United States 

Department of 

Agriculture 

Forest Service 

Forest 

Products 

Laboratory 

Research 

Paper

FPL 455 

Condensation 

Potential in 

High Thermal 

Performance Walls 

Hot, Humid Summer Climate

Gerald E. Sherwood 
Abstract 

As a result of steadily rising energy costs, construction 

practice for light-frame wood structures has changed over 

the past few years. The use of 6-inch-thick walls and 

application of high “R” value, low-permeance sheathings to 

4-inch walls has caused concern for the changing moisture 

patterns that may occur in the wall. Excessive moisture in 

wall cavities can have detrimental effects including decay of 

wood components if the moisture remains for extended 

periods coincident with warm temperatures. To observe 

actual moisture patterns and the potential for condensation 

due to long periods of air conditioning in a hot, humid 

climate, a test structure was constructed near Gulfport, 

Mississippi, for exposure of eight types of insulated wall 

panels at controlled indoor conditions and typical outdoor 

weather conditions. Panels were instrumented with moisture 

sensors and tested without (Phase 1) and with (Phase 2) 

penetrations (electrical outlets) in the indoor surface. 

There was no sustained condensation in any of the walls 

during either winter season. One type of high thermal 

performance wall had sustained condensation during both 

summers, but the wall dried out as the weather became 

cooler, and moisture content of framing never exceeded 

17 percent. Low-permeance sheathing appeared to provide 

resistance to the buildup of moisture during summer in walls 

with high overall “R” values. Penetrating the walls with 

electrical outlets resulted in slightly higher moisture levels in 

all of the walls throughout the year. This paper should be 

useful to building designers, builders, and building code 

officials in establishing vapor retarder requirements for walls. 

Keywords: Condensation, moisture control, vapor retarder, 

air leakage, wood-frame walls, foam sheathing. 

Acknowledgments 

The author acknowledges the valuable contributions of Doug 

Wheeler, formerly of FPL, in instrumenting the test structure 

and providing electronics expertise throughout the 2-year 

study. Thanks also go to Terry Niedziela, also formerly of 

FPL, for his creative solutions to problems during the 

construction, instrumentation, and installation of test panels. 

July 1985 

Sherwood, Gerald E. Condensation potential In high thermal 

performance walls-hot, humid summer climate. Res. Pap. FPL 455. 

Madison, WI: U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory; 1985. 29 p. 

A limited number of free copies of this publication are available to the 

public from the Forest Products Laboratory, One Gifford Pinchot Drive, 

Madison, WI 53705. Laboratory publications are sent to over 1,000 

libraries in the United States and elsewhere. 

The Laboratory is maintained in cooperation with the University of 

Wisconsin. 
Condensation

Potential in

High Thermal

Performance Walls

Hot, Humid Summer Climate1

Gerald E. Sherwood, Engineer 

Forest Products Laboratory, Madison, WI 

Introduction 

High-efficiency thermal insulation systems for wood-frame 

residential construction have become essentially standard 

for many parts of the country in recent years. These 

systems include rigid foam wall sheathing, foil-backed foam 

wall sheathing, or nominal 6-inch wall studs with 6-inch 

insulation batts, all of which provide walls with higher “R” 

values and lower perm values. The higher “R” values will 

result in colder surfaces with greater condensation potential 

and lower perm values will restrict moisture movement. 

Theoretically, all of these systems should result in within-wall 

moisture patterns different from those of conventional walls 

with nominal 4-inch studs and wood or woodbase sheathing 

materials. 

Studies were conducted to evaluate the potential detrimental 

effects of moisture accumulation in wall cavities in both a 

cold climate and in a hot, humid climate with a long 

air-conditioning season. Results from the cold climate were 

reported in a previous paper (Sherwood 1983). Results from 

a hot, humid climate-i.e., Gulfport, Mississippi-are 

reported in this paper. In that location, average temperatures 

during summer months are 80 to 83 °F with frequent highs 

approaching 100 °F. Average relative humidities during 

summer months are 85 percent at 4 a.m. (coolest time of 

day) and 64 percent at 1 p.m. (warmest time of day). 

1

This research was conducted in cooperation with the American Hardboard 

Association, Dow Chemical, U.S.A., Jim Walter Research Corporation, and 

the U.S. Department of Housing and Urban Development. 

