when the vapor retarder 

was placed over the joists, the subflooring MC remained at 

8–12% throughout both years.

In the second study, Duff (1980) investigated the effect of 

insulating the exterior walls of the crawlspace. The concrete-block foundation walls were covered with sections of 

3.5-in.- (90-mm-) thick fiberglass batt insulation, with the 

kraft paper facing the interior of the crawlspace. The vents 

in the foundation walls were sealed and caulked, and the 

access door was insulated and weather-stripped. For the first 

22 months of the study, the ground was covered with 6-mil 

(0.15-mm) polyethylene; for the last 8 months, 10% of the 

ground cover was opened to expose the soil. During the first 

winter of the study, the moisture content of the floor joists 

and subflooring reached a minimum of 9–10%. The moisture content rose during the spring and summer and reached 

a maximum of 15% in the floor joists and 16% in the subflooring in September. These values are a few percentage 

points less than the baseline case in the initial study (Duff 

1978). During the second winter, the floor members dried 

again to 9–10% MC. The polyethylene soil cover was then 

partially removed such that it covered 90% of the crawlspace area. Wood moisture contents rose rapidly the following spring; by August, floor joists peaked at 18% MC and 

subflooring at 17% MC.

Jennings and Moody (1983) reported the results of a survey 

of 36 houses in various locations in Tennessee that were 

investigated due to complaints of moisture-related problems 

following weatherization measures. Nearly 600,000 houses 

were surveyed, of which about half had received energy 

efficiency improvements. The 36 houses reported here obviously did not represent a statistically significant sample. 

Complaints were related both to excessive moisture (such 

as condensation, mold, mildew) and excessive drying (such 

as shrinkage, cracking). Moisture content ranges were reported based on 10 measurements in the wood substructure 

of each house. Of the 36 houses, 23 were constructed on 

crawlspace foundations, five had basement foundations, 

one had a basement combined with a crawlspace, one had a 

crawlspace combined with a slab, and the remaining six had 

slab foundations. The houses on slab foundations by design had no wood substructure; moisture measurements for 

these houses were not reported. In the houses on crawlspace 

foundations, wood substructure moisture contents ranged 

from 7% to 30%; in the houses on basement foundations, 

the corresponding range in moisture contents was from 8% 

to 15%. Visible moisture was reported in most of the crawlspace houses, typically on HVAC ducts and exterior walls, 

and in some cases on floor insulation or floor joists. Visible 

signs of fungal decay were also found in six of the houses 

with crawlspace foundations, typically on floor joists and 

subflooring. When signs of condensation were reported in 

houses with basement or slab foundations, they were usually 

on (above-grade) walls and ceilings.

Moody et al. (1983) (see also Jennings and Moody 1984) 

studied the effects on moisture and heat flux of insulating 

crawlspace walls and eliminating ventilation using four 

houses in Murfreesboro, Tennessee. The houses were in 

the same neighborhood, with the same general terrain, and 

of approximately the same size and age. One house (designated Wyatt) was left uninsulated with no ground vapor 

retarder, while three were insulated with 2-in.- (50-mm-) 

thick foil-backed fiberglass (duct wrap) on the walls of the 

crawlspace. The ground and the foundation walls were covered with 6-mil (0.15-mm) polyethylene (before insulation 

was applied). Two of the houses (including the Wyatt house) 

had ceiling radiant heating and central air conditioning with 

ducts in the crawlspace; two of the houses had air source 

heat pumps with ducts in the crawlspace. The moisture content of the floor joists was measured with a resistance-type 

meter in four locations within each crawlspace on a weekly 

basis for a period of 10 months (January through October).

Prior to the installation of the polyethylene ground cover, 

the floor joists in all four houses had an average moisture 

content of 16% (December). Initially, the vents in the foundation walls were closed in all four houses. The houses with 

ground covers reached minimum MCs of 7–10% between 

February and April and maximum MCs of 13–14% between 

August and October (vents remained closed). The house 

without a ground cover (Wyatt) ranged from 14% MC in 

mid-winter (vents closed) to 18% MC in late summer (vents 

open during summer). This seasonal trend is in agreement 

with the findings of Amburgey and French (1970) and of 

Duff (1978, 1980). It was concluded that removing crawlspace ventilation did not adversely affect moisture conditions in the floor joists.

Moody et al. (1985) (see also Jennings and Moody 1984) 

continued to monitor three of these houses for an additional 

2 years. The Wyatt house was left uninsulated for the first 

year; for the second year, the ground and walls were covered 

with polyethylene and the walls were insulated with 2-in.- 

(50-mm-) thick foil-backed fiberglass (duct wrap). The same 

seasonal trend was observed. For the first year, the floor 
10

General Technical Report FPL–GTR–174

joists in the Wyatt house were between 14% and 18% MC 

and showed a slight decrease to 14–17% MC in the second 

year. The other two houses had floor joist moisture contents 

between 8% and 14%.

Dutt et al. (1988) conducted a field study of crawlspace 

moisture in 15 houses in a development in Toms River, New 

Jersey. The location is 8 miles from the ocean; with a 

high water table, the sandy soil was typically wet. Of the 

15 houses, 6 received a major retrofit including 6-mil 

(0.15-mm) polyethylene covering the ground of the crawlspace to 1 ft (0.3 m) up the walls along with 1-in. (25-mm) 

extruded polystyrene glued to the walls over the polyethylene. The foundation vents were sealed in three of these six 

houses, while the vents were left open in the other three. 

Periodic measurements of moisture content were taken with 

a resistance-type meter in floor joists, rim joists, and support 

beams. The study was carried out for 1 year; moisture contents were monitored every month for the first 3 months and 

then every other month after that.

A consistent seasonal trend was observed in all 15 houses: 

wood moisture content reached a maximum in July and a 

minimum in February, in agreement with the studies above. 

Larger variations were found in the houses without wall 

insulation and with open vents. When the results for various wood members (floor joists, rim joists, and beams) were 

pooled, the most striking difference was found between 

crawlspaces with a ground cover and those without a ground 

cover. The former (Groups 1 and 2 pooled) peaked at an 

average maximum MC of 14%, with an average minimum 

of 11%; the latter (Group 3) had an average maximum of 

19% and an average minimum of 15%. The houses with a 

good ground cover were also those with insulated walls, so 

their individual effects could not be assessed. Results are 

presented with greater detail in Table 3.

Stiles and Custer (1994) conducted a similar study of crawlspace moisture in 17 single-family houses in southern New 

Jersey, also dividing the houses into three groups. The first 

group was left untreated (control). The second group had 

6-mil (0.15-mm) polyethylene covering the ground and 

walls of the crawlspace. The third group had polyethylene 

on the ground and walls, fiberglass insulation on the walls, 

1-in. (25-mm) extruded polystyrene covering the vents, 

and caulking at the rim. Moisture contents were measured 

periodically with a resistance-type meter at the rim joist, 

floor joist at a distance 1 ft (0.3 m) from the rim, and in the 

middle of the crawlspace (in at least three joists), above and 

below the insulation, if present. A baseline was established 

prior to any treatment of the crawlspaces (September), then 

six data sets were collected over a 9-month period that included winter, spring, and summer.

Average moisture contents for various locations and dates 

were reported for each group of crawlspaces, and the results are summarized in Table 4. Individual values as high 

as ~32% MC were measured in two crawlspaces prior to 

treatment. The same seasonal trend of high moisture content in summer and low moisture content in winter was also 

observed here. When moisture contents were normalized 

by the control group and the initial conditions, it was found 

that the full treatment (polyethylene ground and wall cover, 

insulation, and caulking) reduced the floor joist moisture 

contents by 3–5% MC, whereas the polyethylene ground 

and wall cover alone reduced moisture contents by about 

half this amount. The moisture contents of the rim joists, 

however, were not significantly affected by either treatment. 

The authors also found, by using air pressurization tests to 

measure ELAs, that a strong correlation existed between 

moisture content and ELA of the crawlspace to the outside 

for the five crawlspaces that received the full treatment. This 

suggests that a significant source of moisture in the closed 

crawlspaces was the entry of humid air from the outside.

More recently, Davis and Dastur (2004) measured temperature, relative humidity, and wood moisture content in 

12 identical houses in Princeville, North Carolina, over a 

3-year period (see also Advanced Energy 2005c). The study 

was conducted in two phases, with three groups of houses 

in each phase. In the first phase, the control group had 

open foundation wall vents, 6-mil (0.15-mm) polyethylene 

ground cover, and R-19 floor insulation; in the first experiment group (EXP1), the vents were sealed, the ground and 

walls were covered with polyethylene, and the floor was 

left uninsulated; the second experiment group (EXP2) was 

the same as EXP1 except that R-3 rock wool insulation was 

placed on the walls. In the second phase of the study, air 

sealing was done on all 12 houses. In addition, R-19 floor 

insulation was added to the EXP1 group and the rock wool 

insulation in the EXP2 group was replaced with R-13 rigid 

foam insulation. Furthermore, in both experimental groups, 

1 ft3/min (0.5 L/s) of HVAC supply air per 30 ft2 (3 m2) of 

floor area was introduced.

Moisture content measurements were taken periodically 

with a resistance-type meter. The MC values of the floor 

joists in the vented crawlspaces (control group) ranged 

from 9% in winter to 15% in late summer. In the closed 

crawlspaces (EXP1 and EXP2 groups), the floor joist MCs 

remained steady year-round between 9.5% and 11%. These 

differences in wood moisture content between vented and 

closed crawlspaces were attributed to differences in relative humidity of the crawlspace air. The RH in the closed 

crawlspaces stayed below 60% most of the time, whereas 

in the vented crawlspaces it exceeded 80% for a significant 

amount of time during the humid summer months.

In addition to this controlled study, Advanced Energy 

(2005d) conducted a survey of 10 houses with wall-vented 

crawlspaces in the central Piedmont region (Chatham, Durham, and Orange counties) of North Carolina. The study 

protocol was similar to that discussed in Section 2.1 (Advanced Energy 2005a,b). The houses represented a variety 

of construction types and ranged in age from 2 to 9 years. 
 11

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

All houses had HVAC ducts located in the crawlspace. Polyethylene ground covers were present in 7 of the 10 houses, 

with coverage ranging from 60% to 95% of crawlspace area. 

R-19 fiberglass batt insulation was installed in the joist cavities and held in place with wires. Vents in the crawlspace 

foundation walls were fully open in nine of the houses.

Crawlspace inspections and wood moisture content and 

temperature measurements were made between June and 

September 2001. Moisture contents and wood surface temperatures were recorded at 10–12 locations within each 

crawlspace using a resistance-type meter and a non-contact 

thermometer, as described in Section 2.1. Table 5 lists the 

moisture content and surface temperature range for each 

wood member. In nine of the houses, the maximum moisture 

reading was located in a floor joist below the insulation, 

typically in the area of lowest clearance in the crawlspace.

All 10 crawlspaces had visible mold growing on the wood 

framing. Other signs of moisture damage included discoloration and efflorescence on the foundation walls. Water puddles on top of the polyethylene were present in three of the 

crawlspaces and condensation on pipes or ducts was found 

in six. The relative humidity of the crawlspace air varied 

from 73% to 88%, with an average of 79%. This study also 

measured airflows as discussed in Section 2.1 and found that 

air leakage from the crawlspace into the living space was 

significant.

2.3 Marine Climate

Moses and Scheffer (1962) surveyed houses in Southern 

California (March–April) and in Oregon and Washington 

(April–June). For houses with crawlspaces in Southern 

California, the percentages of moisture content readings in 

the 20–30% range were as follows: crawlspaces with dry 

soil, 2%; those with covered damp soil and good ventilation, 

11%; and those with uncovered damp soil and poor ventilation, 0%.7 The corresponding values for Oregon and Washington were 7%, 12%, and 29%, respectively.

Quarles (1989) investigated the combined effects of ventilation area and ground cover level on crawlspace moisture 

conditions in an occupied house in Richmond, California. 

Three different levels of ventilation were investigated: 

1:150 area ratio (net vent area to floor area); 1:1,500 area 

ratio; and no ventilation. The vapor retarder used was 

6-mil (0.15-mm) polyethylene at coverages of 0%, 75%, 

90%, and 100%. Each treatment was monitored until conditions stabilized, which took at least 5 weeks (except one 

case noted below). A total of eight conditions were monitored during the 16-month study (March 1987–June 1988).

Wood moisture contents were measured using resistancetype probes (Duff 1966) inserted in the middle of 2-ft 

(0.6-m) sections of Douglas-fir 2 by 4s,8 which were then 

hung between floor joists, uniformly distributed throughout 

the crawlspace in 20 locations. Temperature sensors were 

placed next to the moisture probes. For comparison with the 

resistance moisture measurements, small wood blocks were 

hung in the crawlspace and were removed periodically for 

gravimetric analysis. The moisture contents determined with 

these two methods were not significantly different. To gauge 

the variation in wood MC in different locations, the 19- by 

42-ft (5.8- by 13-m) crawlspace was divided into six zones, 

7The number of houses in this category was significantly less than in the 

other categories and probably too small a sample to be meaningful.

8Lumber dimensions given throughout this report are strictly nominal 

values in inches.

2

Table 3—Summary of reported moisture contents of wood floor 

members in New Jersey crawlspaces (Dutt et al. 1988) 

Group Ground

cover

Wall 

insulation Vents

Range of 

maximum 

 MC (%) 

Average 

maximum 

MC (%) 

Average 

minimum 

MC (%) 

1 Yes Yes Closed 12–17 14 10 

2 Yes Yes Open 12–17 15 11 

3 No No Open 13–25 19 15 

Table 4—Summary of reported average moisture contents of floor joists and rim 

joists and air temperatures in southern New Jersey crawlspaces (Stiles 

and Custer 1994)

 Moisture content (%) Air temperature (°F (°C)) 

Group Houses Fall Winter Spring Summer Fall Winter Spring Summer

Control 7 15–19 8.5–15 12–14 15–19 68–74

(20–23)

54–59

(12–15)

58

(14)

72

(22)

Moisture

barrier 5 14–20 8–12 9–11 12–16 69–71

(21–22)

60–62

(16–17)

62

(17)

71–72

(22)

Full

treatment 5 15–21 9–13 10–13 12–17 68–71(20-22)

56–61

(13-16)

60

(16)

68–71

(20–22)

2

Table 3—Summary of reported moisture contents of wood floor 

members in New Jersey crawlspaces (Dutt and others 1988) 

Group Ground

cover

Wall 

insulation Vents

Range of 

maximum 

 MC (%) 

Average 

maximum 

MC (%) 

Average 

minimum 

MC (%) 

1 Yes Yes Closed 12–17 14 10 

2 Yes Yes Open 12–17 15 11 

3 No No Open 13–25 19 15 

Table 4—Summary of reported average moisture contents of floor joists and rim 

joists and air temperatures in southern New Jersey crawlspaces (Stiles 

and Custer 1994)

 Moisture content (%) Air temperature (°F (°C)) 

Group Houses Fall Winter Spring Summer Fall Winter Spring Summer

Control 7 15–19 8.5–15 12–14 15–19 68–74

(20–23)

54–59

(12–15)

58

(14)

72

(22)

Moisture

barrier 5 14–20 8–12 9–11 12–16 69–71

(21–22)

60–62

(16–17)

62

(17)

71–72

(22)

Full

treatment 5 15–21 9–13 10–13 12–17 68–71(20-22)

56–61

(13-16)

60

(16)

68–71

(20–22)
12

General Technical Report FPL–GTR–174

each zone measuring 9.5 by 14 ft (2.9 by 4.3 m), and the 

MC readings in each zone were averaged. Relative humidity was determined from dry-bulb and wet-bulb temperature 

measurements. Soil moisture content was also measured 

periodically; statistical analysis showed that soil moisture 

content did not vary significantly during the course of the 

study.

Table 6 shows the wood moisture contents averaged over the 

20 sensors, along with the relative humidity for each vent 

and ground cover condition. The trends in wood moisture 

content ran parallel those in relative humidity. The values 

in zones 1 and 4 (the corners of the crawlspace adjacent to 

the garage) were consistently higher than in the other zones. 

For a given venting ratio, increasing the ground coverage 

clearly decreased the wood moisture content. This suggests 

that evaporation of water from the crawlspace soil was a 

major source of moisture. Unfortunately, this study did not 

examine the role of outside air as a source of crawlspace 

moisture, although the increase in wood moisture content 

and relative humidity between conditions 1 and 2 suggests 

that increased ventilation helps to dry the crawlspace when 

the ground is uncovered. This study did not consider seasonal moisture variation under any given set of vent/ground 

cover conditions.

Moffatt (1992) conducted a study of 10 houses in British Columbia that had crawlspace moisture problems. A 

number of the houses had inadequate or ineffective drainage systems, or a high water table that resulted in very wet 

soil. Wood moisture contents were measured with a resistance-type meter at site visits in July, September, October, 

or January. The measured values of MC in the sill plates 

varied from 10% to ~32%, with an average of 17%. It was 

suggested that sill plate gaskets in use were only partially 

effective in preventing moisture from wicking through concrete foundation walls into the sill plates. In the floor joists, 

the MC varied from 10% to 22%, with an average of 13%. 

Crawlspace air temperatures ranged from 52°F (11°C) in 

January to 72°F (22°C) in July. The importance of an effective moisture barrier (ground cover) was emphasized as the 

greatest single factor influencing the rate of moisture production in crawlspaces.

Additional details of this study were reported by Sheltair 

Scientific Ltd. (1991). Moisture content measurements were 

carried out in one house five times between October 1990 

and March 1991. The average, minimum, and maximum 

MC values were as follows: subflooring, 15%, 11%, 18%; 

floor joists, 15%, 10%, 22%; and rim joists, 23%, 16%, 

27%.

3

Table 5—Reported moisture contents and surface 

temperatures in wood floor members in North 

Carolina crawlspaces during summer months 

(Advanced Energy 2005d)

Wood member MC range (%) Temp. range (°F (°C)) 

Sill plate 13–22 64–72 (18–22) 

Rim joist 11–24 64–74 (18–23) 

Floor joist 11–22 62–74 (17–23) 

Center beam 14–21 60–73 (16–23) 

Subflooring 10–18 61–76 (16–24) 

Table 6—Summary of reported wood moisture contents in a 

California crawlspace (Quarles 1989)

Condition

Vent:floor

area ratio 

Ground

coverage 

(%)

Average 

wood

MC (%) 

Zone 1 

wood

MC (%) 

Average 

crawl space 

RH (%) 

1–initial 1:150 0 12.9 15 86 

2a 1:1500 0 18.0 22 98 

3 1:1500 75 16.1 20 87 

4 1:1500 90 15.0 19 84 

5 1:1500 100 13.8 18 79 

6 0 75 16.2 20 88 

7 0 90 15.1 18 83 

8 0 100 13.1 16 74 

1–final 1:150 0 14.4 18 82 

a

 Condition 2 did not stabilize; it was terminated because excessive mold growth was 

observed in one corner (zone 1) of the crawlspace. 

Table 7—Reported moisture contents of wood members 

during winter in coastal Washington homes with basement 

and crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Sill plate 29 21.6 13 ~36 

Rim joist 22 17.7 11 26 

Subflooring 28 20.3 11 30 

a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 8—Reported moisture contents of wood members 

during winter in Seattle/Olympia homes with basement and 

crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

3

Table 5—Reported moisture contents and surface 

temperatures in wood floor members in North 

Carolina crawlspaces during summer months 

(Advanced Energy 2005d)

Wood member MC range (%) Temp. range (°F (°C)) 

Sill plate 13–22 64–72 (18–22) 

Rim joist 11–24 64–74 (18–23) 

Floor joist 11–22 62–74 (17–23) 

Center beam 14–21 60–73 (16–23) 

Subflooring 10–18 61–76 (16–24) 

Table 6—Summary of reported wood moisture contents in a 

California crawlspace (Quarles 1989)

Condition

Vent:floor

area ratio 

Ground

coverage 

(%)

Average 

wood

MC (%) 

Zone 1 

wood

MC (%) 

Average 

crawl space 

RH (%) 

1–initial 1:150 0 12.9 15 86 

2a 1:1500 0 18.0 22 98 

3 1:1500 75 16.1 20 87 

4 1:1500 90 15.0 19 84 

5 1:1500 100 13.8 18 79 

6 0 75 16.2 20 88 

7 0 90 15.1 18 83 

8 0 100 13.1 16 74 

1–final 1:150 0 14.4 18 82 

a

 Condition 2 did not stabilize; it was terminated because excessive mold growth was 

observed in one corner (zone 1) of the crawlspace. 

Table 7—Reported moisture contents of wood members 

during winter in coastal Washington homes with basement 

and crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Sill plate 29 21.6 13 ~36 

Rim joist 22 17.7 11 26 

Subflooring 28 20.3 11 30 

a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 8—Reported moisture contents of wood members 

during winter in Seattle/Olympia homes with basement and 

crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma
 13

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

Fugler and Moffatt (1994) followed up on the 1991 study in 

British Columbia. Temperature, humidity, and wood moisture content were measured in one house in which remedial 

measures had been applied. A 6-mil (0.15-mm) polyethylene 

ground cover was installed, and passive and active ventilation systems were eliminated. Comparisons can be drawn 

between measured values in March 1991 and March 1993. 

The moisture content of the floor joists dropped from 10% 

to 9% and the rim joists dropped from 14% to 12%. The 

foundations in British Columbia were typically built with 

a wood internal support wall that rested on concrete footings. The moisture contents in these internal support walls 

dropped from 12–15% to 11%, and in the sill plates resting 

on the concrete footings, the MCs dropped from 25–30% to 

12% (measured at the top of the plate).

Flynn et al. (1994) investigated wood moisture content 

along with air temperature and relative humidity in four 

crawlspaces in a six-unit condominium complex in Petaluma, California. This complex had noticeable problems with 

mold and decay of wood members in the crawlspace, likely 

resulting from poor site drainage and lack of a ground cover. 

Although the foundation walls did have vents, the amount 

of ventilation did not satisfy code requirements, and only 

one end unit had cross-ventilation (though ventilation was 

not necessarily a significant factor). Two end units and two 

interior units were selected for study. A 6-mil (0.15-mm) 

polyethylene ground cover was installed in one end unit 

and one interior unit, while the other units were left in their 

original condition.

Wood moisture content was measured with a resistancetype meter in support beams (4 by 8 in. (100 by 200 mm)) 

that ran perpendicular to the floor joists. The joists were 

not monitored because they were not as affected by fungal 

growth as were the beams. Prior to installation of the ground 

cover, the moisture contents ranged from 22% to 27%. 

Afterwards, in the two units with ground covers, the MC 

dropped to 16–18%, while the MC values in the two units 

without ground covers were between 19% and 25%. These 

results were consistent with measured relative humidities of 

79% ± 4% in the interior unit without a ground cover and 

55% ± 17% RH in the interior unit with a ground cover (the 

uncertainties represent the 95% confidence interval). Air 

temperatures in the crawlspaces were also monitored: for 

the period between October and February, the temperatures 

were 65°F ± 5°F (18°C ± 3°C) in the interior unit with a 

ground cover and 64°F ± 5°F (18°C ± 3°C) in the interior 

unit without a ground cover (again, 95% confidence 

intervals).

Several field studies of moisture in houses in the Pacific 

Northwest region were undertaken by Tsongas (1980, 1984, 

1990). Although these studies primarily focused on conditions in walls, crawlspaces were also investigated to a limited extent, and the results were reported in a separate paper 

(Tsongas 1994b). The majority of houses were located in 

marine climates: Portland, Oregon; the Seattle–Olympia, 

Washington, metropolitan area; and the Washington coastal 

area. Inspections were conducted during the winter months. 