Excessive moisture in wall cavities can have several 

detrimental effects. It may decrease the effectiveness of the 

cavity insulation (Joy 1957). If the cavity remains wet for 

extended periods coincident with warm temperatures in the 

wall, wood structural components may decay. Under winter 

conditions, condensation tends to be on sheathings or 

siding. Outdoor temperature and indoor humidity are the 

critical variables since indoor moisture is moving toward the 

drier outdoors and will condense if sheathing or siding are 

below dewpoint temperature. The result may be buckling or 

warping of siding or paint peeling (Anderson and Sherwood 

1974). Under summer conditions, condensation tends to 

occur on the back of gypsum board or on the vapor retarder 

if one is installed. Indoor temperature and outdoor humidity 

are the critical variables since outdoor moisture is moving 

toward the drier air-conditioned space and will condense if 

the gypsum board or vapor retarder are below dewpoint 

temperature. The result may be buckling of interior finish 

materials or mildew and mold on the surface. 

The potential for these detrimental effects can be assessed 

based on measurements of moisture levels at various 

locations in walls exposed on one side to a complete annual 

cycle of outdoor weather conditions while having the 

opposite side exposed to indoor conditions with controlled 

temperature and humidity. A better understanding of the 

moisture patterns in these highly thermal-efficient walls is 

needed in order to establish moisture control practices. 

This study is part of an ongoing program of thermal/moisture 

research at the Forest Products Laboratory (FPL) to 

determine the potential for condensation in walls. Because 

all variables could not be considered in a single study, 

additional studies are planned in both controlled laboratory 

tests and field observations of complete houses. 
Background Materials and Methods 

The results of previous research at FPL on moisture 

condensation in walls have been summarized (Anderson 

and Sherwood 1974). General recommended practice 

applies mostly to cold climates, but there is concern for how 

warm the winter must be to eliminate the need for a vapor 

retarder on the inside face of the wall. There is also concern 

that an outside vapor retarder may be needed during hot, 

humid summers to reduce moisture movement to the interior 

face of the wall. Closed-cell foam sheathings or foil-backed 

foam sheathings act as outside vapor retarders, and could 

reduce moisture movement toward the inside in the summer. 

The fact that moisture reduces the thermal resistance of 

insulating materials was established by Joy (1957) in the 

1950’s. A more recent study by Burch and Treado (1978) 

showed that for certain conditions, condensation occurred as 

a thin film on cold surfaces and had minimal effect on rate of 

heat transfer because it did not wet the insulation. However, 

wet insulation has been found in walls after prolonged 

periods of condensation. In some cases the condensation 

runs to the bottom of the wall cavity, saturating the sole 

plate as well as the lower few inches of insulation. 

Moisture also reduces the thermal resistance of wood and 

wood products. A method for estimating that reduction is 

presented in the Wood Handbook (USDA 1974). More 

serious effects of moisture on wood are dimensional 

changes and the potential for decay, though this author is 

not aware of documented reports of extensive decay in 

wood-frame walls due to condensation. Such decay is a 

greater threat in warm climates than in cold climates 

because decay fungi require temperatures above 40 °F for 

growth (USDA 1974). The most visible problems are mildew 

and paint peeling or blistering. 

Previous air-conditioning studies have been conducted in the 

relatively mild climate of Athens, GA (Duff 1971), but no 

documented studies from hot, humid climates are available. 

The actual moisture patterns through the cross section of a 

variety of walls exposed to outdoor conditions are needed to 

evaluate the effect of construction types. This can best be 

accomplished by exposure structures in more than one 

climate to include the effect of climate on moisture patterns. 

Exposure Structures 

Two structures were built for the purpose of exposing test 

walls to outdoor weather conditions on one side while 

exposing the opposite side to typical indoor conditions. One 

structure was erected near Madison, WI, the other near 

Gulfport, MS. The two locations were planned to provide 

data on moisture patterns in a cold winter climate and in a 

hot, humid climate. This paper is limited to tests at Gulfport. 

Results from the Madison building were discussed in a 

previous report (Sherwood 1983). 

The buildings are long and narrow, 8 feet wide by 48 feet 

long, with the long axis east-west for maximum exposure of 

north and south walls (fig. 1). The center 8-foot-long section 

is an instrument room. The remaining length of the building 

is partitioned every 4 feet resulting in ten 4- by 8-foot rooms 

(fig. 2) connected by doors in partitions. The only exterior 

door is in the instrument room. Support for the roof and 

ceiling is provided by partitions (fig. 3), so exterior wall 

panels can be removed and replaced while the building 

remains intact. Four- by eight-foot wall panels were 

completely instrumented during fabrication and then installed 

by lag bolting them to partitions. Identical panels were 

installed on north and south walls for extremes of exposure. 

Both the ceiling and floor are insulated with R-382

 glass-fiber 

batts to limit heat transfer so the walls would be the major 

element of heat loss from each room. 

Figure 1.—Experimental structure near Gulfport, MS. 

(M150 968-22) 

2

“R” is a measure of insulating value or resistance to heat flow. It is the 

reciprocal of conductance, which is the amount of heat in Btu’s that will flow 

in 1 hour through 1 square foot of homogeneous material per 1 °F 

temperature difference between surface of materials. 

2 
Calculated outdoor 

temp. for freezing at 

Sheathing, insulation & Total “R” sheathing·insulation 

vapor barriers for walls (Calculated) interface, °F 

Fiberboard, R-13 batt, 16.13 27 6-mil polyethylene 

Fiberboard, R-11 blanket, 14.13 26 asphalted paper 

Plywood, R-11 blanket, 13.53 28 asphalted paper 

Fiberboard (6” stud), 

R-19 batt (compressed),

6-mil polyethylene 21.13 28 

N 

Extruded polystyrene foam, 20.27 15 R-13 batt, 6-mil polyethylene

Extruded polystyrene foam, 18.31 13 R-11 blanket, asphalted paper

Foil-backed, GF-reinforced

polyisocyanurate foam w/vent 23.21 9

strip, R-13 batt, 6-mil

polyethylene

Foil-backed, GF-reinforced

polyisocyanurate foam,

R-13 batt, 6-mil polyethylene

Figure 2.—Plan of experimental structure showing variables of construction for each wall panel. 

(ML83 5060) 

3 
Rooms are individually heated by a resistance-type electric 

heater, and individually cooled by a window-type air 

conditioner mounted in the floor. Humidification is available 

by a vaporizing-type humidifier in each room during the 

heating season, but was not needed to maintain a relative 

humidity (RH) above 40 percent. Humidity is not controlled 

during the air-conditioning season. Heaters are controlled by 

wall thermostats to maintain a temperature between 67 and 

70 °F. Air conditioners are set to cycle on at 79 °F and off at 

76 °F. Heating season RH is maintained at a minimum of 

40 ± 5 percent. Ceiling fans operate when either the heater 

or the air conditioner was running. 

End rooms are considered buffers rather than test rooms as 

they have an 8- by 8-foot end wall exposed to the exterior 

and would not have heat loss, heat gain, or water-vapor loss 

comparable to other rooms with only a north and south wall 

exposed. This leaves eight identical rooms in each building 

for test and comparison purposes. Test panels of the same 

construction are inserted on north and south exposures of a 

room, so there is only one type of wall construction for each 

room. 

Figure 3.—Cross section of experimental structure 

showing construction details. (ML83 5067) 

lest Panels 

For this study, all test panels have 1/2-inch gypsum board 

on the inside and 7/16- by 12-inch primed hardboard lap 

siding on the outside. Hardboard was painted after panels 

were fabricated. Full-thickness glass-fiber insulation was 

placed in each wall cavity. One type of panel was framed 

with 2 by 6 studs at 24-inch spacing; all other panels were 

framed with 2 by 4 studs at 16-inch spacing. The primary 

variables are the sheathing material and the vapor retarder 

(fig. 2). In addition, one panel with foil-backed 

polyisocyanurate foam was vented at the top. Polystyrene 

sheathing was in 2- by 4-foot sections; all other sheathings 

were in 4- by 8-foot sections. Sheathing materials included: 

1/2-inch fiberboard, 1/2-inch plywood, 1-inch extruded 

polystyrene foam, and l-inch foil-backed glass-fiber 

reinforced polyisocyanurate foam. Only two types of vapor 

retarders were used: 6-mil polyethylene film continuous over 

the face of the framing (fig. 4), or asphalted kraft paper 

backing on blanket insulation stapled between studs (fig. 5). 