In general, the crawlspaces did not have moisture problems 

despite the wet winter weather: they did not smell musty; 

only a few had standing water; and there were only a few 

cases of minor mold and surface staining on subflooring or 

rim joists, but no cases of wood decay (other than due to 

plumbing leaks).

Moisture contents were measured with a resistance-type 

meter in a few instances. The average MC of sill plates 

(treated wood) was 21%, and this winter average value did 

not drop below 20% by August. Tsongas (1994b) suggested 

that the high moisture levels in the sill plates were caused by 

rain water splashing against the house from the ground. Additional measurements in the San Francisco Bay area found 

that the moisture contents of wood members in crawlspaces 

were generally less than 20%. Attempting to explain the 

lack of moisture problems, Tsongas (1994b) argued that the 

conditions that favor mold and decay—high moisture levels 

coincident with warm temperatures—do not often occur 

in this climate. While the winter is wet and mild, the summer tends to be dry in the Pacific Northwest compared with 

other parts of the country (such as the southeastern United 

States).

Further details of moisture content measurements were 

given for sill plates, rim joists, and subflooring in the coastal 

Washington and Seattle–Olympia houses (Tsongas 1990). 

Tables 7 and 8 list the average, minimum, and maximum 

moisture content values. However, these results are pooled 

and do not differentiate between houses with crawlspaces 

and those with basements. Tsongas did note that sill plates 

over basement foundations were generally drier, possibly 

because most of the basements were heated.

2.4 Cold Climate

Britton (1948) reported the results of investigations into 

crawlspace moisture problems carried out by the U.S. Housing and Home Finance Agency. About 50 WWII housing 

projects were surveyed in Wisconsin, Illinois, Indiana, 

Maryland, Virginia, and Washington, D.C. Although very 

few details were given regarding the construction of these 

houses, Britton described one project in the central states in 

which the crawlspaces were not vented (presumably there 

was no soil vapor retarder). The moisture contents of wood 

members in the foundation were found to be as high as 50% 

(measurement method not specified). In other projects (locations not specified), wood moisture contents over 50% were 

found, and plywood flooring was delaminated and softened 

to the extent that it acted “like a sheet of rubber.”

Diller (1953) investigated the effects of soil cover and 

ventilation in groups of four houses with crawlspaces in 

South Portland, Maine; Hartford, Connecticut; and Wayne, 

Michigan. The studies were conducted as described previously (Section 2.1). The Maine and Michigan houses were 
14

General Technical Report FPL–GTR–174

built on continuous cement block foundations, with 42 in. 

(1.1 m) and 16 in. (0.4 m) of crawlspace clearance, respectively. None of the houses had insulation below the floor. 

The Maine crawlspaces had permanent pools of water, while 

those in Michigan were frequently flooded to a depth of 

2–3 in. (50–75 mm) due to poor drainage. Despite these 

conditions, the crawlspaces with soil covers had average 

moisture contents in the 10–20% range, whereas the uncovered crawlspaces had very wet wood members, reaching 

winter peaks of ~40% and in some cases as high as ~60% 

MC.9 The Connecticut houses had wood pier foundations 

with mineral board skirting and 16 in. (0.4 m) of clearance. The covered crawlspaces had moisture contents in the 

10–15% range. In the uncovered crawlspaces, after an initial 

winter peak of around 30%, the MCs dropped to between 

15% and 22%. Diller suggested that these more favorable conditions were caused by insulation, which had been 

tacked to the bottoms of the joists, coming loose and falling 

to the ground, creating a partial soil cover.

Moses (1954) also studied the effect of a ground cover 

on the moisture content of wood members in houses with 

crawlspaces. In the first part of the study, measurements 

were taken in the crawlspace of a house in Madison, Wisconsin, which did not have a ground cover. Wood moisture 

content was measured using a resistance-type meter, with 

the pins installed permanently in the wood members (rim 

joists, end joists, floor joists, and subflooring) so that readings could be obtained on a weekly basis. Wood surface 

temperature and air temperature were measured with 

MC readings above 30% by electrical resistance are approximate (see 

Section 1.4).

thermocouples, and the air relative humidity was gauged 

by measuring the moisture content of a central joist (it was 

assumed that the wood and air were in equilibrium). The 

measured relative humidity and temperature were then used 

to calculate the dew point temperature.

Data were acquired from October 1944 until June 1945. 

The crawlspace vents were closed and the soil was left 

uncovered during the study; it was observed to be wet and 

sticky, but not muddy or soft. For the rim joists and edge 

joists, most of the readings by far were above 30% MC. 

In one floor joist, about 40% of the readings were above 

30% MC. The moisture content tended to rise above 30% 

when the outside temperature dropped below 40°F (4°C). 

Data from early October to late November showed that the 

MC values rose from ~20% to ~60%9 as the wood surface 

temperature dropped from 63°F (17°C) to 51°F (11°C). During this same period, the wood temperature began to drop 

below the dew point, which likely induced condensation. 

Conversely, for the period from mid-March to mid-April, 

the MC values dropped from nearly 60%9 to ~25% as the 

wood temperature rose from 48°F (9°C) to 58°F (14°C). The 

accuracy of the moisture meter was verified by small wood 

blocks clamped to the rim joist, which were later removed 

for gravimetric analysis.

The second part of the study investigated crawlspaces in two 

suburban Chicago houses between 1948 and 1951. Vents 

were closed during the tests, and 45-lb (2.0-kg/m2) asphalt 

roll roofing was used as a soil cover. The moisture content 

of the crawlspace wood members was measured as before. 

Prior to installation of the soil cover, MC values during winter in the two crawlspaces ranged from as low as ~15% to as 

3

observed in one corner (zone 1) of the crawlspace. 

Table 7—Reported moisture contents of wood members 

during winter in coastal Washington houses with basement 

and crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Sill plate 29 21.6 13 ~36 

Rim joist 22 17.7 11 26 

Subflooring 28 20.3 11 30 

a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 8—Reported moisture contents of wood members 

during winter in Seattle–Olympia houses with basement and 

crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Sill plate 59 21.8 13 ~50 

Rim joist 69 15.9 11 20 

Subflooring 73 17.3 11 24 

a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 9—Reported moisture content range of wood members in 

insulated basements of Minneapolis–St. Paul homes (Anderson 

1989)

Wood member Month

Moisture content 

range (%) 

Insulation cavity framing, above grade March–April 6–18 

Insulation cavity framing, above grade June 6–16 

Insulation cavity framing, below grade March–April 6–24 

Insulation cavity framing, below grade June 7–24 

Rim joist March–April 6–15 

Rim joist June 6–14 

“Interior framing” March–April 6–10 

“Interior framing” June 6–10 

Table 10—Reported moisture contents of wood floor members 

during winter in Montana homes with basement and crawlspace 

foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Sill plate 5 12.7 9.5 15 

Rim joist 18 12.4 8.5 22 

Subflooring 20 15.4 8.0 ~55 a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 11—Reported moisture contents of wood floor members 

during winter in Spokane, Washington, homes with basement 

and crawlspace foundations (Tsongas 1984)

Moisture content (%) 

Wood member Measurements Average Minimum Maximum 

Sill plate 91 14.0 <6 >30 

Rim joist 283 11.3 <6 22 

Subflooring 268 10.6 <6 20 
 15

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

high as ~100%.10 This extreme variation in moisture content 

may have been due to localized variations in temperature: 

the coldest members (rim joists and floor joists near the 

exterior) would be the likely places for condensation. With 

the soil cover in place, MC values during the month of 

June were between 8% and 20%, and the following winter 

remained steady between 6% and 14%. There was a period 

during which the soil covers were partially removed (exposing 20% and 50% of the soil in the two houses); the majority of the readings ranged from 7% to 15%, though a few 

were as high as 30%. The soil covers were then completely 

removed; MC values in July remained low (9–15%), but 

in December, as in the first winter with the soil uncovered, 

MC values again were measured from ~15% to as high as 

~85%.10 The soil covers were finally installed again, and 

during the month of March, values were typically between 

7% and 20% MC. Thus it was concluded that a ground cover (even a partial cover) has a substantial effect on reducing 

the moisture content of wood members in crawlspaces.

Moses and Scheffer (1962) reported moisture content measurements in houses in Chicago, Illinois, and Madison, 

Wisconsin, during the month of January. For houses with 

crawlspaces, the percentages of moisture content readings 

in the 20–30% range were as follows: crawlspaces with dry 

soil, 0%; those with covered damp soil and good ventilation, 65%; and those with uncovered damp soil and poor 

ventilation, 100%. Moisture contents exceeding 30% were 

observed.

Anderson (1989) surveyed 42 houses in the Minneapolis–St. 

Paul, Minnesota, metropolitan area to determine whether insulating the interior of the foundation walls led to moisture 

problems. Several different types of basement foundations 

were included: masonry block, cast concrete, and permanent 

wood foundations. All the houses were less than 5 years old. 

Wood moisture contents were measured with a resistancetype meter in several locations: the framing members in the 

insulation cavity (the type of insulation was not specified), 

rim joists, and “interior framing”—framing exposed only to 

interior air (not in contact with the foundation wall). Table 9 

lists the range of moisture contents for these members in the 

42 houses visited in late March–early April and in the 

16 houses revisited in early June. Values less than 6% MC 

were outside the range of the meter. Excessive wetness 

(with mold growth in some instances) was found in 2 of the 

42 buildings in March–April and in 4 of the 16 buildings 

in June, likely because there was more rainfall in June. The 

excessive moisture levels were associated with water penetrating the foundation due to poor exterior surface drainage 

and lack of gutters.

Tsongas (1994b) surveyed houses in Montana and Spokane, 

Washington, during the winter months. The ground was typically dry, and the crawlspaces in these houses did not have 

10MC readings above 30% by electrical resistance are approximate (see 

Section 1.4).

noticeable moisture problems: there were no occurrences of 

mold or staining and no musty smells. Many crawlspaces 

did not have ground covers, but this did not appear to result 

in any problems. Moisture content measurements were reported for sill plates, rim joists, and subflooring in both the 

Montana houses (Tsongas 1990) and the Spokane houses 

(Tsongas 1984). A resistance-type meter was used, and all 

readings were corrected for temperature. These results were 

pooled and did not differentiate between houses with crawlspaces and those with basements. Tables 10 and 11 list the 

average, minimum, and maximum moisture content values.

2.5 Summary: Floor Members

Table 12 summarizes the studies reporting moisture contents 

in floor members. Tables 13–16 summarize the reported 

moisture contents for each member, including sill plates, rim 

joists, floor joists, and subfloor sheathing. Several conclusions can be drawn from these studies for crawlspace 

foundations.

1. The most extreme moisture contents in wood structural 

members above crawlspace foundations occur when the 

ground is not covered with a vapor diffusion retarder. 

This effect is magnified for sites with poor drainage.

2. Two different seasonal trends have been observed for 

crawlspaces:

a. The moisture content reached a maximum in winter 

and minimum in summer. This trend was observed 

in studies prior to around 1955 in uncovered crawlspaces 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 supplied 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. The buildings were not 

air-conditioned during the summer, and the floor 

framing therefore was probably warmer than the 

crawlspace floor (or below-grade portions of the 

crawlspace walls) for most of the time during 

summer months.

b. The 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 around 1955 in which seasonal 

trends were investigated. These studies included 

various types of crawlspaces (both covered and uncovered, vented and sealed). In many of these studies, the living space above the crawlspace was either known to be, or was probably, air-conditioned 

during the summer. Most likely, the major source 

of crawlspace moisture in these studies was warm, 

humid outdoor air rather than moisture evaporating 

from the soil. In summer, the floor members can 

be cooler than the outdoor air (sometimes cooler 
16

General Technical Report FPL–GTR–174

than the outdoor dew point temperature), especially when the building is air-conditioned. Lower 

outdoor temperatures during fall and winter would 

logically lower the intensity of crawlspace moisture 

sources.

3 Wall Framing and Sheathing

In this section we report measurements of wood moisture 

content and temperature in wall framing and sheathing. In 

addition to climate, major factors that influence conditions 

in wall members are resistance to bulk water intrusion, airtightness of the wall system, water vapor permeances of the 

inner and outer wall assemblies, and thermal characteristics. 

For example, the location of insulation can have a large affect on sheathing temperature and its potential for moisture 

accumulation. Additionally, the way in which the building is 

operated plays an important role; for example, heating, cooling, ventilation, humidification, and dehumidification can 

strongly affect conditions. Gavin (1984) reviewed field studies and outdoor exposure studies as well as laboratory studies and theoretical work related to understanding moisture 

migration in walls. Tsongas (1994a) summarized case studies of moisture problems in walls of residential buildings.

3.1 Hot-Humid Climate

Moses and Scheffer (1962) measured wood moisture contents in the bottom plates and lower ends of studs of exterior 

walls and partition walls of houses with slab-on-grade 

foundations. In the southeastern United States (August–

October), 6% of the houses had exterior wall bottom plates 

in the range of 20–30% MC, whereas all the partition wall 

bottom plates and studs were below 20%. Similarly, in the 

Gulf Coast region (October–December), exterior wall bottom plates were in the range of 20–30% MC in 4% of the 

houses and above 30% MC in 3% of the houses. All the 

4

Rim joist 69 15.9 11 20 

Subflooring 73 17.3 11 24 

a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 9—Reported moisture content range of wood members 

in insulated basements of Minneapolis–St. Paul houses 

(Anderson 1989)

Wood member Month

Moisture content 

range (%) 

Insulation cavity framing, above grade March–April 6–18 

Insulation cavity framing, above grade June 6–16 

Insulation cavity framing, below grade March–April 6–24 

Insulation cavity framing, below grade June 7–24 

Rim joist March–April 6–15 

Rim joist June 6–14 

“Interior framing” March–April 6–10 

“Interior framing” June 6–10 

Table 10—Reported moisture contents of wood floor members 

during winter in Montana houses with basement and 

crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Sill plate 5 12.7 9.5 15 

Rim joist 18 12.4 8.5 22 

Subflooring 20 15.4 8.0 ~55 a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 11—Reported moisture contents of wood floor members 

during winter in Spokane, Washington, houses with basement 

and crawlspace foundations (Tsongas 1984)

Moisture content (%) 

Wood member Measurements Average Minimum Maximum 

Sill plate 91 14.0 <6 >30 

Rim joist 283 11.3 <6 22 

Subflooring 268 10.6 <6 20 

4

Rim joist 69 15.9 11 20 

Subflooring 73 17.3 11 24 

a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 9—Reported moisture content range of wood members 

in insulated basements of Minneapolis–St. Paul houses 

(Anderson 1989)

Wood member Month

Moisture content 

range (%) 

Insulation cavity framing, above grade March–April 6–18 

Insulation cavity framing, above grade June 6–16 

Insulation cavity framing, below grade March–April 6–24 

Insulation cavity framing, below grade June 7–24 

Rim joist March–April 6–15 

Rim joist June 6–14 

“Interior framing” March–April 6–10 

“Interior framing” June 6–10 

Table 10—Reported moisture contents of wood floor members 

during winter in Montana houses with basement and 

crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Sill plate 5 12.7 9.5 15 

Rim joist 18 12.4 8.5 22 

Subflooring 20 15.4 8.0 ~55 a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 11—Reported moisture contents of wood floor members 

during winter in Spokane, Washington, houses with basement 

and crawlspace foundations (Tsongas 1984)

Moisture content (%) 

Wood member Measurements Average Minimum Maximum 

Sill plate 91 14.0 <6 >30 

Rim joist 283 11.3 <6 22 

Subflooring 268 10.6 <6 20 

4

Rim joist 69 15.9 11 20 

Subflooring 73 17.3 11 24 

a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 9—Reported moisture content range of wood members 

in insulated basements of Minneapolis–St. Paul houses 

(Anderson 1989)

Wood member Month

Moisture content 

range (%) 

Insulation cavity framing, above grade March–April 6–18 

Insulation cavity framing, above grade June 6–16 

Insulation cavity framing, below grade March–April 6–24 

Insulation cavity framing, below grade June 7–24 

Rim joist March–April 6–15 

Rim joist June 6–14 

“Interior framing” March–April 6–10 

“Interior framing” June 6–10 

Table 10—Reported moisture contents of wood floor members 

during winter in Montana houses with basement and 

crawlspace foundations (Tsongas 1990)

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Sill plate 5 12.7 9.5 15 

Rim joist 18 12.4 8.5 22 

Subflooring 20 15.4 8.0 ~55 a

 MC readings above 30% by electrical resistance are approximate (see Section 1.4).

Table 11—Reported moisture contents of wood floor members 

during winter in Spokane, Washington, houses with basement 

and crawlspace foundations (Tsongas 1984)

Moisture content (%) 

Wood member Measurements Average Minimum Maximum 

Sill plate 91 14.0 <6 >30 

Rim joist 283 11.3 <6 22 

Subflooring 268 10.6 <6 20 
 17

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

5

Table 12—Summary of studies reporting moisture contents in wood floor members by climatic region 

Moisture content (%) 

Reference Location

Type of 

study Duration Na

Foundationb

Measurement 

locationc

Mean Minimum Maximumd Trends

Hot-humid climate 

Diller 

1953

N. 

Charleston, 

SC

Field 

experiment

3 yr 4 CS SP, girder — 15 ~40 x Max MC in winter with 

uncovered soil 

x Soil cover lowered MC 

Moses & 

Scheffer 

1962

Southeastern 

U.S. 

Survey Aug–Oct 61 B, CS FJ, RJ, SP — <12 30

Moses & 

Scheffer 

1962

Gulf Coast Survey Oct–Dec 14 CS FJ, RJ, SP — 12 <20 

Verrall

1962

Saucier, MS Field 

experiment

Jun–Sep 1 CS SF — 10 30 x Vapor retarder directly 

below floor joists kept MC 

< 15% 

Choong & 

Cassens 

1985

Baton Rouge, 

LA

Field 

experiment

1.5 yr 3 CS blocks 

attached to 

SF

— 8 25 x Ground cover and degree 

of ventilation had no 

significant effect on 

average MC for open pier 

foundation

x No discernable seasonal 

trend

Advanced

Energy 

2005a, b 

North

Carolina 

Survey Jul–Dec 45 CS FJ, RJ, SP, 

SF, beam 

— — >30 x Summer outdoor air 

contained more moisture 

than air in wall-vented 

crawlspace 

x Most CS had RH > 70% 

during summer 

Mixed-humid climate 

Diller 

1946,

1950

Washington,

DC

Field 

experiment

8 yr 4 CS FJ, SP — 12 ~35 x Max MC in winter with 

uncovered soil 

x Soil cover lowered MC 

x Degree of ventilation had 

little effect on MC 

Diller 

1953

Washington,

DC

Field 

experiment

3 yr 4 CS SP, girder — 10 30 x Max MC in winter with 

uncovered soil and vents 

closed 

x Lower MC with soil 

uncovered and vents open

x Lowest MC with soil 

covered

x Ventilation had no 

significant effect when 

soil was covered 

Diller 

1953

Philadelphia, 

PA

Field 

experiment

3 yr 4 CS SP, girder — 10 ~50 x Same as above 

Diller 

1953

Oak Ridge, 

TN

Field 

experiment

3 yr 4 CS SP, girder — 10 ~40 x Same as above 

Moses & 

Scheffer 

1962

Baltimore, 

MD

Survey Aug 48 B, CS FJ, RJ, SP — <12 30

Moses & 

Scheffer 

1962

Memphis, TN Survey Jan 31 CS FJ, RJ, SP — <12 30

Amburgey 

& French 

1970

Raleigh, NC Field 

experiment

14 mo 2 CS FJ, SF, SP — 8.5 22 x Max MC in summer 

x Ground cover lowered 

MC
18

General Technical Report FPL–GTR–174

5

Table 12—Summary of studies reporting moisture contents in wood floor members by climatic region 

Moisture content (%) 

Reference Location

Type of 

study Duration Na

Foundationb

Measurement 

locationc

Mean Minimum Maximumd Trends

Hot-humid climate 

Diller 

1953

N. 

Charleston, 

SC

Field 

experiment

3 yr 4 CS SP, girder — 15 ~40 x Max MC in winter with 

uncovered soil 

x Soil cover lowered MC 

Moses & 

Scheffer 

1962

Southeastern 

U.S. 

Survey Aug–Oct 61 B, CS FJ, RJ, SP — <12 30

Moses & 

Scheffer 

1962

Gulf Coast Survey Oct–Dec 14 CS FJ, RJ, SP — 12 <20 

Verrall

1962

Saucier, MS Field 

experiment

Jun–Sep 1 CS SF — 10 30 x Vapor retarder directly 

below floor joists kept MC 

< 15% 

Choong & 

Cassens 

1985

Baton Rouge, 

LA

Field 

experiment

1.5 yr 3 CS blocks 

attached to 

SF

— 8 25 x Ground cover and degree 

of ventilation had no 

significant effect on 

average MC for open pier 

foundation

x No discernable seasonal 

trend

Advanced

Energy 

2005a, b 

North

Carolina 

Survey Jul–Dec 45 CS FJ, RJ, SP, 

SF, beam 

— — >30 x Summer outdoor air 

contained more moisture 

than air in wall-vented 

crawlspace 

x Most CS had RH > 70% 

during summer 

Mixed-humid climate 

Diller 

1946,

1950

Washington,

DC

Field 

experiment

8 yr 4 CS FJ, SP — 12 ~35 x Max MC in winter with 

uncovered soil 

x Soil cover lowered MC 

x Degree of ventilation had 

little effect on MC 

Diller 

1953

Washington,

DC

Field 

experiment

3 yr 4 CS SP, girder — 10 30 x Max MC in winter with 

uncovered soil and vents 

closed 

x Lower MC with soil 

uncovered and vents open

x Lowest MC with soil 

covered

x Ventilation had no 

significant effect when 

soil was covered 

Diller 

1953

Philadelphia, 

PA

Field 

experiment

3 yr 4 CS SP, girder — 10 ~50 x Same as above 

Diller 

1953

Oak Ridge, 

TN

Field 

experiment

3 yr 4 CS SP, girder — 10 ~40 x Same as above 

Moses & 

Scheffer 

1962

Baltimore, 

MD

Survey Aug 48 B, CS FJ, RJ, SP — <12 30

Moses & 

Scheffer 

1962

Memphis, TN Survey Jan 31 CS FJ, RJ, SP — <12 30

Amburgey 

& French 

1970

Raleigh, NC Field 

experiment

14 mo 2 CS FJ, SF, SP — 8.5 22 x Max MC in summer 

x Ground cover lowered 

MC

6

 

Duff 1978 Athens, GA Test 

structure 

2 yr 1 CS FJ, SF — 8 20 x Max MC in summer 

x Similar MC with 1) soil 

cover + vents closed and 

2) no soil cover + vents 

open

x Vapor retarder directly 

below joists kept MC < 

13%

Duff 1980 Athens, GA Test 

structure 

2.5 yr 1 CS FJ, SF — 9 18 x Max MC in summer 

x Combination of soil cover, 

vents closed and sealed, 

and wall insulation gave 

acceptable MC values 

Jennings

& Moody 

1983

Tennessee Survey Various 

months

36 B, CS Not specified — 7 30 x MCs up to 30% in CS but 

only up to 15% in B 

Moody et 

al. 1983 

Murfreesboro,

TN

Field 

experiment

10 mo 4 CS FJ — 7 18 x Max MC in summer 

x Lower MC with soil 

cover, wall insulation, and 

vents closed than no soil 

cover, no insulation, and 

vents open 

Moody et 

al. 1985 

Murfreesboro,

TN

Field 

experiment

2 yr 3 CS FJ — 8 18 x Same as above 

Dutt et al. 

1988

Toms River, 

NJ

Field 

experiment

1 yr 15 CS FJ, RJ, beam — 10 25 x Max MC in summer 

x Combination of ground 

cover and wall insulation 

lowered MC regardless of 

whether vents were open 

or closed 

Stiles & 

Custer

1994

Southern NJ Field 

experiment

10 mo 17 CS FJ, RJ — 8 ~32 x Max MC in summer 

x Ground cover lowered 

MC in vented CS 

x Ground cover, wall 

insulation, and air sealing 

lowered MC further 

x Correlation between MC 

and air leakage for closed 

CS

Davis & 

Dastur

2004

Princeville, 

NC

Field 

experiment

3 yr 12 CS FJ — 9 15 x Max MC in summer 

x Lower MC and RH in 

closed than in vented CS 

Advanced

Energy 

2005d

Central NC Survey Jun–Sep 10 CS FJ, RJ, SP, 

SF, beam 

— 10 24 x Maximum MC in vented 

CS was often found in a 

floor joist below 

insulation, typically in 

area of lowest clearance to 

ground

Marine climate 

Moses & 

Scheffer 

1962

Southern CA Survey Mar–Apr 190 CS FJ, RJ, SP — <12 30

Moses & 

Scheffer 

1962

OregonWashingtonSurvey Apr–Jun 134 CS FJ, RJ, SP — <12 >30 

Quarles

1989

Richmond,

CA

Field 

experiment

16 mo 1 CS blocks 

attached to 

FJ

— 13 22 x MC decreased with 

increasing ground cover 

for a given level of 

ventilation
 19

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

5

Table 12—Summary of studies reporting moisture contents in wood floor members by climatic region 

Moisture content (%) 

Reference Location

Type of 

study Duration Na

Foundationb

Measurement 

locationc

Mean Minimum Maximumd Trends

Hot-humid climate 

Diller 

1953

N. 

Charleston, 

SC

Field 

experiment

3 yr 4 CS SP, girder — 15 ~40 x Max MC in winter with 

uncovered soil 

x Soil cover lowered MC 

Moses & 

Scheffer 

1962

Southeastern 

U.S. 

Survey Aug–Oct 61 B, CS FJ, RJ, SP — <12 30

Moses & 

Scheffer 

1962

Gulf Coast Survey Oct–Dec 14 CS FJ, RJ, SP — 12 <20 

Verrall

1962

Saucier, MS Field 

experiment

Jun–Sep 1 CS SF — 10 30 x Vapor retarder directly 

below floor joists kept MC 

< 15% 

Choong & 

Cassens 

1985

Baton Rouge, 

LA

Field 

experiment

1.5 yr 3 CS blocks 

attached to 

SF

— 8 25 x Ground cover and degree 

of ventilation had no 

significant effect on 

average MC for open pier 

foundation

x No discernable seasonal 

trend

Advanced

Energy 

2005a, b 

North

Carolina 

Survey Jul–Dec 45 CS FJ, RJ, SP, 

SF, beam 

— — >30 x Summer outdoor air 

contained more moisture 

than air in wall-vented 

crawlspace 

x Most CS had RH > 70% 

during summer 

Mixed-humid climate 

Diller 

1946,

1950

Washington,

DC

Field 

experiment

8 yr 4 CS FJ, SP — 12 ~35 x Max MC in winter with 

uncovered soil 

x Soil cover lowered MC 

x Degree of ventilation had 

little effect on MC 

Diller 

1953

Washington,

DC

Field 

experiment

3 yr 4 CS SP, girder — 10 30 x Max MC in winter with 

uncovered soil and vents 

closed 

x Lower MC with soil 

uncovered and vents open

x Lowest MC with soil 

covered

x Ventilation had no 

significant effect when 

soil was covered 

Diller 

1953

Philadelphia, 

PA

Field 

experiment

3 yr 4 CS SP, girder — 10 ~50 x Same as above 

Diller 

1953

Oak Ridge, 

TN

Field 

experiment

3 yr 4 CS SP, girder — 10 ~40 x Same as above 

Moses & 

Scheffer 

1962

Baltimore, 

MD

Survey Aug 48 B, CS FJ, RJ, SP — <12 30

Moses & 

Scheffer 

1962

Memphis, TN Survey Jan 31 CS FJ, RJ, SP — <12 30

Amburgey 

& French 

1970

Raleigh, NC Field 

experiment

14 mo 2 CS FJ, SF, SP — 8.5 22 x Max MC in summer 

x Ground cover lowered 

MC

7

x Lowest RH in CS for 

combination of full 

ground cover and vents 

closed 

Moffatt

1992

British

Columbia 

Survey Various 

months

10 CS FJ, RJ, SF, 

SP

— 9 ~32 

Sheltair 

Scientific 

1991

British

Columbia 

Survey Oct–Mar 1 CS FJ, RJ, SF — 10 27 

Fugler & 

Moffatt

1994

British

Columbia 

Survey Mar 1 CS FJ, RJ, SP — 9 12 x MC decreased after 

adding ground cover and 

eliminating ventilation 

with outdoor air 

Flynn et 

al. 1994 

Petaluma, CA Field 

experiment

Oct–Feb 4 CS beam — 16 27 x MC decreased after 

adding ground cover 

Tsongas

1990

Washington

Coast

Survey Winter 16 B, CS RJ, SF, SP 20 11 ~36 x High MC in sill plates; 

possibly from splash 

wetting

Tsongas

1990

SeattleOlympia, WA Survey Winter 50 B, CS RJ, SF, SP 18 11 ~50 x Same as above 

Cold climate 

Diller 

1953

S. Portland, 

ME

Field 

experiment

3 yr 4 CS SP, girder — 10 ~55 x Max MC in winter with 

uncovered soil 

x Soil cover lowered MC 

Diller 

1953

Hartford, CT Field 

experiment

3 yr 4 CS SP, girder — 10 ~40 x Same as above 

Diller 

1953

Wayne, MI Field 

experiment

2 yr 4 CS SP, girder — 10 ~60 x Same as above 

Moses 

1954

Madison, WI Field 

experiment

9 mo 1 CS FJ, RJ, SF — 20 ~60 x Max MC near perimeter in 

winter with uncovered soil

Moses 

1954

Chicago, IL Field 

experiment

3 yr 2 CS FJ, RJ, SF — 6 ~100 x Max MC near perimeter in 

winter with uncovered soil

x Soil cover lowered MC 

Moses & 

Scheffer 

1962

Chicago, IL & 

Madison, WI 

Survey Jan 23 CS FJ, RJ, SP — <12 >30 

Anderson

1989

Minneapolis, 

MN

Survey Mar, 

Apr, Jun 

42 B RJ, 

insulation 

cavity 

framing,

interior 

framing

— 6 24 

Tsongas

1990

Montana Survey Winter 20 B, CS RJ, SF, SP 14 8 ~55 

Tsongas

1984

Spokane, WA Survey Winter 103 B, CS RJ, SF, SP 11 <6 >30 x Highest MC in sill plates 

a

 Number of buildings sampled. 

b

 B, basement; CS, crawlspace. 

c

 FJ, floor joist; RJ, rim joist; SF, subfloor sheathing; SP, sill plate. d

 Moisture content measurements above 30% by electrical resistance are approximate (see Section 1.4). 
20

General Technical Report FPL–GTR–174

8

Table 13—Summary of studies reporting moisture contents in sill plates 

Moisture content (%) 

Reference Location

Type of 

study Duration Na Foundationb Mean Minimum Maximumc

Tsongas 1984 Spokane, WA Survey Winter 91 B, CS 14.0 <6 >30 

Tsongas 1990 Montana Survey Winter 5 B, CS 12.7 9.5 15 

Tsongas 1990 Seattle–Olympia, WA Survey Winter 59 B, CS 21.8 13 ~50 

Tsongas 1990 Washington Coast Survey Winter 29 B, CS 21.6 13 ~36 

Moffatt 1992 British Columbia Survey Various months 10 CS 17 10 ~32 

Fugler & Moffatt 1994 British Columbia Survey Mar 1 CS 12 — — 

Advanced Energy 2005d Central NC Survey Jun–Sep >100 CS — 13 22 

a

 Number of measurements. 

b

 B, basement; CS, crawlspace. 

c

 Moisture content measurements above 30% by electrical resistance are approximate (see Section 1.4). 

Table 14—Summary of studies reporting moisture contents in rim joists 

Moisture content (%) 

Reference Location

Type of 

study Duration Na Foundationb Mean Minimum Maximum

Tsongas 1984 Spokane, WA Survey Winter 283 B, CS 11.3 <6 22 

Dutt et al. 1988 Toms River, NJ Field 

Experiment 1 yr >100 CS — 11 16 

Anderson 1989 Minneapolis, MN Survey Mar, Apr, Jun >100 B — 6 15 

Tsongas 1990 Montana Survey Winter 18 B, CS 12.4 8.5 22 

Tsongas 1990 Seattle–Olympia, WA Survey Winter 69 B, CS 15.9 11 20 

Tsongas 1990 Washington Coast Survey Winter 22 B, CS 17.7 11 26 

Sheltair Scientific 1991 British Columbia Survey Oct–Mar 5 CS 23 16 27 

Fugler & Moffatt 1994 British Columbia Survey Mar 1 CS 12 — — 

Stiles & Custer 1994 Southern NJ Field 

experiment 10 mo >170 CS — 10 18 

Advanced Energy 2005d Central NC Survey Jun–Sep >100 CS — 11 24 

a

 Number of measurements. 

b

 B, basement; CS, crawlspace. 

Table 15—Summary of studies reporting moisture contents in floor joists 

Moisture content (%) 

Reference Location

Type of 

study Duration Na Foundationb Mean Minimum Maximum 

Duff 1978, 1980 Athens, GA Test

structure 4.5 yr 1 CS — 9 20 

Moody et al. 1983, 1985 Murfreesboro, TN Field 

experiment 3 yr 4 CS — 7 18 

Dutt et al. 1988 Toms River, NJ Field 

experiment 1 yr 15 CS — 10 23 

Quarles 1989 Richmond, CA Field 

experiment 16 mo 1 CS — 13 22 

Sheltair Scientific 1991 British Columbia Survey Oct–Mar 1 CS 15 10 22 

Moffatt 1992 British Columbia Survey Various months 10 CS 13 10 22 

Fugler & Moffatt 1994 British Columbia Survey Mar 1 CS 9 — — 

Stiles & Custer 1994 Southern NJ Field 

experiment 10 mo 17 CS — 8 21 

Davis & Dastur 2004 Princeville, NC Field 

experiment 3 yr 12 CS — 9 15 

Advanced Energy 2005d Central NC Survey Jun–Sep 10 CS — 11 22 

a

 Number of buildings sampled. 

b

 CS, crawlspace.

Table 16—Summary of studies reporting moisture contents in subfloor sheathing 

Moisture content (%) 

Reference Location

Type of 

study Duration Na Foundationb Mean Minimum Maximumc
 21

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

exterior wall studs and partition wall bottom plates and 

studs were below 20%. The higher moisture levels in exterior wall bottom plates might be attributable to rain wetting 

(direct or splash wetting) of exterior walls or to a higher soil 

moisture content around the building perimeter (and resulting capillary rise through the slab).

Sherwood (1985, 1987) used a test building near Gulfport, 

Mississippi, to investigate moisture conditions in highly 

insulated walls. The 8- by 48-ft (2.4- by 12-m) structure had 

eight identical rooms with north- and south-facing walls that 

could be framed with test panels of various construction. 

All test panels had 1/2-in. (13-mm) gypsum board on the 

inside, full-thickness fiberglass insulation in the cavity, and 

hardboard lap siding on the outside. The primary variables 

investigated were the following:

•	 Sheathing material—1/2-in. (13-mm) fiberboard, 1/2-in. 

(13-mm) plywood, 1-in. (25-mm) extruded polystyrene 

foam, or 1-in. (25-mm) foil-faced glass-fiber-reinforced 

polyisocyanurate foam

•	 Vapor retarder—6-mil (0.15-mm) polyethylene or asphalted kraft paper facing on the insulation (stapled between the studs)11

One test panel was framed with 2 by 6 studs at 24-in. 

(600-mm) spacing and the rest with 2 by 4 studs at 16-in. 

(400-mm) spacing. During the first year of the study, there 

were no penetrations in the gypsum board and vapor retarder; in the second year, a standard duplex electrical outlet was 

installed in each panel to represent the effect of air leakage. 

Inside conditions were maintained at 67–70°F (19–21°C) 

with relative humidity above 40% during the heating season 

and at 76–79°F (24–26°C) during the cooling season, with 

humidity uncontrolled.

11The placement of the vapor retarder against the interior side of the wall is 

atypical for this climate.

Each test panel was instrumented with moisture sensors 

and thermocouples at various locations, including the siding–sheathing interface, the sheathing–insulation interface, 

at the center of the cavity insulation, and at various locations 

within the framing. The moisture sensors (Duff 1966) were 

calibrated to an accuracy of ±2% MC within the range of 

7–20% MC and were corrected for temperature.

Sherwood (1985) found that during the first year (no penetrations in test panels) moisture contents were typically in 

the 8–12% range with an average of 11%. When the walls 

were penetrated with electrical outlets (second year), the 

MCs generally increased into the 12–16% range with an 

average of 14%, with much greater variation in MC readings. The framing in the wall with 2 by 6 studs (which had 

fiberboard sheathing and a 6-mil (0.15-mm) interior polyethylene vapor retarder) had an average MC of about 16% 

at the end of both summers. This wall was the only one with 

wet insulation near the top of the wall during both summers. Where hygroscopic sheathing (plywood or fiberboard) 

was used, the moisture content in the south wall cavities 

was higher than in the north wall cavities. This difference between south and north walls did not occur for foam 

sheathings, implying that increased moisture in the south 

walls was solar-driven. In contrast, the low-permeance foam 

sheathings reduced the movement of moisture into the wall 

cavities during the summer. During the second winter, some 

walls had periods of elevated moisture content (16–20% 

MC), but most readings were between 12% and 16% MC. 

Disassembly of the test panels showed no deterioration of 

the wood framing or wood-based sheathing.

TenWolde and Mei (1986) studied moisture movement in 

the walls of a test building in Beaumont, Texas. The building contained nine instrumented wall panels of various size 

and construction, all facing south. All walls were framed 

with 2 by 4 studs, insulated with fiberglass batts, finished 

9

Advanced Energy 2005d Central NC Survey Jun–Sep 10 CS — 11 22 

a

 Number of buildings sampled. 

b

 CS, crawlspace.

Table 16—Summary of studies reporting moisture contents in subfloor sheathing 

Moisture content (%) 

Reference Location

Type of 

study Duration Na Foundationb Mean Minimum Maximumc

Verrall 1962 Saucier, MS Field 

Experiment Jun–Sep 1 CS — 10 30 

Duff 1978, 1980 Athens, GA Test structure 4.5 yr 1 CS — 8 20 

Tsongas 1984 Spokane, WA Survey Winter 103 B, CS 10.6 <6 20 

Choong & Cassens 1985 Baton Rouge, LA Field 

experiment 1.5 yr 3 CS — 8 25 

Tsongas 1990 Montana Survey Winter 20 B, CS 15.4 8 ~55 

Tsongas 1990 Seattle–Olympia, WA Survey Winter 50 B, CS 17.3 11 24 

Tsongas 1990 Washington Coast Survey Winter 16 B, CS 20.3 11 30 

Sheltair Scientific 1991 British Columbia Survey Oct–Mar 1 CS 15 11 18 

Advanced Energy 2005d Central NC Survey Jun–Sep 10 CS — 10 18 

a

 Number of buildings sampled. 

b

 B, basement; CS, crawlspace. 

c

 Moisture content measurements above 30% by electrical resistance are approximate (see Section 1.4). 
22

General Technical Report FPL–GTR–174

with gypsum board, and clad with hardboard siding. The 

variables investigated were the following:

•	 Wood fiberboard sheathing or aluminum-faced molded 

expanded polystyrene sheathing

•	 Presence of a polyethylene vapor retarder between the 

siding and sheathing

•	 Presence of a ventilated airspace between the siding and 

sheathing

•	 Presence of a polyethylene vapor retarder between the 

insulation and gypsum board

•	 Kraft paper facing or no facing on the insulation

Indoor temperature was maintained at 68–73°F (20–23°C) 

and indoor relative humidity at 50–60%. Although wood 

moisture content was not specifically reported, temperature 

and relative humidity were monitored in the wall panels using thermocouples and modified wood electrical resistance 

sensors (Duff 1966) between April and November 1984. 

The wood moisture sensors measured RH in the 40–100% 

range with a reported accuracy of ±10% RH, which roughly 

corresponds to moisture contents of 8–30%.

Condensation of moisture was observed during afternoons 

in one test panel (designated S9) with fiberboard sheathing 

(no exterior vapor retarder) and a polyethylene vapor retarder between the insulation and gypsum board. However, 

the moisture evaporated at night. None of the other panels 

showed evidence of condensation. This confirms the finding 

of Sherwood (1985) that in hot-humid climates, solar radiation can drive moisture from hygroscopic sheathing into the 

wall cavity when an exterior vapor retarder is absent, and 

the moisture can condense when an interior vapor retarder is 

present.

Mei (1988) modified this test facility in Beaumont, Texas, to 

investigate brick facades and plywood siding and to further 

explore the effect of a ventilated airspace between the cladding and exterior sheathing. After the hardboard siding was 

replaced with 3/8-in. (9-mm) plywood on the panel that had 

experienced cyclical condensation and evaporation (S9 mentioned above), this condensation disappeared. In one panel 

with brick cladding, fiberboard sheathing, no exterior vapor 

retarder, and a polyethylene interior vapor retarder, a daily 

cycle was observed of very high humidity levels (>90% RH) 

in the evening that then dropped overnight. A ventilated airspace between cladding and sheathing was found to reduce 

the flow of heat and moisture into the wall cavity.

During the 1990s, moisture problems ranging from wet 

sheathing to completely rotted walls were uncovered in 

hundreds of houses with an exterior insulation and finish system (EIFS) in Wilmington, North Carolina (Nisson 

1995). Although the number of EIFS-clad houses in the 

United States was estimated to be 150,000 in 1997, the 

extent of damage was not known but was thought to be concentrated in the hot, humid coastal regions of the Southeast 

and not limited to North Carolina (Best and Wardell 1997). 

The design of this cladding system consisted of rigid foam 

panels applied over exterior sheathing (commonly woodbased, though gypsum board and cement board were also 

used), a reinforcing mesh, and a synthetic plaster base coat 

and finish coat. Rainwater that penetrated the cladding system—usually around penetrations in walls such as windows 

and doors or at interfaces with roofs and decks—became 

trapped inside the wall, leading to high moisture contents 

and decay of wood sheathing and, in some cases, framing 

members.

Crandell and Kenney (1996) investigated eight EIFS-clad 

houses in Wilmington that were representative of the moisture problems in that area. Moisture content readings in the 

wood-based sheathing below window and door openings 

ranged from 18% to greater than 50% with a resistancetype meter. In most cases, wood decay in walls was not 

readily detected by visual observation from either side of 

the wall. Moisture accumulation in walls was attributed 

to rainwater intrusion through improper sealing at joints 

around windows, doors, and other penetrations; improperly 

sloped horizontal EIFS surfaces; inadequate flashing at roof 

lines, dormers, decks, fireplace chases, and other points; 

and window frames that leaked into wall cavities. In New 

Hanover County, it was reported that as many as 16% of 

properly caulked windows had internal leaking through window frames. It was also noted that the county building code 

required an interior vapor retarder, which, when combined 

with EIFS cladding, yielded walls that were unable to dry by 

vapor diffusion in either direction.

Lstiburek (1995) argued that failures resulted from a fundamental flaw in the design of EIFS—the lack of a drainage 

plane. That is, the design did not include a way for water 

that had penetrated the cladding to drain to the outside or to 

be dried by airflow, and the low permeance of the exterior 

base and finish coats prevented drying by vapor diffusion. 

He proposed details that included drainage planes between 

the coatings and rigid insulation, between the rigid insulation and the sheathing, and within the rigid insulation. 

Williams and Williams (1998) suggested that the details, 

methods, and procedures for waterproofing EIFS-clad residential construction had never been sufficiently developed. 

They pointed out the route of water ingress at a typically 

constructed window sill and proposed new remedial designs 

for window head and sill details that would direct water to 

the exterior of the EIFS rather than between the cladding 

and sheathing.

3.2 Mixed-Humid Climate

Weber and Reichel (1942) compared the performance of 

seven different sections of 2 by 4 construction in the north 

wall of a test structure in Washington, D.C. The variables of 

interest in the study were the following:

•	 Vapor retarder (presence, type, and location)

•	 Type of exterior sheathing (fiber insulation board or 

southern yellow pine)
 23

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

The wall cavities were not insulated. The various sections 

were monitored for a 14-day period in mid-winter, during 

which the average outdoor temperature was 28°F (–2°C). 

The indoor temperature was maintained at 75°F (24°C) during the day and 60°F (16°C) during the night, with a mean 

of 71°F (22°C). The indoor RH was maintained at a mean 

of 70%. Temperatures were measured daily with thermocouples at eight locations within each wall section. Moisture 

levels were monitored gravimetrically using photographic 

blotting paper of known RH–MC relationship. These papers 

were placed within the wall cavities and at the exterior side 

of the sheathing and were removed and weighed daily. They 

were accessed from the interior through a removable section 

of sheathing or from the exterior through a small door cut in 

the siding, which was then sealed with rubber gasketing.

Temperature profiles indicated that the fiber insulation board 

exterior sheathing raised the cavity temperature by about 

5°F (3°C) compared with the pine sheathing. The placement 

of a vapor retarder was found to have a significant effect: 

condensation was observed in all sections except those that 

had a vapor retarder on the warm side of the cavity. In one 

section with a vapor retarder on the warm side of the cavity, 

the RH of the air in the cavity was 55%, and the sheathing 

moisture content was reported to be ~9%. With no vapor 

retarder, the RH in the wall cavity reached 93%, and condensation was observed on the exterior sheathing. In the 

sections where a vapor retarder was placed anywhere on the 

cold side of the cavity (that is, between the cavity and exterior sheathing, between the sheathing and siding, or on both 

sides of the sheathing), the cavity RH reached 100% and 

condensation was observed. Sheathing moisture contents 

were reported between 6% and 60%.12

Moses and Scheffer (1962) surveyed houses in Baltimore, 

Maryland (August), and Memphis, Tennessee (January). For 

houses with slab foundations, all the MC readings in studs 

were below 20%. Exterior wall bottom plates with MCs in 

the 20–30% range were found in 12% of the houses in Baltimore and only 2% of those in Memphis.

Using an air-conditioned test house in Athens, Georgia, Duff 

(1971) measured temperature and moisture conditions on 

the interior and exterior sides of an insulated wall cavity. 

The 24- by 24-ft (7.3- by 7.3-m) structure contained 4- by 

8-ft (1.2- by 2.4-m) removable test sections on each of the 

four sides. These sections were constructed with wood lap 

siding, 3/8-in. (9-mm) plywood sheathing, fiberglass batt 

insulation, 6-mil (0.15-mm) polyethylene interior vapor 

retarder, and 1/2-in. (13-mm) gypsum board. The study was 

conducted to determine whether the interior vapor retarder 

would contribute to high moisture conditions in the walls of 

an air-conditioned building during the summer. The 

interior of the test house was conditioned at 75°F ± 2°F 

(24°C ± 1°C) with humidity uncontrolled. Temperature and 

12The measurement technique for wood MC was not reported; it was most 

likely gravimetric.

moisture content in the test panels were measured with thermocouples and wood moisture probes (Duff 1966). Based 

on measurements in the month of June, the moisture content 

at the plywood–insulation interface was fairly constant at 

about 12%, whereas a maximum of 24% MC was reached 

between the insulation and the interior vapor retarder. The 

peak in moisture content coincided with the peak in temperature (~90–100°F (32–38°C) depending on weather conditions and wall orientation) on the exterior wall surface during daylight hours and typically lasted only about 4 hours. 