Although the asphalted kraft paper could be installed by the 

recommended method of lapping all joints over studs, in field 

practice it is often stapled between studs with no laps 

(fig. 5). That method was followed to simulate typical field 

conditions. 

Figure 4.—Six-mil polyethylene film being applied as 

a continuous vapor retarder on an experimental 

panel. Lead wires to moisture sensors and 

thermocouples are brought through a small slit that is 

thoroughly caulked to preserve the integrity of the 

vapor retarder. (M147 188-11) 

4 
Each test panel was instrumented with moisture sensors at 

11 locations in the wall (fig. 6). A thermocouple was also 

placed at each moisture sensor location. At heights of 1 and 

7 feet above the floor, moisture measurements were made 

at the siding-sheathing interface, at the sheathing-insulation 

interface, at the center of the cavity insulation, and in the 

adjacent stud. Sensors were also located in the center of the 

top plate, the center of the sole plate, and between siding 

and sheathing at the midheight of the wall. Since the 

purpose of the study was to monitor the moisture content 

(MC) of wood components, there was no moisture sensor 

placed at the vapor retarder interface. Brief periods of 

condensation could have occurred there and been 

undetected unless the condensation affected MC of 

insulation or ran down to the sole plate. Lead wires from all 

these data points were brought into the room through the 

vapor retarder and gypsum board at two points (1 and 7 feet 

above the floor). The punctures in the vapor retarders were 

caulked around each wire individually (fig. 4). 

All test panels were without open punctures in the gypsum 

board or vapor retarder for the first year-Phase l-of the 

study. In the second year of testing–Phase 2–a standard 

duplex electrical outlet was installed in each wall panel to 

observe the effect of air leakage into the wall cavity. In 

conventional construction, joints around windows or at 

baseboards and other discontinuities in the vapor retarder 

may result in additional leakage. For this study the electrical 

outlet was selected as uniform penetration to provide air 

leakage for comparison purposes. 

After installation of test panels, all joints with floor, ceiling, 

and partitions were caulked. On the outside, vertical joints 

between panels were caulked, and the joint between floor 

framing and the bottom edge of the wall panel was caulked. 

Six-mil polyethylene taped to each face of the partitions 

extends out between adjoining panels to prevent transfer of 

moisture between panels (fig. 1). 

Figure 6.—Moisture sensor locations (·) in each test 

panel: Four sensors are in the framing; two are in the 

center of insulation; two are at the 

Figure 5.—Asphalted kraft paper vapor retarder insulation-sheathing interface; and three are at the 

stapled to sides of studs. (M147 191-11) sheathing-siding interface. (ML83 5062) 

5 
Data Acquisition 

Moisture Content 

Moisture conditions were measured at 176 locations in the 

walls (fig. 6) using small wood sensors. The MC of the wood 

sensor was converted to MC of the members in which they 

were imbedded, or MC at the interface between two 

materials based on the RH of the air in the immediate 

vicinity. The RH in the rooms and outdoors was also 

recorded. 

The sensors were calibrated wood elements in which 

electrical resistance changed with MC of the wood. 

Construction and details of operation of this sensor are 

given by Duff (1966). The sensors were calibrated in 

humidity rooms to an accuracy of ±2 percent MC over an 

RH range of 35 to 90 percent, which corresponds to an MC 

in the wood sensor of 7 to 20 percent. Determination of MC 

beyond these limits was less accurate due to difficulties in 

measuring extreme ranges of resistance and beads of 

condensed water often present on surfaces at sensor 

readings of 20 percent or higher. 

To effectively measure the very high resistance inherent in 

the sensor and to accurately transmit data to the logger, 

amplifiers were located as close to each sensor as practical; 

their output was connected to the data logger and calibrated 

(fig. 7). 

The resistance readings were first converted to MC for the 

sensor species and corrected for temperature effects. 

Further conversions were then made to provide the MC of 

the species of the wood sensor in structural members or to 

provide the RH of ambient air conditions. 

Temperature 

Temperature measurements were made at each wood 

sensor with a type T (copper-constantan) thermocouple and 

used for the temperature corrections. 

Data Recording 

All of the moisture and temperature data were digitized and 

recorded on cassette tape using a multichannel, 

programmable data logger. Readings were made three times 

per day at 1 a.m., 9 a.m., and 5 p.m. 