During the night, the exterior temperature fell below the 

interior temperature, and the MC at the insulation–polyethylene interface typically fell back to 12% or lower. The most 

likely mechanism for this transient moisture transfer was 

that heating of the exterior surface drove moisture towards 

the cooler interior.

Duff (1972) used the same test house to monitor moisture 

and temperature conditions over a 1-year period. Test sections (same dimensions as above) were constructed with 

wood lap siding, 3/4-in. (19-mm) fiberboard sheathing, 

fiberglass batt insulation between 2 by 4 studs spaced 16-in. 

(400-mm) on center, and 1/2-in. (13-mm) gypsum board. 

These sections were installed in each of the four walls of the 

structure, facing each of the four cardinal directions. Each 

test section had three panels (no penetrations): the first had 

a 6-mil (0.15-mm) polyethylene vapor retarder positioned 

at the interior (between insulation and gypsum board), the 

second had the same positioned at the exterior (between 

insulation and fiberboard, also covering the studs on each 

side), and the third had no vapor retarder. The interior of the 

test house was conditioned at 75°F ± 3°F (24°C ± 2°C) yearround. During the heating season the interior humidity was 

maintained at 30% ± 5% RH but was uncontrolled during 

the cooling season. Wood moisture probes (Duff 1966) and 

thermocouples were used to monitor moisture content and 

temperature in the siding, fiberboard, studs, and at the interface between the insulation and gypsum board. Monthly 

maximum moisture contents were reported.

In the panel with the interior vapor retarder, moisture content in the fiberboard sheathing reached 14% in the winter 

and stayed at 11–12% the rest of the year. At the center of 

the 2 by 4 studs, MC remained at about 10% throughout the 

year. Maximum moisture content was 23% at the insulation–polyethylene interface for a very short duration (several hours) during the summer. In the panel with the exterior 

vapor retarder, sheathing MC remained between 12% and 

15%, with the minimum in September and the maximum in 

March. During the winter, a maximum of 25% MC in the 

sheathing was found in the panel with no vapor retarder, 

and this high moisture level (>20%) persisted for about 

4 months. The direction of exposure was found to have an 

effect: the east-facing panel reached 25% MC, followed 

by the north-facing panel at 24%, the south-facing panel at 

23%, and the west-facing panel at 22%. These differences 
24

General Technical Report FPL–GTR–174

were attributed to drying effects of wind and solar radiation. 

Sheathing MC dropped to 10–12% in the summer in all panels with no vapor retarder. At the center of the 2 by 4 studs, 

MC varied from 10% in summer to 13% in winter.

3.3 Marine Climate

Moses and Scheffer (1962) surveyed houses with slab 

foundations in Southern California (March–April) and in 

Oregon and Washington (April–June). In California, 14% of 

the houses had exterior wall bottom plates with MCs in the 

20–30% range (sample size N = 74) and 20% had partition 

wall bottom plates with MCs in this range (though only 10 

partition walls were sampled). Exterior wall and partition 

wall studs were all below 20% MC (N = 14). Houses in Oregon and Washington had bottom plate moisture contents all 

below 20% (N = 20).

Tsongas (1980) surveyed 93 houses in Portland, Oregon, to 

determine whether houses retrofitted with wall insulation 

without a vapor retarder developed moisture problems. Of 

these 93 houses, 25 were uninsulated (control group), 

43 had urea-formaldehyde foam insulation, 10 had cellulose 

insulation, and 15 had mineral wool insulation. Almost all 

the houses had wood siding and #15 felt weather barrier; the 

majority had lapboard sheathing, and the rest had plywood 

sheathing. The majority of houses were heated with gas or 

oil, though some were electrically heated. The average number of occupants was three persons. Fan depressurization 

tests showed that the houses had fairly high air exchange 

rates, with an average of 16 air changes per hour at 50 Pa. 

However, the average indoor relative humidity was rather 

high at 56% RH.

In a preliminary survey, two locations for opening the walls 

of each house were determined. The objective was to find 

regions with the highest moisture levels that were not affected by bulk water leakage (12% of the houses had leaks 

in the walls). A capacitive surface-type moisture meter was 

used to scan the walls from the inside. Two locations were 

selected for each house, and at each location, two openings 

were made from the outside: one just below the top plate 

and one just above the bottom plate. The moisture contents 

of the sheathing and framing members were measured with 

a resistance-type meter at both the surface and the interior. 

Furthermore, small samples were taken for gravimetric analysis in the laboratory. Data were collected between February 

and early April 1979.

Table 17 lists the average, minimum, and maximum moisture contents for each wood member at the surface and interior determined from moisture meter readings. Gravimetric 

results were generally in good agreement. Average MC 

values ranged from about 11% to 14%, with an average of 

11.8% for all houses. No significant difference was found 

between insulated and uninsulated houses, or between houses with different types of insulation. Wood interior moisture 

content values were consistently higher than surface values. 

The maximum reading was 20% MC. However, some of 

the measurements were not included because some type 

of bulk water leak was found that would have biased the 

data. Moisture contents as high as 30% were found in such 

instances. Leaks were also found with the surface moisture 

meter when selecting sites for opening, and these wet locations were avoided.

Tsongas (1990) conducted another field study of indoor 

moisture problems in new houses in the Pacific Northwest. 

Inspections and moisture measurements were made at 

16 houses in the coastal western Washington area, 50 in the 

Seattle–Olympia metropolitan area, and 20 in Montana (see 

Section 3.4). These houses ranged from a few months to a 

few years old. They were relatively airtight and highly insulated (at least R-19). All were electrically heated; most had 

an air-to-air heat exchanger for ventilation, and some had a 

dehumidifier. In the first phase of the study, the houses were 

selected, the occupants were interviewed, wall sections were 

opened, and moisture content measurements were taken 

during the winter of 1987. In phase 2 (summer 1988), the 

28 wettest houses were revisited, and remedial measures 

were taken in the 10 houses with the worst moisture problems. In phase 3 (winter 1989), the 16 wettest houses were 

again revisited.

Tables 18 and 19 list the average, minimum, and maximum 

moisture contents for each wood member measured during 

winter 1987 (January–March). Average values range from 

14% to 21%. The walls of the coastal Washington houses 

were generally 3–4% higher in MC than those of the Seattle–Olympia houses. In both regions, the sheathing was 

significantly wetter than the various framing members. A 

correlation was found between high wood moisture content 

and high indoor relative humidity. Ventilation systems (spot 

exhaust fans and air-to-air heat exchangers) were often 

not working properly or not used. Tsongas suggested that 

portable dehumidifiers may be necessary to control indoor 

moisture in marine climates where the outdoor RH averages 

~90% during fall, winter, and spring.

During summer 1988, the 14 wettest houses in the Seattle–

Olympia area were revisited. While average and minimum 

values were not reported, the maximum sheathing MC had 

dropped to 17% (from ~50%13), and all the framing MC 

values were less than 17% (down from a maximum of 26%). 

In the 12 houses in the coastal Washington area that were 

revisited, the sheathing had dried to 23% or lower (from 

32%) and the studs to 25% or lower (from ~40%13), though 

a similar high value of 29% MC was found in a top plate.

In a separate paper, Tsongas (1991) compared the houses in 

this study that had exterior insulating sheathing with those 

that did not. Either foil-faced polyisocyanurate, extruded 

polystyrene, or expanded polystyrene insulation was present 

13MC readings above 30% by electrical resistance are approximate (see 

Section 1.4).
 25

Review of In-Service Moisture and Temperature Conditions in Wood-Frame Buildings

10

Table 17—Reported moisture contents of wall members during winter 

in Portland, Oregon, houses (Tsongas 1980) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximum

Bottom plate surface 173 11.7 8 20 

Bottom plate interior 173 12.6 7 20 

Top plate surface 110 10.9 7 15 

Top plate interior 104 11.7 8 16 

Stud surface, bottom 176 11.7 6 19 

Stud interior, bottom 175 12.4 7 19 

Stud surface, top 178 10.8 6 17 

Stud interior, top 176 11.6 7 17 

Sheathing surface, bottom 140 12.2 6 18 

Sheathing interior, bottom 5 13 11 16 

Sheathing surface, top 144 12.0 6 18 

Sheathing interior, top 5 14 11 15 

Table 18—Reported moisture contents of wall members 

during winter in coastal Washington houses (Tsongas 1990) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Bottom plate 31 18.9 10 28 

Top plate 15 19.6 12 29 

Left stud 48 18.4 10 ~40 

Right stud 48 18.9 12 34 

Sheathing 41 20.6 9 32 

a

 MC readings above 30% by electrical resistance are approximate 

 (see Section 1.4). 

Table 19—Reported moisture contents of wall members 

during winter in Seattle–Olympia houses (Tsongas 1990) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Bottom plate 90 15.5 10 26 

Top plate 48 14.1 8 21 

Left stud 150 14.7 10 26 

Right stud 150 14.8 9 25 

Sheathing 101 18.1 9 ~50 

a

 MC readings above 30% by electrical resistance are approximate (see 

Section 1.4). 

10

Table 17—Reported moisture contents of wall members during winter 

in Portland, Oregon, houses (Tsongas 1980) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximum

Bottom plate surface 173 11.7 8 20 

Bottom plate interior 173 12.6 7 20 

Top plate surface 110 10.9 7 15 

Top plate interior 104 11.7 8 16 

Stud surface, bottom 176 11.7 6 19 

Stud interior, bottom 175 12.4 7 19 

Stud surface, top 178 10.8 6 17 

Stud interior, top 176 11.6 7 17 

Sheathing surface, bottom 140 12.2 6 18 

Sheathing interior, bottom 5 13 11 16 

Sheathing surface, top 144 12.0 6 18 

Sheathing interior, top 5 14 11 15 

Table 18—Reported moisture contents of wall members 

during winter in coastal Washington houses (Tsongas 1990) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Bottom plate 31 18.9 10 28 

Top plate 15 19.6 12 29 

Left stud 48 18.4 10 ~40 

Right stud 48 18.9 12 34 

Sheathing 41 20.6 9 32 

a

 MC readings above 30% by electrical resistance are approximate 

 (see Section 1.4). 

Table 19—Reported moisture contents of wall members 

during winter in Seattle–Olympia houses (Tsongas 1990) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Bottom plate 90 15.5 10 26 

Top plate 48 14.1 8 21 

Left stud 150 14.7 10 26 

Right stud 150 14.8 9 25 

Sheathing 101 18.1 9 ~50 

a

 MC readings above 30% by electrical resistance are approximate (see 

Section 1.4). 
in 15 of the 50 houses in the Seattle–Olympia area, none of 

16 houses in coastal Washington, and 10 of the 20 houses in 

Montana (see Section 3.4). The presence of exterior insulating sheathing was found to greatly reduce wall moisture levels. Furthermore, a higher R-value (greater thickness) of the 

exterior insulating sheathing correlated with a lower moisture level. Two possible explanations were offered. First, 

the exterior insulating sheathing keeps the wall cavity wood 

members warmer, reducing the accumulation of moisture 

within the cavity. Second, the insulating sheathing acts as a 

break between the siding and the wall cavity, preventing the 

transfer of moisture from wet siding into the wall cavity. A 

disconcerting finding was that increased wall cavity insulation (not including exterior or interior insulating sheathing) 

correlated with increased wall moisture levels. It was argued 

that increasing the amount of wall cavity insulation has the 

effect of lowering the temperature in the outer layers of the 

wall cavity.

During the 1990s in Vancouver, British Columbia, an inordinate number of major decay problems were discovered 

in wood-frame condominiums due to rainwater intrusion 

(Kadulski 1998, Morrison Hershfield Ltd. 1998). Best 

(2000) reported that an estimated 50–90% of the buildings 

constructed in Vancouver since 1985 were experiencing 

moisture problems. Many buildings developed extensive rot 

in the wood framing members and sheathing after less than 

8 years. Of all the problems reported, 25% were related to 

windows themselves and their interface with the walls; 25% 

had to do with poor flashing of horizontal surfaces or the 

interfaces between horizontal and vertical surfaces such as 

guard-rail–wall interfaces; 17% were associated with decks, 

balconies, walkways, and their interfaces with walls; and 

the remaining 33% were spread between roof–wall flashing, 

poor gutter–downspout installation and maintenance, poor 

vents, and other problems. An important finding was that the 

proportion of walls with problems dropped with increasing 

width of the roof overhang. Finally, walls clad with stucco 

had a proportionally greater incidence of problems than 

walls with other types of cladding.

Lawton (1999a,b) described the main factors relating to the 

Vancouver crisis. As mentioned above, smaller overhangs 

on three- and four-story condominiums resulted in greater 

exposure to rain than “traditional” residential construction. 

A second significant factor was the use of complicated features that were more difficult to construct in a manner that 

did not leak. Ninety percent of the problems were associated with joints and interface details at decks, balconies, 

stairs, handrails, windows, and other penetrations. Windows 

typically lacked a projecting sill with drip edge and, when 

exposed to high volumes of water, leaked into the walls. 

The third major factor was poor quality in water protection 

detailing (see also Dell and Liaw 1998), which resulted 

from a combination of inadequate design detailing, poor 

construction, and inadequate site supervision and inspection. 

Additional factors were changes in stucco construction that 

made the cladding less forgiving: (1) the lack of a drainage 

space between the stucco and sheathing paper, resulting 

from changes in the metal lath and (2) the reduction in drying potential due to acrylic finishes. While the magnitude of 

these changes is debated (other cladding systems had water 

intrusion problems as well), they likely compounded the 

increased exposure to wetting.

A similar crisis, though less severe, has plagued wood-frame 

buildings in Seattle, Washington (Best 2000, Desjarlais et 

al. 2001). An estimated 20% of the multi-story, multi-family residential structures built in Seattle between 1984 and 

1998 had wall moisture damage. Sources of water intrusion 

were similar to those described for Vancouver. The extent 

of moisture damage was less severe in Seattle because of 

several factors:

•	 Seattle gets less rain (38–42 in. (0.97–1.1 m) per year) 

than Vancouver (40–90 in. (1.0–2.3 m) per year).

•	 Size of roof overhangs tended to be larger in Seattle, 

helping to shield the walls from wind-driven rain.

•	 Rate of new construction in Seattle was not as great as 

in Vancouver, and Seattle did not experience the same 

shortage of skilled labor and site inspectors as in Vancouver.

•	 The building industry in Seattle was more regulated 

than in Vancouver (builders were required to register 

with the State and carry a certain level of liability 

insurance).

Moisture content measurements have not been reported for 

either Vancouver or Seattle.

Murray and Tichy (2006) recently reported initial results 

from a test facility with 12 south-facing wall sections in 

Puyallup, Washington (Seattle metropolitan area). The 

variables of interest were as follows:

•	 Cladding type—cement stucco or fibercement lap 

siding

•	 Cladding ventilation—none, vented (3/4-in. (19-mm) 

cavity between sheathing and cladding open at the bottom only), or ventilated (3/4-in. (19-mm) cavity open at 

both top and bottom)

•	 Structural sheathing—7/16-in. (11-mm) oriented strandboard (OSB) or 15/32-in. (12-mm) plywood

•	 Framing and cavity insulation—2 by 6 studs with R-21 

glass fiber batt insulation, 2 by 4 studs with R-13 batts 

and R-5 rigid foam sheathing over the structural sheathing, or 2 by 4 studs with R-11 batts

•	 Interior vapor retarder—polyethylene, “smart” vapor 

retarder (synthetic polymer sheet with RH-dependent 

permeance), kraft paper facing, or paint

All wall sections had two layers of 60-minute building paper 

over the exterior sheathing. The interior temperature and 

relative humidity were maintained at 69°F (21°C) and 

26
50–55% RH, respectively. Data were reported from the 

period between October 1, 2003, and September 13, 2004. 

Wood moisture contents were measured using electrical resistance at unspecified locations.

Several types of unvented stucco-clad wall sections are 

discussed first. A wall with construction details common 

in Seattle prior to 1985 featured 2 by 4 framing, plywood 

sheathing, and R-11 batt insulation with kraft paper facing. For comparison, a more contemporary wall had 2 by 6 

framing, OSB sheathing, unfaced R-21 batt insulation, and 

a polyethylene vapor retarder. Both wall sections were without rigid foam sheathing. The moisture performance was 

similar: MC values were below 19% except for a few hours 

out of the entire test period. The combination of R-5 rigid 

foam sheathing over OSB, 2 by 4 framing with R-13 batt 

insulation, and the “smart” vapor retarder resulted in moisture contents averaging between 8% and 10.5%, and never 

exceeding 14%. A wall section identical to the one described 

above with 2 by 6 construction but with the “smart” vapor 

retarder instead of polyethylene had peak moisture contents 

approaching 20% (the location was unspecified but was 

likely in the OSB sheathing).

The effect of a cavity between the stucco cladding and 

sheathing is discussed next. The wall sections were identical to the one described above with 2 by 6 framing and 

polyethylene vapor retarder aside from the 3/4-in. (19-mm) 

cavity. The fully ventilated stucco cladding (open at the top 

and bottom) performed better than the vented (open only 

at the bottom) and non-vented claddings. An air pressure 

difference of 5 Pa was measured between the bottom and 

top of the ventilated wall. The vented stucco was similar in 

moisture performance to the non-vented stucco. Although 

specific MC values were not given, it can be deduced from 

the values above that they were below 19%.

Walls with fibercement lap siding were all fairly dry, and the 

vented and ventilated walls were slightly dryer than the nonvented. In general, fibercement lap siding performed better 

than stucco, though the fully ventilated stucco-clad wall 

performed as well as those with fibercement lap siding. The 

wall with no vapor retarder had moisture content readings 

over 30% for several months in winter. The RH remained 

near 100%, and mold growth was observed on the OSB 

sheathing; however, this wall also dried quickly in spring. 

The “smart” vapor retarder allowed more moisture into wall 

cavities during winter than did polyethylene but also dried 

more quickly in spring. On stucco-clad walls, plywood allowed more moisture into the cavity than did OSB, but no 

differences between plywood and OSB were found for walls 

with fibercement lap siding.

3.4 Cold Climate

Teesdale (1959) used a test house near Madison, Wisconsin, 

to study moisture accumulation in wall sheathing. Three different wall panels were constructed with 2 by 4 studs, gypsum lath and plaster, wood sheathing (type not specified), 

tarred felt building paper, and painted wood bevel siding. 

Three stud cavities were uninsulated, three had loose-fill 

insulation with no vapor retarder, and three had loose-fill insulation with asphalt-coated kraft paper as a vapor retarder. 

The interior environment was maintained at ~40% RH and 

~70°F (21°C).

Moisture content was measured gravimetrically in removable sections of the wood sheathing at the top and bottom of 

each wall for an 8-month period from October 1938 to early 

June 1939. The sheathing had previously been conditioned 

to 6% MC. On the uninsulated cavities, the sheathing MC 

peaked at 24% (bottom) and 16% (top) in January and fell 

to 13% in June at both locations. On the insulated cavities 

without a vapor retarder, the sheathing rose to 47% MC 

(top) and 38% (bottom) in March, dropping down to 18% 

(top) and 15% (bottom) in June. Finally, on the insulated 

cavities with a vapor retarder, the sheathing MCs at the top 

and bottom were 18% and 13% in March and 13% and 11% 

in June, respectively.

Moses and Scheffer (1962) reported moisture content measurements in houses with slab foundations in Chicago, Illinois, and Madison, Wisconsin, during the month of January. 

In all cases (only 6 houses), the exterior wall bottom plates 

had MC values below 20%.

Duff (1968) measured moisture contents in a test house near 

Madison, Wisconsin, during two consecutive winters. Removable 8- by 8-ft (2.4- by 2.4-m) wall sections were constructed with 2 by 4 studs, 1/2-in. (13-mm) gypsum board, 

fiberglass insulation with kraft paper facing, 3/8-in. (9-mm) 

exterior plywood sheathing, and various types of siding. 

The variable of interest in this study was the installation of 

a 6-mil (0.15-mm) polyethylene vapor retarder. In one section, the polyethylene was installed over the inside surface 

(between gypsum board and kraft paper); a second section 

had no polyethylene with the kraft paper removed; and a 

third section had the polyethylene and kraft paper punctured 

and four 1/4-in. (6-mm) holes drilled in the gypsum board to 

simulate the effect of an electrical outlet. Prior to the second 

winter, these holes were plugged. The interior environment 

was maintained at 30% ± 5% RH and ~72°F (22°C).

Moisture content and temperature were monitored at 2-day 

intervals with wood moisture probes (Duff 1966) and thermocouples in wall framing and sheathing. The sensors were 

positioned at the bottom, middle, and top of each section, 

and those in the studs were placed 3/8 in. (9 mm) from the 

inside and outside. In general, moisture content increased 

with increasing height in any given section. Duff attributed 

this effect to convective airflow within the wall cavity: 

warm air rises and absorbs moisture along the inside of 

the wall cavity and then deposits this moisture on the cold 

sheathing surface, where it cools and flows downward. The 

moisture content values reported below are from the probes 

at the top of each section.

 27
During the first winter, the sheathing moisture contents were 

12–13% in the section with the “good” vapor retarder, as 

high as 24% in the section with the broken vapor retarder, 

and exceeding 30% (the maximum was estimated to be 

nearly 60%14) in the section without a vapor retarder. During the second winter, the sheathing MC was about 14–15% 

in the section with the “good” vapor retarder. The warm 

side of the stud remained constant at 7–8% MC in winter 

and rose to 11% in spring. The cold side of the stud stayed 

at 13–14% MC in winter and at 13% in spring. In the section with the broken vapor retarder with plugged holes, the 

sheathing stayed constant at 15–16% MC. In the section 

with no vapor retarder, the sheathing rose to a maximum of 

~40% MC14; the warm side of the stud stayed constant at 

7–9% in winter, rising to 10% in spring; and the cold side of 

the stud rose to a maximum of 17% in winter (5 weeks after 

the ~40% MC peak in the sheathing), dropping to 10% in 

spring.

Wang (1981) carried out a series of wood-frame house 

inspections in various locations in the United States and 

Canada, most of which were in the cold climatic region. 

Locations included Marion, Illinois; Boston, Massachusetts; Midland, Michigan; St. Paul, Minnesota; Charlotte, 

North Carolina; Portsmouth, New Hampshire; Rochester, 

New York; Cleveland and Columbus, Ohio; and Madison, 

Wisconsin. Moisture contents were measured with a resistance-type meter in wall framing members during winter 

months between 1974 and 1979. More than 70 houses were 

inspected, covering a variety of construction types and 

ranging in age from 0 to 10 years. Moisture contents were 

mostly between 7% and 12%; all were below 15%. No appreciable difference in moisture content was found between 

walls sheathed with extruded polystyrene insulation, wood 

fiberboard, and plywood. No signs of condensation, water 

staining, wood decay, or fungal growth were observed.

Marshall Macklin Monaghan Ltd. (1983) conducted a survey of 201 government-financed housing units with reported 

moisture damage in Canada, including single-family houses, 

semi-detached row houses, and multi-family dwellings. 

The sample of housing units was selected according the 

geographic distribution of reported problems. The majority was located on the Atlantic coast (Newfoundland, New 

Brunswick, and Nova Scotia) and the Pacific coast (British 

Columbia). The average age of the buildings was less than 

5 years. Single point-in-time moisture content measurements were made in wall framing and sheathing (the time 

of year was not specified). Specific wood moisture contents 

were not given; instead, proportions of the readings below 

or above specified moisture levels were reported. These are 

shown in Table 20. High moisture content readings in first 

floor framing were correlated with high indoor relative humidity. The higher incidence of moisture accumulation in 

second floor framing was thought to be due to air 

14MC readings above 30% by electrical resistance are approximate (see 

Section 1.4).

exfiltration. Moisture accumulation in wall sheathing was 

correlated with exposure to wind-driven rain and exposure 

to wind (which can induce air leakage).

Sherwood (1983, 1987) studied moisture conditions in 

highly insulated walls with a test building near Madison, 

Wisconsin. The structure was identical to that described 

above for Gulfport, Mississippi (Sherwood 1985), having 

eight rooms with north- and south-facing walls containing 

test panels of various construction. This study also had a 

2-year duration, with no penetrations in the gypsum board 

for the first year and electrical outlets in each panel for the 

second year.

During the first summer, moisture contents were typically 

in the 8–12% range. In winter, condensation was found in 

walls with kraft paper vapor retarders (stapled to the sides 

of the studs) in conjunction with fiberboard or polystyrene 

sheathing but not plywood sheathing. No condensation was 

found in walls with polyethylene vapor retarders, regardless 

of the type of sheathing. In the walls in which condensation 

was found, moisture contents exceeded 20% for no more 

than 6 weeks, and the location of increased moisture content was generally at the sheathing–siding interface or the 

sheathing–insulation interface. The framing MC never exceeded 12%. Winter temperatures recorded at the sheathing–

insulation interface generally were between 20°F (–7°C) 

and 50°F (10°C) depending on the outside temperature and 

the type of construction.

When the walls were penetrated with electrical outlets (second year), moisture contents at the sheathing–insulation 

interface generally rose above 16% in winter. Localized 

condensation was found on the sheathing surface behind the 

electrical outlet for panels with R-13 or R-19 batt insulation 

during January through March, but not in panels with lowdensity R-11 blanket insulation. By early April, all moisture 

content readings were below 11%. Again, the framing moisture content did not increase significantly at any time.

Tsongas (1984) surveyed 103 houses in Spokane, Washington, to determine whether houses retrofitted with wall 

insulation without a vapor retarder developed moisture 

problems. Of these 103 houses, 24 were uninsulated (control group), 61 had cellulose insulation, 11 had fiberglass 

insulation, and 7 had rock wool insulation. The majority of 

the houses had wood siding, the rest had metal or vinyl siding. Almost all had #15 felt or building paper as the weather 

barrier. Lapboard sheathing was the most common, though 

a few houses had plywood or wood fiberboard sheathing. 

The majority of houses were electrically heated (most with 

baseboards, some with forced air). The average number of 

occupants was three persons, and the average indoor relative 

humidity was 47% RH.

In a preliminary survey, three locations for opening the 

walls of each house were determined. The objective was to 

find regions with the highest moisture levels that were not 

28
affected by bulk water leakage. Typical locations were near 

bathrooms or kitchens, near electrical outlets, away from 

heat sources, and in areas showing evidence of possible 

moisture damage (blistering paint, warped siding, discoloration, mold or mildew, and termite or dry rot damage). 

All locations were selected to be near the floor line and 

were opened from the outside. The moisture content of the 

sheathing and framing members were measured with a resistance-type meter both at the surface and the interior. Data 

were collected between December 1982 and February 1983.

Table 21 lists the average, minimum, and maximum moisture contents for each wood member at the surface and interior. The average MC values range from about 10% to 13%, 

with an average of 11.3% for all houses. No significant 

difference was found between insulated and uninsulated 

houses, or between houses with different types of insulation. 

Sheathing moisture content values were consistently higher 

than those for framing members. Readings above 20% MC 

were attributed mainly to wetting by water dripping from 

the roof and splashing back against the house, wood members in contact with the ground, and bulk water leaks.

Platts (1988) summarized the findings of a survey of older 

houses with retrofit-insulated walls in Canada in which 

1,900 houses were examined during October, when the walls 

should not have been affected by the usual pattern of winter 

moisture accumulation. Moisture contents were measured 

in the bottom plates in 10 locations in each house. Excessive moisture contents (near or above 30% MC) were found 

in the walls of 1–3% of the houses in the inland provinces 

but as many as 15% in Newfoundland and 12% in British 

Columbia. The higher incidence of moisture problems in 

coastal provinces is in agreement with the findings of Marshall Macklin Monaghan Ltd. (1983) discussed above. Most 

of the moisture problems were related to rainwater intrusion 

or excessive indoor humidity in winter. In the coastal provinces, a significant source of moisture accumulation in walls 

was wind-induced exfiltration of humid indoor air through 

leaks on the lee side of the house.

Tsongas (1990) conducted a field study of indoor moisture 

problems in new houses in the northwestern United States 

(as described in Section 3.3). Inspections and moisture 

measurements were made at 20 houses in several cities in 

Montana during winter 1987. Table 22 lists the average, 

minimum, and maximum moisture contents for each wood 

member. The sheathing, with an average of 15.5% MC, 

was significantly wetter than the various framing members, 

which all averaged 12–13% MC. However, when the two 

wettest houses were revisited during August 1988, the highest moisture content measured in the walls (sheathing and 

framing) was 7.5% MC, which indicated that walls dried out 

satisfactorily during warm weather.

Tsongas and Nelson (1991) reported a separate investigation 

of one of the houses in Helena, Montana, from the study 

mentioned above. Following the initial visit on February 18, 

11

Table 20—Proportion of moisture content measurements in wall 

members in Canadian houses below 15% MC and above 22% MC 

(Marshall Macklin Monaghan 1983) 

Percentage of MC readings 

Wood member Measurements MC < 15% MC > 22% 

Stud, inside face, 1st floor 127 75 3 

Stud, inside face, 2nd floor 58 62 14 

Sheathing 18 61 11 

Table 21—Reported moisture contents of wall members during 

winter in Spokane, Washington, houses (Tsongas 1984) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximum 

Bottom plate, surface 304 10.9 6.5 26 

Bottom plate, interior 304 11.0 6.5 26 

Right stud, surf 305 10.6 6.5 22 

Right stud, int 305 10.8 6.5 22 

Left stud, surf 304 10.6 6.5 24 

Left stud, int 303 10.9 6.5 24 

Sheathing, surfa 283 12.7 6.5 >30 

Sheathing, int 283 13.2 6.5 >30 

a

 Warm side. 

Table 22—Reported moisture contents of wall members 

during winter in Montana houses (Tsongas 1990) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Bottom plate 38 12.0 8.0 19 

Top plate 17 12.6 6.0 24 

Left stud 57 12.0 6.0 22 

Right stud 57 12.7 6.0 34 

Sheathing 30 15.5 8.0 ~50 

a

 MC readings above 30% by electrical resistance are approximate 

 (see Section 1.4). 

11

Table 20—Proportion of moisture content measurements in wall 

members in Canadian houses below 15% MC and above 22% MC 

(Marshall Macklin Monaghan 1983) 

Percentage of MC readings 

Wood member Measurements MC < 15% MC > 22% 

Stud, inside face, 1st floor 127 75 3 

Stud, inside face, 2nd floor 58 62 14 

Sheathing 18 61 11 

Table 21—Reported moisture contents of wall members during 

winter in Spokane, Washington, houses (Tsongas 1984) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximum 

Bottom plate, surface 304 10.9 6.5 26 

Bottom plate, interior 304 11.0 6.5 26 

Right stud, surf 305 10.6 6.5 22 

Right stud, int 305 10.8 6.5 22 

Left stud, surf 304 10.6 6.5 24 

Left stud, int 303 10.9 6.5 24 

Sheathing, surfa 283 12.7 6.5 >30 

Sheathing, int 283 13.2 6.5 >30 

a

 Warm side. 

Table 22—Reported moisture contents of wall members 

during winter in Montana houses (Tsongas 1990) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Bottom plate 38 12.0 8.0 19 

Top plate 17 12.6 6.0 24 

Left stud 57 12.0 6.0 22 

Right stud 57 12.7 6.0 34 

Sheathing 30 15.5 8.0 ~50 

a

 MC readings above 30% by electrical resistance are approximate 

 (see Section 1.4). 

 29
1987, a second visit was made on February 20, and moisture 

contents were measured at 120 locations in the exterior wall 

sheathing (at the top and bottom of every cavity). Three locations exceeded 40% MC, four locations were between 30 

and 40% MC, and 11 locations were between 20 and 30% 

MC.15 It was discovered that the interior relative humidity 

was quite high: the air-to-air heat exchanger was set to run 

when the humidistat read 40% RH; however, the humidistat 

was not calibrated and the unit turned on at 55% RH. On 

a later visit in mid-April, depressurization measurements 

showed that the house was fairly airtight, with 1.2 air changes per hour at 50 Pa. Infrared thermography was used during 

house pressurization and depressurization to locate air leakage sites and paths. Numerous leaks were found both inside 

and outside, resulting from improper sealing or poor quality 

of work. The main leakage sites were at penetrations for the 

air-to-air heat exchanger ducts, electrical service box, windows, exterior lights, and ceiling penetrations for vent stacks 

and electrical wiring. A correlation was established between 

the locations of air leaks and the locations of high sheathing 

moisture content, implying that the source of moisture in the 

wall sheathing was exfiltration of humid indoor air.

TenWolde et al. (1995) investigated the relationship between 

air leakage and moisture conditions in the walls of a test 

building near Madison, Wisconsin. Of particular interest 

was the construction practice of ventilating the stud spaces, 

which was common in manufactured houses. The 50- by 8-ft 

(15- by 2.4-m) test building had two 20-ft (6-m) sections 

(east and west rooms) and one central 10-ft (3-m) section. 

The east and west rooms were maintained at 35% RH and 

45% RH, respectively, and 70°F (21°C) during the winter, 

but temperature and humidity were not controlled during 

spring and fall. East and west rooms contained replicates 

of 10 different wall modules. All walls were framed with 

2 by 6 studs, 16-in. (400-mm) on center, and insulated with 

R-19 fiberglass with kraft paper facing. The variables under investigation were type of siding, presence and type of 

sheathing, presence of wall cavity (stud-space) ventilation, 

presence of an electrical outlet, and presence of a gasket 

15MC readings above 30% by electrical resistance are approximate (see 

Section 1.4).

around the outlet cover plate. Because the walls were intended to be representative of modestly priced manufactured 

housing, 6 of the 10 module designs used combination 

sheathing–siding. The other four module designs had separate sheathing and siding; two had steel siding, and the other 

two had waferboard siding. In these four modules a ventilation space between the sheathing and siding was formed 

intentionally with spacers (with waferboard siding) or provided by the siding corrugations (with steel siding).

Relative humidity and temperature were measured at the 

inside surface of the exterior sheathing with wood electrical 

resistance probes (Duff 1966) and thermocouples. The average RH from November through February varied from less 

than 40% (~8% MC) to as high as 90% (~20% MC) for the 

different wall modules. The average for all modules in the 

low-humidity room was 64% RH, while that for the highhumidity room was 75% RH. Values of 100% RH (indicating condensation) were recorded for short periods in some 

modules and for extended periods in several others. Visual 

inspection after completion of the study found water staining or mold in the modules that had sustained periods of 

condensation. All these modules were in the high-humidity 

room. The only walls in the high-humidity room that consistently remained free of condensation were those constructed 

with separate sheathing and with intentionally-installed 

spacers between the sheathing and siding; these walls did 

not have ventilated stud spaces.

By comparing the different types of construction in the wall 

modules, several conclusions were reached. In walls without 

electrical outlets, stud-space ventilation did promote drying. In walls with outlets in the low-humidity room, cavity 

ventilation either promoted drying or had no effect. In walls 

with outlets in the high-humidity room, cavity ventilation 

had no apparent influence on wall moisture conditions. The 

presence of mold and evidence of sustained condensation in 

at least two wall modules with stud-space ventilation demonstrated that ventilation of the insulated cavity was not a 

dependable moisture-control strategy. Air pressure differential measurements indicated that the prevailing direction of 

air leakage across the gypsum board was infiltrative. If the 

prevailing direction had been exfiltrative, the walls would 

likely have been wetter.

30

11

Sheathing, surf283 12.7 6.5 >30 

Sheathing, int 283 13.2 6.5 >30 

a

 Warm side. 

Table 22—Reported moisture contents of wall members 

during winter in Montana houses (Tsongas 1990) 

Moisture content (%) 

Wood member Measurements Average Minimum Maximuma

Bottom plate 38 12.0 8.0 19 

Top plate 17 12.6 6.0 24 

Left stud 57 12.0 6.0 22 

Right stud 57 12.7 6.0 34 

Sheathing 30 15.5 8.0 ~50 

a

 MC readings above 30% by electrical resistance are approximate 

 (see Section 1.4). 
Investigations of decayed plywood sheathing in manufactured houses in Wisconsin have been reported by several 

authors (TenWolde 1988, Merrill and TenWolde 1989, 

Tsongas and Olson 1995, TenWolde 2000). Typical wall 

construction included 2 by 4 framing with 1/2-in. (13-mm) 

gypsum board, 1/2-in. (13-mm) exterior plywood sheathing, 

asphalt-coated building paper, and hardboard lap siding. The 

wall cavity was filled with mineral fiber insulation that had 

a kraft paper exterior facing and an interior facing of kraft 

paper, polyethylene, or aluminum foil. Merrill and TenWolde (1989) found through site inspections that decay occurred in fewer than half the houses and was located mainly 

in the plywood sheathing, with very few instances of damage to wood framing. They observed a correlation between 

incidence of moisture problems and occupant density. Airtightness measurements indicated that ventilation rates were 

very low. This resulted in high indoor humidities that led to 

condensation in the walls during winter.

Moisture contents were measured in 17 of these manufactured houses by Tsongas and Olson (1995). A surface capacitance-type meter was used to scan for MC values up to 

20%; a resistance-type meter with a reported range between 

6% and 60% MC was used for higher moisture contents.15

Measurements were made at various times, including 

winter, early spring, and summer. In many cases, the 

plywood sheathing was found to be very wet, with MC 

values exceeding 20%, especially at the upper portions of 

gable end walls and non-gable end walls outside bedroom 

closets. Moisture contents greater than 60%16 were reported 

during the summer, when the walls have typically dried out 

in this climate. On many houses the plywood sheathing was 

decayed, delaminated, and moldy. In only a few instances 

were the wood framing members decayed. The authors 

argued that the exterior building paper acted as a vapor 

retarder that trapped moisture in the plywood. However, 

TenWolde (2000) analyzed the design of the houses with a 

computer model and found that only a combination of indoor humidity control, a permeable exterior weather barrier, 

and elimination of indoor air leakage through the wall could 

eliminate all potential for decay and mold growth.

Rose and McCaa (1998) measured moisture and temperature 

conditions in a test house in Champaign, Illinois, during 

three consecutive winters. The setup was designed to focus 

on water vapor diffusion and to exclude air leakage through 

the walls. Removable 8- by 8-ft (2.4- by 2.4-m) wall sections were constructed with 2 by 4 studs spaced 16 in. 

(400 mm) on center, 1/2-in. (13-mm) gypsum board with 

two coats of latex paint, 7/16-in. (11-mm) OSB sheathing, 

and vinyl siding. The variables of interest in this study were 

the following:

•	 Insulation material—fiberglass or cellulose

•	 Vapor retarder—none, polyethylene, or kraft-paper 

facing

16MC readings above 30% by electrical resistance are approximate (see 

Section 1.4).

•	 Encapsulation materials

•	 Method of attaching the kraft paper facing—unstapled, 

face-stapled, or inset-stapled

•	 Location within the wall cavity

A total of 16 different wall samples were monitored. The 

interior environment was maintained at 70°F (21°C) with 

50–55% RH during the first winter and 40% RH during the 

second and third winters. Heating was by electrical resistance with fans for circulation; there was no mechanical 

pressurization or depressurization of the building. The test 

panels were on the south side of the building (the north side 

could not be used); since the more severe moisture conditions occur on shaded walls, a nylon tarpaulin was installed 

from the fascia to the ground as a sun shield.

Moisture content and temperature were monitored hourly 

with wood moisture probes (Duff 1966) and thermocouples. 

The sensors were positioned at the center and edge of the 

framing cavities, 2 ft (0.6 m) below the top plate and 

2 ft (0.6 m) above the bottom plate. In addition, 2-in.- 

(50-mm-) diameter holes were cut in the OSB sheathing 

and plugs were fit snugly into the holes. The plugs were removed periodically for gravimetric moisture content determination. The correlation between the electrical resistance 

and gravimetric moisture content measurements was not 

very strong. 

Although the resistance measurements did not give consistent outputs from year to year, they were reliable for sideby-side comparison of different insulation–vapor retarder 

installations.

In general, moisture content and temperature increased with 

increasing height in any given cavity. During the first winter 

with the interior RH at 50–55%, the sheathing MC varied 

from 8–12% for cavities with vapor retarders to 24–35% for 

cavities without vapor retarders. The plugs without vapor 

protection actually delaminated from excess moisture (they 

were replaced the following winter). During the second and 

third winters with the interior RH at 40%, all the sheathing 

MCs were between 6% and 12%. Following the study, the 

walls were disassembled and checked for mold growth and 

signs of moisture. The cavities without vapor retarders had 

mold distributed uniformly on the sheathing, as well as corrosion on metal fasteners. The cavities with polyethylene 

showed no mold growth. All the fiberglass-insulated cavities 

with kraft paper facings showed some mold growth on the 

sheathing near the tops of the cavities where the electrical 

cables penetrated the drywall. Although the penetrations 

were caulked, the mold growth pattern indicated there was 

some air leakage.

Straube and Burnett (1998) investigated the role of ventilation in the drying of masonry-clad wood-frame walls using 

a test building in Waterloo, Ontario. Two different types of 

4- by 8-ft (1.2- by 2.4-m) wall sections were constructed. 

Both types had 3.3-in. (85-mm) brick veneer with open head 

joints (24 in. (600 mm) on center) at both top and bottom, a 

 31
1.2-in. (30-mm) air space, 2 by 4 framing (16 in. (400 mm) 

on center) filled with low-density batt insulation, a 6-mil 

(0.15-mm) polyethylene vapor retarder, and painted gypsum 

board. The combination of interior drywall and polyethylene was tested and confirmed to be airtight. The variables 

of interest in this study included permeance of the exterior 

sheathing and whether the air space was vented or unvented.

Wall type A had 2-in. (50-mm) mineral fiberboard insulation over exterior gypsum board sheathing. Wall type B 

was constructed with asphalt-impregnated building paper 

over 1-1/4-in. (32-mm) extruded polystyrene sheathing. 

The water vapor permeance of wall type B was about 2% of 

that of wall type A. One panel of type A was placed on an 

east-facing wall. Four panels of type B were placed on walls 

facing all four cardinal directions. Each panel was instrumented with temperature sensors, relative humidity sensors, 

and moisture pins for electrical resistance measurements of 

wood moisture content near the center of the studs and top 

and bottom plates. The interior environment was maintained 

at 70°F ± 2°F (21°C ± 1°C) and 50% ± 5% RH for a twoyear period. The air space in panel A was vented for the first 

year, and its vents were sealed airtight for the second year.

The moisture content in the framing of the wall panels with 

polystyrene exterior sheathing (low permeance, type B) 

remained at 9–11% MC for the 2-year period. During the 

year that panel A was vented, the framing moisture content 

was in the same range except for a rise to 15% MC for a 

~10-day period in the summer. This increase was attributed 

to moisture being driven from the brick cladding by solar 

heating and permeating the exterior sheathing.17 When the 

vents in panel A were sealed the following summer, the 

framing MC rose to 18–20%. Occasional condensation was 

found on the interior polyethylene vapor retarder in this 

panel. Measurements showed that the water vapor concentration in the air space between the brick veneer and the 

exterior sheathing depended significantly on ventilation: the 

unvented air space had 44% more moisture than did outside 

air, whereas the vented air space had only 11% more moisture than did outside air when averaged over the summer 

months.

In response to reported wall moisture problems in Alberta 

similar to those in British Columbia (see Section 3.3), 

Building Envelope Engineering, Inc. (1999) inspected 

41 houses and 9 multi-unit residential buildings located primarily in Calgary and Edmonton. All buildings were under 

10 years old. The sampling was not random but was biased 

towards houses that had a higher occurrence of moisture 

problems than the general Alberta population. The 

17The study did not discuss the possibility that the brick veneer was wet 

by rain water that ran off the roof and splashed from the ground. The test 

building was constructed with a hip roof, minimal overhang, and no gutters. If splash wetting did occur, the degree of wetting of a particular wall 

section would have depended on its distance from the corner because the 

amount of run-off from a hip roof is greatest at the center of the wall. The 

study did not specify the horizontal locations of wall panels A and B.

buildings were representative of typical materials and methods used in single-family and low-rise multi-family woodframe construction in Alberta. Although wood moisture 

contents were not reported, most moisture-related failures 

were found to stem from inadequacies in either design or 

construction. Rainwater intrusion was a contributing factor 

in 91% of problems, most of which occurred at window and 

door perimeters and decks. Buildings with stucco cladding 

had a higher incidence of rainwater intrusion problems than 

did buildings with vinyl siding. Condensation within wall 

cavities due to indoor relative humidities above 30% during 

winter was a factor in 14% of all problems.

Similarly, Chouinard and Lawton (2001) reported detailed 

investigations of bulk water entry and wood decay in three 

multi-story wood-frame housing complexes built in the 

late 1980s. The first was a condominium in Bedford, Nova 

Scotia. Significant water penetration at windows and warping of cladding at balcony walls were reported a few years 

after construction, and wood decay was found in framing 

members. A few years after remediation, new problems 

arose from water leakage through improperly nailed cedar 

shingles and at window corners. Moisture contents in wood 

sheathing and framing members often exceeded 25%. The 

second case was a housing cooperative in Dartmouth, Nova 

Scotia. Water intrusion problems were discovered 9 years 

after construction and were mainly due to poor detailing 

around balconies and patio doors. Moisture contents in wall 

sheathing and framing and floor members were above 25% 

where wood was rotted or bulk water was present, but in 

areas with no damage, MCs were in the 10–13% range. The 

third case was a townhouse complex in Ottawa, Ontario. 

The partial collapse of a second-floor balcony 12 years after 

construction led to the discovery of extensive decay of framing members in the first- and second-floor exterior walls. 

Water entry was attributed to deficiencies in flashing details 

at windows and doors. Curiously, the tops of windows and 

doors were wrapped with polyethylene, which actually collected water and then redistributed it to the surrounding 

framing members. In all three cases, bulk water intrusion 

was related to inadequate detailing.

Cautley (2004) compared moisture performance of wall sections in an unoccupied house in Madison, Wisconsin. Walls 

were constructed with 2 by 6 framing, 16 in. (400 mm) on 

center, fiberglass insulation, interior gypsum board, an exterior housewrap, and vinyl siding. The variables of interest 

were the following:

•	 Wall orientation—west- or north-facing

•	 Exterior sheathing—1/2-in. (13-mm) OSB, 1/2-in. 

(13-mm) extruded polystyrene (XPS), 1-in. (25-mm) 

XPS, or combination of 1/2-in. (13-mm) OSB and 

1/2-in. (13-mm) XPS

•	 Interior vapor retarder—none or polyethylene

•	 Airtightness of the assembly—continuous bead of 

sealant applied around perimeter of insulated cavity or 

32
air leakage paths intentionally left to both interior and 

exterior

Wood moisture content was monitored for 12 different stud 

cavities in framing members and OSB sheathing by electrical resistance measurements. Stainless steel pins were 

installed in the bottom plates, top plates, and studs at midheight at locations 1/2 in. (13 mm) from the gypsum board 

and 1/2 in. (13 mm) from the sheathing. Pins were installed 

in OSB sheathing near the bottom plate, top plate, and at 

mid-height. Temperature was measured with thermocouples 

at the same locations. Data were acquired over a 10-month 

period (November 2000 through August 2001) at intervals 

of 15 min. The interior of the house was initially humidified to ~50% RH during November. Subsequently, the indoor RH was gradually decreased to a minimum of ~20% 

in March, after which it was increased again. The average 

indoor temperature and relative humidity over this November–March period were 71°F (22°C) and 29% RH. Temperature and humidity were uncontrolled during the summer.

In walls that did not incorporate a polyethylene vapor retarder, a trend was observed of high moisture content at 

or near the sheathing during winter, followed by drying in 

spring and summer. A similar trend was apparently also 

observed in walls with a polyethylene vapor retarder, provided they were not airtight. Average winter framing moisture contents near the sheathing ranged from 8% to ~22%, 

the driest winter conditions being associated with airtight 

walls with a polyethylene vapor retarder. In all walls except 

airtight walls with a polyethylene vapor retarder, a higher 

moisture content was observed during winter in the bottom 

plates near the exterior (>25% MC) than at any other location in the framing. Moisture levels in OSB sheathing were 

similar to those in outer portions of the framing, but there 

was no discernable spatial variation in sheathing moisture 

content. It was also noted that in most cases, the temperature was slightly higher at the bottom than at mid-height or 

at the top. The high moisture content in the outer bottom 

plates was thus attributed to condensation dripping from the 

sheathing (whether OSB or XPS).

No benefit of insulating sheathing (XPS) was observed on 

wintertime moisture conditions at the most critical location in the framing (the bottom plate). However, more rapid 

springtime dissipation of moisture was observed in airtight 

walls sheathed with XPS than in comparable walls sheathed 

with OSB. In a similar manner, the influence of air leakage on wall moisture conditions was not straightforward; 

in some cases it apparently exacerbated winter moisture 

accumulation, but it also evidently accelerated springtime 

moisture dissipation.

During spring and summer, a few episodes of rapid, localized wetting (>30% MC) followed by several days of drying 

were observed in framing below windows near the sheathing 

(different locations than the 12 cavities discussed above). 

Each wetting event was preceded by rainfall. Window type 

and installation method were not reported.

Decay in wall framing and sheathing was reported for 

houses built in Woodbury, Minnesota, during the 1990s 

(Holladay 2006a). Most of the problems occurred in 

wood-frame houses with stucco cladding, OSB sheathing, 

fiberglass batt insulation, and interior polyethylene vapor 

retarder. Although water damage occurred in houses with 

claddings other than stucco, the percentage of houses needing repair was 51% for stucco and about 3% for other claddings. The extensive decay clearly points to within-wall 

moisture conditions in excess of fiber saturation coinciding 

with temperatures that would support decay.

Holladay (2006a) listed nine factors that have been suggested by experts as contributing to the failures. In the majority of cases, the high moisture levels were attributable to 

bulk water leakage, while in some cases high indoor relative 

humidity during the winter in conjunction with air exfiltration was cited as the source of moisture. According to the 

City of Woodbury (2005), “Window leaks, a lack of kickout 

flashing, improper deck flashing, and grade above the wood 

framing are the primary causes that account for the majority 

of the damage. All other causes are secondary.” In contrast 

to the stated opinion of the City, Bomberg et al. (2005a,b) 

suggested that two additional factors came into play: “The 

stucco cladding, which is a major reservoir of moisture, is 

not properly separated from the wall sheathing to prevent 

transfer of exterior moisture to the inner part of the wall assembly,” and “The exterior wall has limited drying ability 

to the exterior and interior. This fact increases the effect of 

water entrapment and further compounds the effects of the 

first two problems.”

3.5 Summary: Wall Framing and Sheathing

Table 23 summarizes studies that report moisture contents in 

wall framing and sheathing for each climatic region. Again, 

moisture conditions span a very wide range. Tables 24–27 

summarize moisture contents for various members, including bottom plates, top plates, studs, and wall sheathing. 

Several conclusions can be drawn from the literature.

1. High moisture contents in walls may occur in response 

to rainwater intrusion. The moisture content depends on 

the magnitude of the leakage and the rate at which 

drying occurs.

2. High moisture levels may occur in exterior wall sheathing in cold and marine climates during winter. Exfiltration of humid indoor air and diffusion of moisture 

through insulated wall cavities lacking effective air barriers and vapor retarders allow moisture to accumulate 

in the cold sheathing. The severity of this problem correlates with indoor humidity levels maintained during 

the winter.

3. In cold and marine climates, the sheathing usually dries 

during the spring and reaches a minimum during the 

 33
12

Table 23—Summary of studies reporting moisture contents in wall framing and sheathing by climatic region 

Moisture content (%) 

Reference Location

Type of 

study Duration Na

Measurement 

locationb Mean Minimum Maximumc Trends

Hot-humid climate 

Moses & 

Scheffer 

1962

Southeastern 

US

Survey Aug–Oct 78 BP, St — <12 30

Moses & 

Scheffer 

1962

Gulf Coast Survey Oct–Dec 255 BP, St — <12 >30 

Sherwood

1985, 1987 

Gulfport, MS Test structure 2 yr 8 BP, Sh, St, TP — 8 >20 x Higher MC in walls 

penetrated by electrical 

outlets

x Highest summer MC in 

walls with fiberboard 

sheathing

x Higher summer MC in 

walls with wood-based 

sheathing than with 

exterior insulating 

sheathing

TenWolde

and Mei 

1986

Beaumont, TX Test structure Apr–Nov 9 RH sensors at 

interfaces 

between 

materials in 

wall cavity 

— — — x Walls remained dry with 

an exterior vapor 

retarder or without any 

vapor retarder either 

inside or outside 

x Wall with interior vapor 

retarder experienced 

daily cyclical 

condensation

Mei 1988 Beaumont, TX Test structure Jun–Oct 9 RH sensors at 

interfaces 

between 

materials in 

wall cavity 

— — — x Ventilation between 

cladding and sheathing 

reduced moisture flow 

into wall cavity 

x Daily cyclical 

condensation at interior 

vapor retarder in wall 

with brick veneer but not 

plywood siding 

Crandell & 

Kenney 

1996

Wilmington,

NC

Survey Aug 8 Sh 18 ~50 x Bulk water intrusion 

related to inadequate 

detailing 

Mixed-humid climate 

Weber & 

Reichel 

1942

Washington,

DC

Test structure 2 wk 

(winter)

7 Sh — 6 ~60 x Condensation in wall 

cavities without interior 

vapor retarder 

Moses & 

Scheffer 

1962

Baltimore, MD Survey Aug 97 BP, St — <12 30

Moses & 

Scheffer 

1962

Memphis, TN Survey Jan 59 BP, St — <12 30

Duff 1971 Athens, GA Test structure Jun 4 Sh — 10 13 x Sensors at interface 

between insulation and 

interior vapor retarder 

showed daily period of 

high RH 

Duff 1972 Athens, GA Test structure 1 yr 12 Sh, St — 10 25 x Highest MC occurred in 

sheathing during winter 

34
12

Table 23—Summary of studies reporting moisture contents in wall framing and sheathing by climatic region 

Moisture content (%) 

Reference Location

Type of 

study Duration Na

Measurement 

locationb Mean Minimum Maximumc Trends

Hot-humid climate 

Moses & 

Scheffer 

1962

Southeastern 

US

Survey Aug–Oct 78 BP, St — <12 30

Moses & 

Scheffer 

1962

Gulf Coast Survey Oct–Dec 255 BP, St — <12 >30 

Sherwood

1985, 1987 

Gulfport, MS Test structure 2 yr 8 BP, Sh, St, TP — 8 >20 x Higher MC in walls 

penetrated by electrical 

outlets

x Highest summer MC in 

walls with fiberboard 

sheathing

x Higher summer MC in 

walls with wood-based 

sheathing than with 

exterior insulating 

sheathing

TenWolde

and Mei 

1986

Beaumont, TX Test structure Apr–Nov 9 RH sensors at 

interfaces 

between 

materials in 

wall cavity 

— — — x Walls remained dry with 

an exterior vapor 

retarder or without any 

vapor retarder either 

inside or outside 

x Wall with interior vapor 

retarder experienced 

daily cyclical 

condensation

Mei 1988 Beaumont, TX Test structure Jun–Oct 9 RH sensors at 

interfaces 

between 

materials in 

wall cavity 

— — — x Ventilation between 

cladding and sheathing 

reduced moisture flow 

into wall cavity 

x Daily cyclical 

condensation at interior 

vapor retarder in wall 

with brick veneer but not 

plywood siding 

Crandell & 

Kenney 

1996

Wilmington,

NC

Survey Aug 8 Sh 18 ~50 x Bulk water intrusion 

related to inadequate 

detailing 

Mixed-humid climate 

Weber & 

Reichel 

1942

Washington,

DC

Test structure 2 wk 

(winter)

7 Sh — 6 ~60 x Condensation in wall 

cavities without interior 

vapor retarder 

Moses & 

Scheffer 

1962

Baltimore, MD Survey Aug 97 BP, St — <12 30

Moses & 

Scheffer 

1962

Memphis, TN Survey Jan 59 BP, St — <12 30

Duff 1971 Athens, GA Test structure Jun 4 Sh — 10 13 x Sensors at interface 

between insulation and 

interior vapor retarder 

showed daily period of 

high RH 

Duff 1972 Athens, GA Test structure 1 yr 12 Sh, St — 10 25 x Highest MC occurred in 

sheathing during winter 

13

in wall without vapor 

retarder 

x Daily periodic high RH 

occurred at interface 

between insulation and 

interior vapor retarder 

during summer 

Marine climate 

Moses & 

Scheffer 

1962

Southern CA Survey Mar–Apr 98 BP, St — <12 >30 

Moses & 

Scheffer 

1962

Oregon & 

Washington

Survey Apr–Jun 20 BP — <12 <20 

Tsongas

1980

Portland, OR Survey Feb–Apr 93 BP, Sh, St, TP 11.8 6 20 x MC > 30% due to bulk 

water leakage not 

included 

x No significant difference 

in MC between insulated 

and uninsulated homes 

Tsongas

1990

Washington

Coast

Survey Winter 16 BP, Sh, St, 

TP 

19.2 9 ~40 

Tsongas

1990

SeattleOlympia, WA Survey Winter 50 BP, Sh, St, 

TP 

15.4 8 ~50 

x Correlation between 

high indoor RH and high 

wood MC 

x Sheathing generally 

wetter than framing 

x Lower MC in walls with 

exterior insulating 

sheathing

Murray & 

Tichy 2006 

Puyallup, WA Test structure 1 yr 12 Not specified — 8 >30 x Highest MC in wall 

without vapor retarder 

x Low MC in walls with 

exterior insulating 

sheathing

x Low MC in walls with 

ventilated space between 

cladding and sheathing 

Cold climate 

Teesdale 

1959

Madison, WI Test structure Oct–Jun 3 Sh — 11 47 x MC increased in order of

insulated wall with 

vapor retarder < 

uninsulated wall (no 

vapor retarder) < 

insulated wall with no 

vapor retarder 

Moses & 

Scheffer 

1962

Chicago, IL & 

Madison, WI 

Survey Jan 6 BP — <12 <20 

Duff 1968 Madison, WI Test structure 2 yr 3 Sh, St — 7 ~60 x MC increased in order of

wall with “good” vapor 

retarder < wall with 

punctured vapor retarder 

< wall with no vapor 

retarder 

Wang 1981 Various Survey Winter >70 St — 7 <15 x No appreciable 

differences in MC 

between walls sheathed 

with extruded 

polystyrene, wood 

fiberboard, and plywood

 35
12

Table 23—Summary of studies reporting moisture contents in wall framing and sheathing by climatic region 

Moisture content (%) 

Reference Location

Type of 

study Duration Na

Measurement 

locationb Mean Minimum Maximumc Trends

Hot-humid climate 

Moses & 

Scheffer 

1962

Southeastern 

US

Survey Aug–Oct 78 BP, St — <12 30

Moses & 

Scheffer 

1962

Gulf Coast Survey Oct–Dec 255 BP, St — <12 >30 

Sherwood

1985, 1987 

Gulfport, MS Test structure 2 yr 8 BP, Sh, St, TP — 8 >20 x Higher MC in walls 

penetrated by electrical 

outlets

x Highest summer MC in 

walls with fiberboard 

sheathing

x Higher summer MC in 

walls with wood-based 

sheathing than with 

exterior insulating 

sheathing

TenWolde

and Mei 

1986

Beaumont, TX Test structure Apr–Nov 9 RH sensors at 

interfaces 

between 

materials in 

wall cavity 

— — — x Walls remained dry with 

an exterior vapor 

retarder or without any 

vapor retarder either 

inside or outside 

x Wall with interior vapor 

retarder experienced 

daily cyclical 

condensation

Mei 1988 Beaumont, TX Test structure Jun–Oct 9 RH sensors at 

interfaces 

between 

materials in 

wall cavity 

— — — x Ventilation between 

cladding and sheathing 

reduced moisture flow 

into wall cavity 

x Daily cyclical 

condensation at interior 

vapor retarder in wall 

with brick veneer but not 

plywood siding 

Crandell & 

Kenney 

1996

Wilmington,

NC

Survey Aug 8 Sh 18 ~50 x Bulk water intrusion 

related to inadequate 

detailing 

Mixed-humid climate 

Weber & 

Reichel 

1942

Washington,

DC

Test structure 2 wk 

(winter)

7 Sh — 6 ~60 x Condensation in wall 

cavities without interior 

vapor retarder 

Moses & 

Scheffer 

1962

Baltimore, MD Survey Aug 97 BP, St — <12 30

Moses & 

Scheffer 

1962

Memphis, TN Survey Jan 59 BP, St — <12 30

Duff 1971 Athens, GA Test structure Jun 4 Sh — 10 13 x Sensors at interface 

between insulation and 

interior vapor retarder 

showed daily period of 

high RH 

Duff 1972 Athens, GA Test structure 1 yr 12 Sh, St — 10 25 x Highest MC occurred in 

sheathing during winter 

14

Marshall

Macklin 

Monaghan

1983

Canada—

various

provinces

Survey Not 

specified 

201 Sh, St — <15 >22 x High MC readings 

correlated with high 

indoor RH 

x Higher MCs in second 

floor than first floor 

x Moisture accumulation 

in sheathing correlated 

with exposure to winddriven rain and exposure 

to wind 

Sherwood

1983, 1987 

Madison, WI Test structure 2 yr 8 BP, Sh, St, TP — 8 >20 x Condensation found in 

walls with kraft paper 

but not polyethylene 

vapor retarders 

x When electrical outlets 

were added, localized 

condensation found on 

sheathing surface behind 

outlet

Tsongas

1984

Spokane, WA Survey Dec–Feb 103 BP, Sh, St 11.3 6.5 >30 x No significant difference 

in MC between insulated 

and uninsulated walls, or 

between walls with 

different types of 

insulation

x Sheathing consistently 

wetter than framing 

x MC > 20% attributed 

mainly to splash wetting, 

wood members in 

contact with ground, and 

bulk water leaks 

Platts 1988 Canada—

various

provinces

Survey Oct 1900 BP, Sh, St — — >30 x Higher incidence of high 

MC in coastal provinces 

than inland provinces 

x High MC attributed to 

rainwater intrusion or 

excessive indoor RH in 

winter

x In coastal provinces, 

high MC attributed to 

wind-induced 

exfiltration of humid 

indoor air through leaks 

on lee side of house 

Tsongas

1990

Montana Survey Winter 20 BP, Sh, St 12.8 6 ~50 x Sheathing wetter than 

framing

x All wood members dried 

to MC < 8% by august 

Tsongas & 

Nelson 1991 

Helena, MT Forensic Feb–Apr 1 Sh — 6 ~50 x Locations of high MC 

correlated with locations 

of major air leakage 

TenWolde

et al. 1995 

Madison, WI Test structure Nov–Feb 10 Sh, inside 

surface 

— <8 >30 x MC generally increased 

as indoor RH increased 

from 35% to 45% RH 

x Condensation observed 

only when indoor RH 

was 45% 

x Ventilation of the 

insulated cavity did not 

36
12

Table 23—Summary of studies reporting moisture contents in wall framing and sheathing by climatic region 

Moisture content (%) 

Reference Location

Type of 

study Duration Na

Measurement 

locationb Mean Minimum Maximumc Trends

Hot-humid climate 

Moses & 

Scheffer 

1962

Southeastern 

US

Survey Aug–Oct 78 BP, St — <12 30

Moses & 

Scheffer 

1962

Gulf Coast Survey Oct–Dec 255 BP, St — <12 >30 

Sherwood

1985, 1987 

Gulfport, MS Test structure 2 yr 8 BP, Sh, St, TP — 8 >20 x Higher MC in walls 

penetrated by electrical 

outlets

x Highest summer MC in 

walls with fiberboard 

sheathing

x Higher summer MC in 

walls with wood-based 

sheathing than with 

exterior insulating 

sheathing

TenWolde

and Mei 

1986

Beaumont, TX Test structure Apr–Nov 9 RH sensors at 

interfaces 

between 

materials in 

wall cavity 

— — — x Walls remained dry with 

an exterior vapor 

retarder or without any 

vapor retarder either 

inside or outside 

x Wall with interior vapor 

retarder experienced 

daily cyclical 

condensation

Mei 1988 Beaumont, TX Test structure Jun–Oct 9 RH sensors at 

interfaces 

between 

materials in 

wall cavity 

— — — x Ventilation between 

cladding and sheathing 

reduced moisture flow 

into wall cavity 

x Daily cyclical 

condensation at interior 

vapor retarder in wall 

with brick veneer but not 

plywood siding 

Crandell & 

Kenney 

1996

Wilmington,

NC

Survey Aug 8 Sh 18 ~50 x Bulk water intrusion 

related to inadequate 

detailing 

Mixed-humid climate 

Weber & 

Reichel 

1942

Washington,

DC

Test structure 2 wk 

(winter)

7 Sh — 6 ~60 x Condensation in wall 

cavities without interior 

vapor retarder 

Moses & 

Scheffer 

1962

Baltimore, MD Survey Aug 97 BP, St — <12 30

Moses & 

Scheffer 

1962

Memphis, TN Survey Jan 59 BP, St — <12 30

Duff 1971 Athens, GA Test structure Jun 4 Sh — 10 13 x Sensors at interface 

between insulation and 

interior vapor retarder 

showed daily period of 

high RH 

Duff 1972 Athens, GA Test structure 1 yr 12 Sh, St — 10 25 x Highest MC occurred in 

sheathing during winter 

15

consistently promote 

drying whereas 

ventilation between 

siding and sheathing was 

effective

Tsongas & 

Olson 1995 

Wisconsin Forensic Winter, 

spring,

summer 

17 Sh — — ~80 x Plywood sheathing had 

mold, decay, 

delamination, but 

framing was generally 

not harmed 

Rose & 

McCaa 1998 

Champaign, IL Test structure 3 yr 16 Sh — 6 35 x MC and temperature 

increased with 

increasing height in any 

given cavity 

x Cavities without vapor 

retarders had high winter 

MC, mold growth, and 

nail corrosion 

x MC generally decreased 

as indoor RH decreased 

from 50% to 40% RH 

Straube & 

Burnett

1998

Waterloo, ON Test structure 2 yr 2 BP, St, TP — 9 20 x Low MC in framing of 

wall with lowpermeance sheathing 

x Ventilation between 

brick cladding and highpermeance sheathing 

reduced framing MC 

during summer 

Chouinard

& Lawton 

2001

Nova Scotia, 

Ontario

Forensic — 3 Framing and 

Sh

— 10 >25 x Bulk water intrusion 

related to inadequate 

detailing 

Cautley 

2004

Madison, WI Field 

experiment

10 mo 1 BP, Sh, St, 

TP, window 

framing

— <8 >30 x Lowest winter MCs in 

airtight walls with 

polyethylene vapor 

retarder 

x In walls without airtight 

construction or vapor 

retarder, high MC in 

winter followed by 

drying in springtime 

x In same walls, highest 

framing MC found in 

outer bottom plates 

x Leaky walls dried faster 

than airtight walls 

x Rapid, localized wetting 

observed in framing 

below windows 

following rainfall 

a

 For surveys and forensic investigations, N is number of buildings sampled; for test structures, N is number of wall configurations. b

 Moisture content measurements above 30% by electrical resistance are approximate (see Section 1.4). c

 BP, bottom plate; Sh, exterior sheathing; St, stud; TP, top plate.

 37
summer, unless there is excessive rainwater intrusion. 

However, in some cases, the design or construction of 

the wall prevents effective drying.

4. In cold and marine climates, framing members in insulated walls generally do not accumulate as much 

moisture as the exterior sheathing. During winter, the 

cold side of framing members tends to be wetter than 

the warm side for the reasons discussed above. Drying 

usually occurs in spring and summer.

5. In hot-humid climates, the significant moisture transport mechanisms (besides rainwater intrusion) are infiltration of humid outdoor air and inward diffusion of 

water vapor through wall cavities lacking effective air 

barriers and (exterior) vapor retarders, the opposite of 

cold climates in winter. In addition, solar radiation can 

drive moisture from the exterior sheathing into the wall 

cavity. When a vapor retarder is placed on the interior 

(cold side) of an insulated wall cavity, condensation can 

occur on the vapor retarder.

6. All studies that have investigated the effect of a ventilated space between the sheathing and cladding have 

found that this feature reduces moisture accumulation 

in the wall.

7. When bottom plates are in direct contact with concrete 

slab foundations (no capillary break), moisture can be 

absorbed by the bottom plate, resulting in MCs near 

30%.

4 Roof Framing and Sheathing

In this section we discuss measurements of wood moisture 

content and temperature in roof framing and sheathing. 

Again, many factors influence the conditions in roof members: climate, including solar radiation, outdoor temperature, 

and humidity; the design and construction of the roof, including attic ventilation; and the properties of the materials 

and roof system as a whole, including moisture transfer 

from the living space to the attic by diffusion or air leakage. 

Several types of roof construction are common: 

(1) attics, in which insulation is placed over a flat ceiling; 

(2) cathedral ceilings, in which the ceiling is sloped (usually 

parallel to the roof but not necessarily); and (3) for lack of a 

better term, “cathedralized” attics, in which the ceiling is flat 16

Table 24—Summary of studies reporting moisture contents in bottom plates 

Moisture content (%) 

Reference Location Type of study Duration Na Foundationb Mean Minimum Maximum 

Moses & Scheffer 1962 Southeastern U.S. Survey Aug–Oct 73 Slab — <12 30

Moses & Scheffer 1962 Gulf Coast Survey Oct–Dec 249 Slab — <12 >30 

Moses & Scheffer 1962 Baltimore, MD Survey Aug 54 Slab — <12 30

Moses & Scheffer 1962 Memphis, TN Survey Jan 58 Slab — <12 30

Moses & Scheffer 1962 Southern CA Survey Mar–Apr 84 Slab — <12 >30 

Moses & Scheffer 1962 Oregon & Washington Survey Apr–Jun 20 Slab — <12 <20 

Moses & Scheffer 1962 Chicago, IL & Madison, WI Survey Jan 6 Slab — <12 <20 

Tsongas 1980 Portland, OR Survey Feb–Apr 346 B, CS 12.2 7 20 

Tsongas 1984 Spokane, WA Survey Winter 608 B, CS 11.0 6.5 26 

Tsongas 1990 Montana Survey Winter 38 B, CS 12.0 8 19 

Tsongas 1990 Seattle–Olympia, WA Survey Winter 90 B, CS 15.5 10 26 

Tsongas 1990 Washington Coast Survey Winter 31 B, CS 18.9 10 28 

Cautley 2004 Madison, WI Field experiment 10 mo. — — — <8 >25 

a

 Number of measurements. 

b

 B, basement; CS, crawlspace. 

Table 25—Summary of studies reporting moisture contents in top plates 

Moisture content (%) 

Reference Location Type of 

study Duration Na Mean Minimum Maximum 

Tsongas 1980 Portland, OR Survey Feb–Apr 214 11.4 7 16 

Tsongas 1990 Montana Survey Winter 17 12.6 6 24 

Tsongas 1990 Seattle–Olympia, WA Survey Winter 48 14.1 8 21 

Tsongas 1990 Washington Coast Survey Winter 15 19.6 12 29 

Cautley 2004 Madison, WI Field 

experiment 10 mo. — — <8 >14 

a

 Number of measurements. 

38
17

Table 26—Summary of studies reporting moisture contents in studs 

Moisture content (%) 

Reference Location Type of study Duration Na Mean Minimum Maximumb

Moses & Scheffer 1962 Southeastern U.S. Survey Aug–Oct 5 — <12 <20 

Moses & Scheffer 1962 Gulf Coast Survey Oct–Dec 6 — <12 <20 

Moses & Scheffer 1962 Baltimore, MD Survey Aug 41 — <12 <20 

Moses & Scheffer 1962 Memphis, TN Survey Jan 1 — 12 <20 

Moses & Scheffer 1962 Southern CA Survey Mar–Apr 14 — <12 <20 

Duff 1968 Madison, WI Test structure 2 yr 3 — 7 17 

Duff 1972 Athens, GA Test structure 1 yr 12 — 10 13 

Tsongas 1980 Portland, OR Survey Feb–Apr 705 11.6 6 19 

Wang 1981 Various Survey Winter 70 — 7 <15 

Marshall Macklin Monaghan 1983 Canada—various provinces Survey Not specified 201 — <15 >22 

Tsongas 1984 Spokane, WA Survey Winter 1,217 10.7 6.5 24 

Tsongas 1990 Montana Survey Winter 114 12.4 6 34 

Tsongas 1990 Seattle–Olympia, WA Survey Winter 300 14.8 9 26 

Tsongas 1990 Washington Coast Survey Winter 96 18.6 10 ~40 

Straube & Burnett 1998 Waterloo, ON Test structure 2 yr 2 — 9 20 

Cautley 2004 Madison, WI Field experiment 10 mo. 12 — <8 >16 

a

 For surveys, N is number of buildings sampled or number of measurements; for test structures and field experiments, N is number of wall configurations. b

 MC readings above 30% by electrical resistance are approximate (see Section 1.4). 

Table 27—Summary of studies reporting moisture contents in wall sheathing 

Moisture content (%) 

Reference Location Type of study Duration Na Mean Minimum Maximumb

Weber & Reichel 1942 Washington, DC Test structure 2 wk (winter) 7 — 6 ~60 

Teesdale 1959 Madison, WI Test structure Oct–Jun 3 — 11 47 

Duff 1968 Madison, WI Test structure 2 yr 3 — 12 ~60 

Duff 1971, 1972 Athens, GA Test structure 1 yr 12 — 10 25 

Tsongas 1980 Portland, OR Survey Feb–Apr 294 12.2 6 18 

Marshall Macklin Monaghan 

1983

Canada—various 

provinces Survey Not specified 201 — <15 >22 

Sherwood 1983 Madison, WI Test structure 2 yr 8 — 8 >20 

Tsongas 1984 Spokane, WA Survey Winter 566 13.0 6.5 >30 

Sherwood 1985 Gulfport, MS Test structure 2 yr 8 — 8 >20 

Tsongas 1990 Montana Survey Winter 30 15.5 8 ~50 

Tsongas 1990 Seattle–Olympia, WA Survey Winter 101 18.1 9 ~50 

Tsongas 1990 Washington Coast Survey Winter 41 20.6 9 32 

Tsongas & Nelson 1991 Helena, MT Forensic Feb–Apr 120 — 6 ~50 

TenWolde et al. 1995 Madison, WI Test structure Nov–Feb 10 — <8 >30 

Tsongas & Olson 1995 Wisconsin Forensic Winter, spring, summer 17 — — ~80 

Crandell & Kenney 1996 Wilmington, NC Forensic Aug 8 — 18 ~50 

Rose & McCaa 1998 Champaign, IL Test structure 3 yr 16 — 6 35 

Cautley 2004 Madison, WI Field experiment 10 mo. 1 — <8 >25 

a

 For surveys, field experiments, and forensic investigations, N is number of buildings sampled or number of measurements; for test structures, N is number of wall 

configurations. 

b

 MC readings above 30% by electrical resistance are approximate (see Section 1.4). 

Table 28—Reported moisture content ranges in roof 

bf hG iNJb

 39
but insulation is placed beneath the roof sheathing, making 

the attic part of the conditioned space. Rose (1995b) 

provides a historical overview of attic ventilation, and 

TenWolde and Rose (1999) and Lstiburek (2006b) discuss 

various design options in relation to advantages and disadvantages of ventilation.

4.1 Mixed-Humid Climate

Harrje et al. (1986) monitored moisture conditions in two 

houses with attics in New Jersey. The first house (denoted 

G) had plywood roof sheathing; the second (P) had tongueand-groove lumber decking. Moisture content was measured 

with two types of electrical resistance probes in the roof 

sheathing, rafters, and ceiling joists, with generally good 

agreement between the two probes. Both houses showed a 

seasonal trend, with high moisture contents in winter that 

fell during spring, reached minima in summer, and rose 

again in fall. Table 28 summarizes the measurements in the 

G house and indicates the importance of solar radiation in 

drying the sheathing, as seen in the different winter maxima 

for the north and south slopes. Table 29 summarizes the data 

for the P house, which had previously experienced attic condensation problems, with ceiling damage and mold growth 

on the underside of the roof where air leakage occurred 

from a bathroom into the attic. No wood decay or structural 

damage was observed, likely because the high moisture 

contents coincided with cold temperatures. Unfortunately, 

temperatures were not reported.

Levins et al. (1989) measured moisture contents of wood 

truss members during winter in three houses near Oak 

Ridge, Tennessee. These houses were constructed with attics 

containing radiant barriers. Several variables were investigated:

•	 Indoor relative humidity—45% or 55% RH

•	 Placement of the radiant barrier—above the insulation 

or attached to the trusses

•	 Presence of a vapor retarder (kraft paper facing on the 

insulation)

•	 Amount of attic ventilation—1:150 or 1:300 area ratio 

(net vent area to floor area)

Moisture content values were generally in the 7–11% range, 

with only slight differences due to indoor relative humidity and ventilation. Temperature and relative humidity were 

also measured at the bottom of the insulation, at the top of 

the insulation, and in air a foot above the radiant barrier. A 

diurnal cycle was observed in which moisture condensed on 

the bottom surface of the radiant barrier when the outdoor 

temperature dropped below 35°F (2°C) and then dissipated 

in the afternoon. (The radiant barrier had perforations of 

0.040-in. (1-mm) diameter that allowed water vapor to escape.) They did not observe any wet insulation or staining 

on the ceiling. They found that more moisture condensed in 

the house when the indoor RH was 55% than 45%. Reducing the attic ventilation area did not significantly change the 

moisture levels. More condensation was observed over the 

bathrooms than elsewhere; this was attributed to air leakage 

through penetrations for plumbing and exhaust fan ducting.

Winandy et al. (2000) monitored conditions in the sheathing 

and rafters of five test structures in Starkville, Mississippi, 

over a 4-year period. The 12- by 16-ft (3.7- by 4.9-m) structures had a 3:12 pitched roof with black fiberglass shingles 

and felt over 3/4-in. (19-mm) plywood sheathing supported 

by 2 by 6 rafters. The structures differed significantly from 

residential attic construction: they were essentially enclosed, 

unventilated sheds with no ceiling and thus no ceiling insulation (radiative exchange between the roof sheathing and 

the floor was possible). Temperatures were measured in 

various locations with thermocouples, and moisture content 

readings were acquired every 2 weeks with a resistancetype meter. Two of the structures were humidified to above 

85% RH during most of the day, while the remaining three 

structures were not humidified. The moisture content of the 

plywood roof sheathing in the “dry” structures was between 

2% and 5% during summer and between 3% and 8% during 

winter. Maximum temperatures at the top of the plywood, 

bottom of the plywood, and within the rafters were 172°F 

(78°C), 145°F (63°C), and 136°F (58°C), respectively. Corresponding minimum temperatures were –2°F (–19°C), 

1°F (–17°C), and 3°F (–16°C). In the “wet” structures, the 

plywood MCs ranged from 4% to 12% in summer and from 

6% to 20% in winter. Maximum temperatures at the top of 

the plywood, bottom of the plywood, and within the rafters 

were 165°F (74°C), 136°F (58°C), and 129°F (54°C), respectively. Corresponding minimum temperatures were 0°F 

(–18°C), 3°F (–16°C), and 5°F (–15°C).

4.2 Hot-Dry Climate

Moisture problems were reported in new energy-efficient 

houses with wood truss-framed low-slope roofs and lightcolored membrane roofing in the desert southwest (Holladay 

2006b). The roof sheathing was typically OSB with R-38 

fiberglass batt insulation directly beneath it. Measurements 

showed that during winter, the OSB sheathing was 5–7°F 

(3–4°C) colder than the outdoor air at night, which was attributed to radiant heat loss from the roof surface. Moisture 

contents greater than 30% were measured, the fiberglass insulation was found to be totally wet, and mold was growing 

on the bottom side of the sheathing. The relatively cold roof 

sheathing was evidently acting as a condensing surface at 

night. The primary moisture source was not identified with 

certainty,18 but unexpectedly high indoor humidities were 

observed by at least one investigator. Construction moisture 

from concrete, adobe block, and wet-spray cellulose insulation were implicated as sources for indoor humidity. The 

white roof surfaces, which can reflect as much as 80% of the 

incident solar radiation, apparently did not absorb enough 

heat during the daytime for the OSB to dry out.

18The article by Holladay (2006b) made no mention of whether nighttime 

surface temperatures dropped as low as exterior dew point temperature, or 

whether overnight dew formation on exterior roof surfaces was observed. 

40
4.3 Marine Climate

Tsongas and Bolme (1988) conducted a field investigation 

of roof moisture damage in three state-owned buildings in 

Trail, Oregon. The buildings had low-sloped roofs of woodframe construction with plywood inner and outer sheathing, R-11 fiberglass batt insulation, and a rubber membrane 

covering the outer plywood surface. Moisture contents 

were measured with a resistance-type meter in the plywood 

sheathing and in laminated wood beams. Values from as low 

as 7% MC to as high as 30% MC were observed. The wet 

sites were attributed to leakage of water into the roof cavity 

through cracks in the deteriorated rubber membrane. Wood 

decay and structural damage in the roof were also observed.

Sheltair Scientific Ltd. (1997) measured temperature and 

moisture conditions in two pairs of houses in Vancouver, 

British Columbia. Each pair was constructed by the same 

builder, one with attic ventilation and the other without. 

Moisture contents were measured in roof sheathing and 

truss members using a resistance-type meter on a monthly 

basis from September through March. The highest moisture 

levels were found in the sheathing of north-facing slopes in 

midwinter, the same trend observed by Harrje et al. (1986). 

In the first pair of houses, the sheathing rose from a low of 

8% MC in September to highs of 25% MC and 30% MC in 

February for the nonvented and vented attics, respectively. 

By March, these levels fell to 17–19% MC. In the second 

pair, the September MC values were 10–11% and rose to 

18% and 15% for the nonvented and vented attics, respectively. The houses with higher indoor relative humidity levels showed higher moisture levels in attic wood members, 

irrespective of attic ventilation. Truss members typically 

remained at lower moisture contents than sheathing, with 

values between 8% and 14% MC, and no significant differences between vented and nonvented attics.

4.4 Cold Climate

Jordan et al. (1948) investigated the effects of attic ventilation and the presence of a ceiling vapor retarder on roof 

sheathing moisture content. Three adjacent houses in Madison, Wisconsin, were monitored several times a week during 

the month of February. Temperature and relative humidity 

were measured in various locations in the living space and 

attic. In addition, moisture content was measured gravimetrically using thin wood strips that were installed below 

the roof sheathing. All houses had loose-fill insulation above 

the ceiling to depths of 5–6 in. (130–150 mm). 

Table 30 gives wood moisture contents and attic relative 

humiditys for each house. House A was known to have had 

prior condensation both in the attic and in the walls, which 

was related to high humidity in the living space. House B 

was the only one with a vapor retarder between the living 

space and attic, and there were no signs of condensation 

problems. The attic vents were closed partway through the 

study. House C also did not have evidence of prior attic 

condensation. The authors attributed the low attic relative 

humidity and moisture content to the low humidity in the 

living space rather than attic ventilation. The wood moisture 

content and attic relative humidity for the three houses correlated well with the relative humidity in the living space.

Marshall Macklin Monaghan Ltd. (1983) conducted a survey of 201 government-financed housing units with reported 

18

Table 28—Reported moisture content ranges in roof 

members of house G in New Jersey by season 

(Harrje et al. 1986) 

Moisture content range (%) 

Roof member Winter Spring Summer Fall 

Sheathing, north slope 13–18 7–13 5–7 7–13 

Sheathing, south slope 9–11 7–10 5–7 7–10 

Rafters 9–11 7–10 5–7 7–10 

Table 29—Average, minimum, and maximum moisture 

contents reported in roof members of house P in New 

Jersey (Harrje et al. 1986)

Moisture content (%) 

Date Measurements Average Minimum Maximuma

3/7/1983 22 21.2 14 ~40 

5/2/1983 16 12.0 11 14 

5/9/1983 9 11.1 10 12 

5/23/1983 9 11.7 11 13 

a

 MC readings above 30% by electrical resistance are approximate (see 

Section 1.4). 

Table 30—Reported roof sheathing moisture contents and relative 

humidity of attic air during February in houses in Madison, Wisconsin 

(Jordan et al. 1948) 

Moisture content (%) 

House

Vent area 

ratio

Vapor

retarder Average Minimum Maximum

Average 

RH (%) 

A 1:770 No 18.2 16.5 19.4 84 

B --- Yes 11.1 10.1 11.5 59 

C 1:430 No 10.5 9.9 11.1 55 

Table 31—Reported “typical” moisture content ranges 

in roof members of Ontario and Prince Edward Island 

houses by season (Buchan, Lawton, Parent 1991) 

Moisture content range (%) 

Roof member Winter Spring Summer Fall 

Sheathing, “wet attics” >30 10–15 ~10 10–20 

Sheathing, “dry attics” 12–16 10–12 8–10 10–12 

Trusses, “wet attics” 15–40 12–20 8–10 10–12 

Trusses, “dry attics” 9–12 8–10 ~8 8–10 

18

Table 28—Reported moisture content ranges in roof 

members of house G in New Jersey by season 

(Harrje et al. 1986) 

Moisture content range (%) 

Roof member Winter Spring Summer Fall 

Sheathing, north slope 13–18 7–13 5–7 7–13 

Sheathing, south slope 9–11 7–10 5–7 7–10 

Rafters 9–11 7–10 5–7 7–10 

Table 29—Average, minimum, and maximum moisture 

contents reported in roof members of house P in New 

Jersey (Harrje et al. 1986)

Moisture content (%) 

Date Measurements Average Minimum Maximuma

3/7/1983 22 21.2 14 ~40 

5/2/1983 16 12.0 11 14 

5/9/1983 9 11.1 10 12 

5/23/1983 9 11.7 11 13 

a

 MC readings above 30% by electrical resistance are approximate (see 

Section 1.4). 

Table 30—Reported roof sheathing moisture contents and relative 

humidity of attic air during February in houses in Madison, Wisconsin 

(Jordan et al. 1948) 

Moisture content (%) 

House

Vent area 

ratio

Vapor

retarder Average Minimum Maximum

Average 

RH (%) 

A 1:770 No 18.2 16.5 19.4 84 

B --- Yes 11.1 10.1 11.5 59 

C 1:430 No 10.5 9.9 11.1 55 

Table 31—Reported “typical” moisture content ranges 

in roof members of Ontario and Prince Edward Island 

houses by season (Buchan, Lawton, Parent 1991) 

Moisture content range (%) 

Roof member Winter Spring Summer Fall 

Sheathing, “wet attics” >30 10–15 ~10 10–20 

Sheathing, “dry attics” 12–16 10–12 8–10 10–12 

Trusses, “wet attics” 15–40 12–20 8–10 10–12 

Trusses, “dry attics” 9–12 8–10 ~8 8–10 

 41
moisture damage throughout Canada (as described in 

Section 3.4). Single point-in-time moisture content measurements were made in roof sheathing and framing members of 

houses with accessible attics (the time of year was not specified). Of the 44 readings taken, 57% were below 15% MC 

and 14% were above 22% MC. High moisture readings were 

associated with colder weather, wind-driven rain, and low 

solar gain. Poor attic ventilation was commonly found in 

houses with high attic moisture. Mold was present in all the 

attics sampled, and 39% of these showed “major” growth, 

where mold covered more than 50% of the sheathing.

An attic moisture survey of 15 houses in Ottawa, Ontario, 

and 5 houses in Charlottetown, Prince Edward Island, was 

conducted by Buchan, Lawton, Parent Ltd. (1991). These 

20 houses spanned a range of ages, types of construction, 

and types of attic ventilation. Moisture contents were measured in attic wood members with a resistance-type meter; 

temperatures were measured with thermocouples. Locations 

monitored included gable ends, ceiling joists, and trusses. 

In addition, roof sheathing was monitored with a surface 

moisture probe. Other parameters were measured, including 

indoor relative humidity and temperature and air exchange 

rates for the house and attic.

The same seasonal trend observed in the studies above was 

also found here: wood moisture levels tended to increase in 

the winter and fall rapidly in spring. The houses were categorized as “wet”—having values above 30% MC in 

the sheathing and at least one framing member—or “dry”

—having all values at or below 20% MC. Four houses fell 

into the “wet” category, six into the “dry” category, and 

ten were in between. Table 31 lists the range of moisture 

contents typically found in the “wet” and “dry” attics. A significant observation was that high attic moisture levels were 

correlated with high indoor humidity levels, in agreement 

with the findings of Sheltair Scientific Ltd. (1997).

Rose (1992) measured temperature and sheathing moisture 

content in attic assemblies of a test facility in central Illinois. The structure had eight separate bays, each 20 by 

8 ft (6.1 by 2.4 m), that were configured to test various types 

of residential construction. Five bays had flat-ceiling trussframed construction and three had cathedral ceilings. Other 

variables under study were the following:

•	 Venting—with or without ridge and soffit vents

•	 Airtightness of the ceiling—capped or uncapped 1.5-in. 

(38-mm) round openings in ceilings that tested as essentially airtight with the openings capped19

•	 Placement of insulation in cathedral ceilings—with or 

without an airspace between the top of the insulation 

and bottom of the roof sheathing (termed “slotted” 

versus “stuffed”)

19Although all ceilings had these controllable openings, the effect of 

capping versus uncapping them was more extensively investigated for 

cathedral ceilings than for attic spaces.

The roof pitch was 5:12, the sheathing was 7/16-in. (11-mm) 

OSB, and the shingles were reinforced asphalt triple-tab, either black or white. The interior of each bay was faced with 

1/2-in. (12-mm) gypsum board. Each bay was individually 

conditioned: in summer, the indoor temperature was maintained at 75°F (24°C) with humidity uncontrolled; in winter, 

the bays were heated to 70°F (21°C) and humidified to 50% 

RH.

Temperature at the top and bottom sides of the OSB sheathing was measured in several locations using thermocouples. 

Sheathing moisture content was monitored using modified 

wood electrical resistance probes (Duff 1966) both at the 

surface and embedded in the OSB. As was observed in other 

studies, sheathing moisture conditions peaked during the 

winter. When openings in the ceiling were capped, thereby 

blocking airflow between the conditioned room and the attic 

or cathedral ceiling cavity, the moisture content of the roof 

sheathing was between 10% and 18% for most of the winter, 

with a minimum of 8% and a maximum of 22%. Flat-ceiling 

attics (which had capped openings) showed little difference 

in moisture content whether vented or not vented; in contrast, cathedral ceilings with capped openings generally had 

drier sheathing if ventilated than if not.

When openings in cathedral ceilings were uncapped during 

the winter, thereby allowing exfiltration from the conditioned room, sheathing moisture contents increased. Moisture conditions depended on three factors: location within 

the cathedral ceiling cavity relative to the uncapped opening, whether the cavity was vented or unvented, and whether 

there was an airspace between the insulation and the sheathing. In addition, these three factors apparently interacted. 

Nevertheless, Rose was able to conclude that the vented–

slotted configuration maintained the driest conditions during 

the winter when an opening in the ceiling was present. Some 

of the less desirable cathedral ceiling configurations resulted 

in sheathing moisture contents exceeding 30% for significant periods when an opening in the ceiling was present. An 

overarching finding of the study was that moisture and temperature conditions in cathedral ceiling cavities were more 

variable than in attics, and thus less likely to remain within 

a moderate range.

Maximum summer temperatures were reported for the top 

side of the roof sheathing. The maximum values, which 

varied with shingle color, type of construction (attic or cathedral ceiling) and ventilation, ranged from 148°F (64°C) 

to 186°F (86°C).

Rose (1995a) also investigated “cathedralized” attic construction in which the insulation was placed against the 

underside of the roof sheathing rather than on top of the flat 

ceiling. He modified two bays of the test facility described 

above and measured temperature and moisture content in 

the sheathing over a 2-year period. The variables in this 

study were the following:

42
•	 Ventilation or no ventilation of the cavity

•	 Continuous air chute construction or stuffed insulation 

construction

•	 Open joints in exposed kraft paper facing of insulation 

or taped joints

Sheathing moisture contents averaged around 16% during 

the winter. The variable with the largest impact on sheathing 

moisture was the air chute (with or without venting). The 

overall average for the cavities with air chutes was ~10% 

MC, with values from less than 6% to ~15%, whereas for 

the cavities without air chutes the average was ~22% MC, 

with values from less than 6% to greater than 30%. Taping 

the kraft facing of the insulation and venting the cavity had 

only small effects on moisture levels.

Summer average temperatures at the top of the sheathing were between 83°F (28°C) and 88°F (31°C), whereas 

maximum temperatures ranged from 150°F (66°C) to 178°F 

(81°C). The temperature performance of cathedralized attic 

assemblies was similar to that of cathedral ceilings. Venting 

did help maintain lower temperatures, though the reduction 

in temperature was seen most strongly on the parts of roof 

closest to the eaves.

Further measurements were carried out in this test facility 

and were used to verify a roof temperature and moisture 

model (TenWolde 1997). Data were reported only for Bay 2, 

which had flat-ceiling attic construction, black shingles, solid vinyl soffit panels, no ridge vent, and no ceiling penetrations. Moisture contents in the sheathing of the north-facing 

roof slope were between 7% and 11% during February and 

between 5% and 8% during May. Temperatures measured at 

the top surface of the sheathing in May cycled between lows 

of 40–60°F (4–16°C) at night and highs between ~70°F 

(21°C) and ~140°F (60°C) during the day.

Winandy et al. (2000) monitored conditions in the sheathing 

and rafters of five test structures near Madison, Wisconsin, 

over an 8-year period. The structures and test methodology were the same as described in Section 4.1, except that 

one structure had white shingles rather than black. One of 

the black-shingled structures was humidified to above 85% 

RH for most of the day from April to October. The moisture content of the plywood roof sheathing was typically 

between 13% and 16% during winter and between 8% and 

12% during summer. No difference between humidified and 

non-humidified buildings was reported. Maximum temperatures for the top of the plywood, the bottom of the plywood, 

and the rafters were 147–167°F (64–75°C), 127–138°F (53–

59°C), and 120–129°F (49–54°C), respectively; the higher 

temperatures were observed in roofs with black shingles. 

There was little variation in minimum temperatures at the 

various measurement locations, with values between –38°F 

(–39°C) and –26°F (–32°C). Similarly, yearly average temperatures were between 50°F (10°C) and 55°F (13°C).

4.5 Very Cold Climate

Sheltair Scientific Ltd. (1997) measured temperature and 

moisture conditions in two pairs of houses in Edmonton, 

Alberta. Each pair was constructed by the same builder, one 

with attic ventilation and the other without. Moisture contents were measured in roof sheathing and truss members 

using a resistance-type meter on a monthly basis from September through March. In the first pair of houses, the sheathing moisture content rose from lows of 8% and 12% MC 

in September and October to highs of 13% and 18% MC in 

January and February for the nonvented and vented attics, 

respectively. By March, these levels fell to 9% MC. In the 

second pair, the minimum MC values were 10% and 7% and 

rose to 16% and 10% for the nonvented and vented attics, 

18

5/23/1983 9 11.7 11 13 

a

 MC readings above 30% by electrical resistance are approximate (see 

Section 1.4). 

Table 30—Reported roof sheathing moisture contents and relative 

humidity of attic air during February in houses in Madison, Wisconsin 

(Jordan et al. 1948) 

Moisture content (%) 

House

Vent area 

ratio

Vapor

retarder Average Minimum Maximum

Average 

RH (%) 

A 1:770 No 18.2 16.5 19.4 84 

B --- Yes 11.1 10.1 11.5 59 

C 1:430 No 10.5 9.9 11.1 55 

Table 31—Reported “typical” moisture content ranges 

in roof members of Ontario and Prince Edward Island 

houses by season (Buchan, Lawton, Parent 1991) 

Moisture content range (%) 

Roof member Winter Spring Summer Fall 

Sheathing, “wet attics” >30 10–15 ~10 10–20 

Sheathing, “dry attics” 12–16 10–12 8–10 10–12 

Trusses, “wet attics” 15–40 12–20 8–10 10–12 

Trusses, “dry attics” 9–12 8–10 ~8 8–10 

18

5/23/1983 9 11.7 11 13 

a

 MC readings above 30% by electrical resistance are approximate (see 

Section 1.4). 

Table 30—Reported roof sheathing moisture contents and relative 

humidity of attic air during February in houses in Madison, Wisconsin 

(Jordan et al. 1948) 

Moisture content (%) 

House

Vent area 

ratio

Vapor

retarder Average Minimum Maximum

Average 

RH (%) 

A 1:770 No 18.2 16.5 19.4 84 

B --- Yes 11.1 10.1 11.5 59 

C 1:430 No 10.5 9.9 11.1 55 

Table 31—Reported “typical” moisture content ranges 

in roof members of Ontario and Prince Edward Island 

houses by season (Buchan, Lawton, Parent 1991) 

Moisture content range (%) 

Roof member Winter Spring Summer Fall 

Sheathing, “wet attics” >30 10–15 ~10 10–20 

Sheathing, “dry attics” 12–16 10–12 8–10 10–12 

Trusses, “wet attics” 15–40 12–20 8–10 10–12 

Trusses, “dry attics” 9–12 8–10 ~8 8–10 

 43
respectively. Truss members typically remained at lower 

moisture contents than sheathing, with values between 8% 

and 13% MC, although one value of 15% MC was observed 

in a vented attic. The houses with higher indoor relative 

humidity levels showed higher moisture levels in attic wood 

members, irrespective of attic ventilation.

4.6 Summary: Roof Framing and Sheathing

Table 32 summarizes reported moisture contents in roof 

framing and sheathing for each climatic region. Several 

conclusions can be drawn from the literature.

1. High moisture contents in roofs may occur in response 

to rainwater intrusion. The moisture content depends on 

the magnitude of the leakage and the rate at which drying occurs.

2. A trend of higher moisture content during winter and 

lower moisture content during summer has been observed in roof framing and sheathing in all studies in 

which seasonal trends were investigated. Higher moisture contents have generally been found in sheathing 

than in rafters or trusses.

3. Wintertime moisture accumulation in roof members is 

worse when indoor humidity levels are high and solar 

gain is low. The dominant moisture transport mechanism is likely exfiltration of humid indoor air through 

ceilings lacking effective air barriers. Ventilation of the 

attic space has not been a dependable method of controlling moisture levels in roofs when air leakage 

is significant.

5 Exposed Wood Decks

Although they did not specifically monitor wood decks, 

Moses and Scheffer (1962) did measure moisture contents 

of exposed wood posts and columns, porch and step rails, 

and step carriages (stringers) and treads. The data consist 

of single point-in-time measurements and were undoubtedly influenced by the preceding weather. In all the regions 

surveyed (Baltimore, Maryland; southeastern United States; 

Gulf Coast; Memphis, Tennessee; California; Oregon; 

Washington; Madison, Wisconsin; and Chicago, Illinois), 

moisture contents above 30% were observed regularly, and 

the majority of readings were generally between 20% and 

30% MC. In general, higher moisture contents were spatially located near the bases of posts and columns and near 

joints in rails; however, moisture contents at the centers of 

stair treads were similar to those near the ends, probably 

because of surface checks.

Lebow and Lebow (2007) recently reviewed studies that 

report moisture content of pine sapwood exposed outdoors 

in locations around the world. Several of these studies were 

conducted with exposed decks in North America and are 

summarized here.

Gaby and Duff (1978) constructed an exposed deck and railing assembly at a test building in Athens, Georgia, from

2 by 6 boards of untreated southern yellow pine (SYP).20

Moisture content was measured in various locations using 

electrical resistance moisture probes (Duff 1966). Moisture 

content and temperature data were acquired every hour during periods of rain and every six hours during dry periods. 

In addition to weather conditions, moisture content depended on the location within the board and the condition of the 

surface. During and after wetting, the bottom of the board 

was drier than the top. When surface checks were present, the rate at which water was absorbed increased. Water 

absorption was also rapid in the end grains of joints in the 

lower parts of rail members. Although data were acquired 

over 18 months, moisture contents were reported only for a 

10-day period, with values between 10% and 30% MC. The 

upper limit of the moisture probes was 30%; higher levels 

were truncated at this value. Several alternative construction 

details for railings were shown to avoid water retention at 

end-grain surfaces.

In current construction practice, lumber that is intended 

for outdoor use is typically pressure-treated with chemical preservatives. Treated lumber generally has a very high 

initial moisture content (> 50% MC) unless it is kiln-dried 

after treatment (KDAT) or air-dried after treatment (ADAT) 

prior to construction (McDonald et al. 1996). The American 

Wood-Preservers’ Association specifies that lumber and 

plywood dried after treatment shall not exceed 19% MC 

and 18% MC, respectively (AWPA 2006). However, KDATgrade lumber in reality may not always meet the specification: Shupe et al. (2001) showed that the actual moisture 

content of KDAT No. 1 SYP treated with chromated copper 

arsenate (CCA) and kiln-dried untreated SYP from three 

suppliers ranged from 12% to well above 30% MC and that 

the deviation from the specified moisture content increased 

with the thickness of the lumber for nominal 1-, 2-, and 

4-in. boards. They also showed that the CCA treatment does 

not affect the wood’s hygroscopicity: the ratios of adsorption EMC to desorption EMC did not differ for treated and 

untreated wood. Recent submission of amine copper-based 

preservatives (copper azole and copper HDO) report no 

difference in hygroscopicity between treated and untreated 

wood (Lebow 2006). To our knowledge there are no published data on sorption behavior of other wood preservatives. Although the active ingredients in the wood preservatives discussed above do not significantly affect the response 

of wood to water vapor, some formulations do include water 

repellents that are intended to impede the uptake of liquid 

water.

Zahora (1992) compared the field performance of KDAT 

and ADAT SYP deck boards treated with CCA only and 

with CCA plus a water repellent (WR) additive. Eight decks 

20The use of southern pine that was not pressure-treated reflected practice 

for stairs and railings in the South (Verrall 1966) prior to the introduction 

of CCA-treated material in the residential market (which occurred in the 

mid-1970s). Southern pine was typically treated by soak, dip, or brush with 

site-prepared mixtures of pentachlorophenol concentrate in fuel oil carrier, 

with variable results.

44
19

Table 32—Summary of studies reporting moisture contents in roof framing and sheathing by climatic region 

Moisture content (%) 

Reference Location 

Type of 

study Duration Na

Measurement 

locationb Mean Minimum Maximumc

Trends

Mixed-humid climate 

Harrje et al. 

1986

New Jersey Field 

experiment

1 yr 2 J, R, Sh — 5 ~40 x Max MC in winter; min 

in summer 

x North slope wetter than 

south

Levins et al. 

1989

Oak Ridge, 

TN

Field 

experiment

Winter 3 T — 7 11 x Daily cycle of 

condensation on bottom 

surface of radiant barrier 

in attic 

Winandy et al. 

2000

Starkville, 

MS

Test

structure 

4 yr 5 Sh — ~2 ~20 x MC higher in 

humidified buildings 

x Max MC in winter; min 

in summer 

Hot-dry climate 

Holladay 2006b Arizona Forensic --- --- Sh — — >30 x Nighttime condensation 

in newly-constructed 

roofs with low solar 

gain

Marine climate 

Tsongas & 

Bolme 1988 

Trail, OR Forensic --- 3 Sh — 7 30 x High MC attributed to 

bulk water leakage into 

roof through cracks in 

deteriorated rubber 

membrane 

Sheltair 

Scientific 1997 

Vancouver, 

BC

Field 

experiment

Sep–Mar 4 T, Sh — 8 14 x Sheathing generally 

wetter than framing 

x Highest MC found in 

sheathing of northfacing slopes in 

midwinter 

x Houses with higher 

indoor RH had higher 

MC, irrespective of attic 

ventilation

Cold climate 

Jordan et al. 

1948

Madison, WI Field 

experiment

Feb 3 Sh 13.3 10 19 x Sheathing MC and attic 

air RH correlated with 

RH in living space 

Marshall

Macklin 

Monaghan 1983 

Canada—

various

provinces

Survey --- 44 Framing, Sh — <15 >22 x High MC associated 

with cold weather, winddriven rain, and low 

solar gain 

Buchan, 

Lawton, Parent 

1991

Charlottetow

n, PEI & 

Ottawa, ON 

Survey 1 yr 20 J, T, Sh — 8 ~80 x Max MC in winter; min 

in summer 

x High MC correlated 

with high indoor RH 

Rose 1992 Central IL Test 

structure 

2 yr 8 Sh — 8 >30 x Max MC in winter; min 

in summer 

x MC increased when 

small air leak was 

introduced from living 

space into attic 

x MC and temperature in 

cathedral ceiling cavities 

 45
with sets of end-matched boards of each treatment were 

constructed in Harrisburg, North Carolina, and monitored 

periodically over 1 year. Moisture content was measured 

at three depths with an electrical resistance-type meter. 

The CCA-only boards fluctuated rapidly in moisture content, from below 10% to over 30%, with MC correlating 

closely with rainfall. Peaks as high as ~40–50% MC were 

observed.21 In contrast, the CCA–WR boards showed a 

much narrower variation, with values between 7% and 15% 

MC. The boards with CCA–WR treatment also showed less 

checking and cupping than those with only the CCA 

treatment.

Cui and Zahora (2000) conducted a similar study at the 

same location comparing SYP lumber with CCA-only, 

CCA–WR, alkaline copper quat (ACQ)-only, and 

ACQ–WR treatments. Data were reported for three periods: 

(A) September–November 1997; (B) May–June 1998; and 

(C) February–March 2000. Moisture content was measured 

by gravimetric analysis with matched sets of 2-ft (0.6-m) 

unfastened 2 by 6 boards exposed horizontally over deck 

framing. Four decks were constructed from 4-ft (1.2-m) 

2 by 6 boards, and moisture content was determined by 

electrical resistance using pairs of stainless steel pins inserted from the undersides of the boards.22 Reported data 

are summarized in Table 33. Several trends are evident. The 

boards without water repellent additives again showed large 

21MC readings above 30% by electrical resistance are approximate (see 

Section 1.4).

22MC readings were apparently corrected for ACQ-treated boards, but the 

study establishing this correction was not referenced.

fluctuations in moisture content, with values ranging from 

~8% to over 30%. Both the ACQ–WR and CCA–WR treatments apparently improved in water repellency over time, 

with lower moisture content values and smaller fluctuations 

during the later measurement periods.23 This effect was seen 

more strongly for the ACQ–WR treatment.

On the basis of these studies, the following conclusions can 

be drawn:

1. Untreated boards and pressure-treated boards without 

water repellent additives exhibit large fluctuations in 

moisture content with weather. Water absorption occurs 

more rapidly when the wood surface is checked.

2. Treating boards with a preservative and water repellent 

additive significantly dampens these fluctuations. The 

water repellency of certain treatments improves over 

time. Water repellents also minimize checking and 

cupping.

6 Permanent Wood Foundations

Baker and Gjovik (1977) inspected nine houses with 

permanent wood foundations (PWFs) in Charlottesville and 

Verona, Virginia, that were treated with CCA. They also 

inspected seven houses with PWFs in the Minneapolis–St. 

Paul, Minnesota, area that were treated with ammonical 

copper arsenate (ACA). The foundations included full 

basements, crawlspaces, and a slab-on-grade. The purpose 

23The rainfall during each period was not reported; however, the large MC 

fluctuations in the boards without the WR additive in each period imply 

that rainfall was sufficient to test the water repellency. 19

Table 32—Summary of studies reporting moisture contents in roof framing and sheathing by climatic region 

Moisture content (%) 

Reference Location 

Type of 

study Duration Na

Measurement 

locationb Mean Minimum Maximumc

Trends

Mixed-humid climate 

Harrje et al. 

1986

New Jersey Field 

experiment

1 yr 2 J, R, Sh — 5 ~40 x Max MC in winter; min 

in summer 

x North slope wetter than 

south

Levins et al. 

1989

Oak Ridge, 

TN

Field 

experiment

Winter 3 T — 7 11 x Daily cycle of 

condensation on bottom 

surface of radiant barrier 

in attic 

Winandy et al. 

2000

Starkville, 

MS

Test

structure 

4 yr 5 Sh — ~2 ~20 x MC higher in 

humidified buildings 

x Max MC in winter; min 

in summer 

Hot-dry climate 

Holladay 2006b Arizona Forensic --- --- Sh — — >30 x Nighttime condensation 

in newly-constructed 

roofs with low solar 

gain

Marine climate 

Tsongas & 

Bolme 1988 

Trail, OR Forensic --- 3 Sh — 7 30 x High MC attributed to 

bulk water leakage into 

roof through cracks in 

deteriorated rubber 

membrane 

Sheltair 

Scientific 1997 

Vancouver, 

BC

Field 

experiment

Sep–Mar 4 T, Sh — 8 14 x Sheathing generally 

wetter than framing 

x Highest MC found in 

sheathing of northfacing slopes in 

midwinter 

x Houses with higher 

indoor RH had higher 

MC, irrespective of attic 

ventilation

Cold climate 

Jordan et al. 

1948

Madison, WI Field 

experiment

Feb 3 Sh 13.3 10 19 x Sheathing MC and attic 

air RH correlated with 

RH in living space 

Marshall

Macklin 

Monaghan 1983 

Canada—

various

provinces

Survey --- 44 Framing, Sh — <15 >22 x High MC associated 

with cold weather, winddriven rain, and low 

solar gain 

Buchan, 

Lawton, Parent 

1991

Charlottetow

n, PEI & 

Ottawa, ON 

Survey 1 yr 20 J, T, Sh — 8 ~80 x Max MC in winter; min 

in summer 

x High MC correlated 

with high indoor RH 

Rose 1992 Central IL Test 

structure 

2 yr 8 Sh — 8 >30 x Max MC in winter; min 

in summer 

x MC increased when 

small air leak was 

introduced from living 

space into attic 

x MC and temperature in 

cathedral ceiling cavities 

20

more variable than in 

attics 

Rose 1995a Central IL Test 

structure 

2 yr 4 Sh — <6 >30 x Lower MC in cavities 

with air chute between 

sheathing and insulation 

in cathedralized attics 

TenWolde 1997 Central IL Test 

structure 

Feb & May 1 Sh — 5 11 

Winandy et al. 

2000

Madison, WI Test 

structure 

8 yr 5 Sh — 6 20 x Max MC in winter; min 

in summer 

Very cold climate 

Sheltair 

Scientific 1997 

Edmonton,

AB

Field 

experiment

Sep–Mar 4 T, Sh — 7 18 x Sheathing generally 

wetter than framing 

x Highest MC found in 

sheathing of northfacing slopes in 

midwinter 

x Houses with higher 

indoor RH had higher 

MC, irrespective of 

attic ventilation a

 For field experiments, forensic investigations, and surveys, N is number of buildings sampled; for test structures, N is number of roof configurations. b

 J, ceiling joists; R, rafters; T, trusses; Sh, sheathing. 

c

 Moisture content measurements above 30% by electrical resistance are approximate (see Section 1.4).

Table 33—Moisture contents of unfastened SYP 

boards by gravimetric analysis and of fastened SYP 

deck boards by electrical resistance with four 

treatments over three time periods (see text) in 

North Carolina (Cui and Zahora 2000) 

Gravimetric MC (%) Resistance MC (%)

Period A B C A C 

ACQ 13–41 8–27 8–43 12–28 17–29 

ACQ–WR 15–27 11–16 9–15 13–27 10–14 

CCA 11–34 7–16 8–26 9–27 10–18 

CCA–WR 11–16 8–13 8–11 11–17 8–11 

46
of the study was to observe the condition of galvanized steel 

nails in the foundation to determine the extent of corrosion 

and to measure wood moisture contents. The moisture content readings were taken using a resistance-type meter above 

grade and at depths no deeper than 2 ft (0.6 m) below grade. 

The MC in sheathing and framing above grade ranged from 

10% to 18%, whereas below grade it ranged from 10% to 

over 30%. Many of the foundations were protected by a 

polyethylene film, though in some instances the film was 

broken. There was not sufficient data to determine whether 

polyethylene in good condition was effective at protecting 

the wood from excessive moisture.

Moisture contents of framing and sheathing in a full basement PWF have been measured at the Forest Products Laboratory’s Advanced Housing Research Center (unpublished 

data). The plywood sheathing and 2 by 8 framing members 

were treated with CCA. The highest moisture contents, in 

excess of ~50%,21 were observed in foundation wall bottom plates during the later stages of construction, and soon 

thereafter. These high moisture contents reflected the construction sequence, in which gutter installation was the last 

construction step, followed by landscape grading. Spatial 

variability at this stage was very high, sometimes exceeding 30 percentage points in bottom plates. For calendar year 

2006, which spanned between 51 and 63 months after construction, bottom plate moisture contents were significantly 

lower with less spatial variability: values were between 16% 

and ~35%. For the same calendar year, above-grade sheathing MC values ranged from 12% to 20%, and below-grade 

sheathing values ranged from 12% to ~35%.

On the basis of these two studies, it can be concluded that 

moisture contents of permanent wood foundations apparently span a larger range for framing and sheathing below 

grade than above grade. Factors such as site drainage, soil 

conditions, and the presence of a protective film may affect 

moisture levels.

Conclusions

This literature review reports in-service moisture and 

temperature conditions for floor, wall, and roof structural 

members of wood-frame buildings and wood decks and permanent wood foundations. Because the data reported in the 

literature may not be statistically representative of woodframe buildings in general, it cannot be determined if a bias 

exists towards either higher or lower moisture contents. 

However, the studies include common construction found 

throughout North America, and the data therefore represent 

conditions that can occur. Moisture contents vary widely, 

spanning a range from as low as 2% to well above 30%.

The most extreme moisture contents in structural members 

above crawlspaces occur when the ground is not covered 

with a vapor diffusion retarder, especially at sites with poor 

drainage. Two different seasonal trends have been observed 

for crawlspaces. Studies prior to around 1955 (when buildings were typically not air-conditioned) in both mixed-humid and cold climates observed a maximum moisture content in winter and minimum in summer. When crawlspace 

vents were closed during winter, the uncovered soil supplied 

moisture that condensed on the coldest wood members near 

the exterior. In the spring, the moisture was driven off as 

wood temperatures rose and crawlspace vents were opened. 

Studies after around 1955 in the hot-humid and mixed-humid climatic regions in various types of crawlspaces found 

that moisture content peaked in summer with a minimum in 

winter. Moisture from the soil and from warm, humid outdoor air entering the crawlspace was absorbed by the floor 

members, which were cooled by air-conditioning.

High moisture levels in walls occur in response to rainwater 

intrusion. When bottom plates are in direct contact with 

concrete slab foundations (no capillary break), MCs may 

approach 30%. High moisture contents have been found in 

exterior wall sheathing in cold and marine climates during 

winter where exfiltration and vapor diffusion are not impeded. The severity of this problem correlates with indoor 

humidity levels maintained during the winter. In these climates, framing members in insulated walls generally do not 

accumulate as much moisture as does exterior sheathing. 

The cold side of framing members tends to be wetter than 

the warm side. The sheathing and outer portions of framing 

members usually dry during the spring and reach a minimum moisture content during the summer (unless there is 

excessive rainwater intrusion). However, in some cases, the 

design or construction of the wall prevents effective drying. 

In hot-humid climates, the significant moisture transport 

mechanisms (besides rainwater intrusion) are infiltration of 

humid outdoor air and diffusion of water vapor inward, the 

opposite of cold climates in winter. In addition, solar radiation can drive moisture from the exterior sheathing into the 

wall cavity. When a vapor retarder is placed on the interior 

(cold side) of the wall cavity, condensation can occur on the 

vapor retarder. In a number of climates, studies that have 

investigated the effect of a ventilated space between the 

sheathing and cladding have found that this feature reduces 

moisture accumulation in the wall.

21

Table 33—Moisture contents of unfastened SYP 

boards by gravimetric analysis and of fastened SYP 

deck boards by electrical resistance with four 

treatments over three time periods (see text) in 

North Carolina (Cui and Zahora 2000) 

Gravimetric MC (%) Resistance MC (%)

Period A B C A C 

ACQ 13–41 8–27 8–43 12–28 17–29 

ACQ–WR 15–27 11–16 9–15 13–27 10–14 

CCA 11–34 7–16 8–26 9–27 10–18 

CCA–WR 11–16 8–13 8–11 11–17 8–11 

 47
The highest moisture contents in roof sheathing (aside 

from bulk leakage) occur during winter for all the climatic 

regions reported here. Rafters and trusses generally do not 

accumulate as much moisture as does sheathing, but they 

do exhibit the same trend of high moisture content in winter 

and low moisture content in summer. Wintertime moisture 

accumulation in roof members is greater when indoor humidity levels are high and solar gain is low. The dominant 

moisture transport mechanism is likely exfiltration of humid 

indoor air through ceilings lacking effective air barriers. 

Ventilation of the attic space has not been a dependable 

method of controlling moisture levels in roofs when air 

leakage is significant.

Exposed wood decks constructed with untreated boards or 

pressure-treated boards without water-repellent additives 

exhibit large fluctuations in moisture content with weather. 

Water absorption occurs more rapidly when the wood surface is checked. Treating the boards with a preservative and 

water-repellent additive significantly dampens these fluctuations. The water repellency of certain treatments improves 

over time. Water repellents also minimize checking and 

cupping.

Moisture contents in permanent wood foundations are generally higher below grade than above grade. Factors such as 

site drainage, soil conditions, and the presence of a protective film may affect the moisture levels.

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