Figure 7.—Flow chart of instrumentation and data 

recording system. (ML83 5063) 

6 
Results 

Examination of the building after completion of the test 

revealed that panel 7S had a large gap between the sole 

plate and the sheathing, as the result of damage during 

construction. Since this allowed outdoor air movement into 

the wall cavity, data from this panel are not reported in the 

results. 

Phase 1 — No Penetrations 

During the first year of operation, there was little change in 

moisture levels in the fall and spring seasons, thus moisture 

levels will be discussed only for winter (December 

1979-February 1980) and for summer (June-August 1980). 

Heating and cooling degree-days for the time periods are 

shown in table 1. During winter, the north exposure is the 

most severe, so plots of MC in north panels are shown for 

December 1979 and January and February 1980 (figs. 8-15). 

For summer, the south exposure is generally more critical, 

so plots of MC in south panels are shown for June, July, 

and August 1980 (figs. 8-15). High and low outdoor 

temperatures for these time periods are recorded in 

figure 16. 

Although south facing walls have been considered more 

critical for summer condensation, the results of this study 

showed that only the panels with wood-base sheathing had 

greater moisture rise in south walls than in north walls. This 

trend did not exist in walls with foam sheathing. A possible 

explanation is that when the sun heats the sheathing on the 

south wall, water evaporates from both surfaces of the 

sheathing and, thus, moisture is drawn into the center of the 

wall. When the sun goes down, water is reabsorbed from 

outside air so there is a net flow of water into the wall cavity 

roughly equal to the amount that evaporated into the cavity 

earlier. This type of moisture flow happens only if the 

sheathing is hygroscopic. Since wood is hygroscopic and 

foam is not, the difference in north and south walls is seen 

only in the walls with wood-base sheathing. 

The MC at most data points in the building remained low 

(< 12 pct) throughout the winter. Only three moisture probes 

rose above the 12 percent level (table 2), and these were 

not sustained for long time periods. All of these moisture 

probes were located in framing members: one each in 

panels 4N, 8N, and 9N. Since there was no rise in moisture 

level of the wall cavity, these peaks were apparently not the 

result of condensation but were probably the result of some 

outside moisture influence. The results of the study indicated 

that winter condensation is not a problem with any of the 

types of walls tested in the climate of Gulfport, MS. 

However, all of the walls tested had some type of vapor 

retarder. 

Table 1.—Summary of heating and cooling degree-days for 

the study time periods 

Heating Cooling 

Month degree-days degree-days 

(65 °F base) (80 °F base) 

PHASE 1 

December 1979 416 

January 1980 323 

February 1980

June 1980 

444 

62 

July 1980 

August 1980 

128 

83 

PHASE 2 

December 1980 441 

January 1981 578 

February 1981

June 1981 

322 

60 

July 1981 98 

During the summer, all of the 4-inch walls with no foam 

sheathing (2S, 3S, and 4S) had MC’s in the low range at all 

data points (table 2) with one exception. Panel 2S had one 

probe with MC’s slightly into the moderate (12-16 pct) range, 

but this was located between sheathing and siding, which 

indicates some interaction with outside moisture such as rain 

or humid air, rather than wall-cavity moisture. All of the 

panels with foam sheathing (6S, 8S, and 9S) had MC’s 

reaching moderate or high (16-20 pct) levels in the center of 

the insulation, but only one of these panels had elevated 

MC’s that were sustained beyond two readings (table 1) and 

condensation (>20 pct MC) was not indicated at any time. 

All framing remained at low MC for those panels with foam 

sheathing. The one panel that had condensation as well as 

a rise in MC of framing members was 5S. 

Panel 5S had indication of condensation in the insulation 

near the top of the wall during most of the summer. In 

addition, the sheathing and framing near the top showed 

substantial increases in MC, though only into the moderate 

range. In September, the wall was opened for a visual check 

and the insulation was found to be wet enough that water 

could be squeezed from it. Direct measurements with a 

moisture meter showed the studs and top plate had MC’s of 

15 to 16 percent which generally verified moisture-probe 

readings. The wet moisture probe in panel 5S was replaced 

to avoid future malfunction due to fungal growth. 

7 
For comparison, a panel with foam sheathing and a 4-inch 

wall panel with fiberboard sheathing were opened and 

inspected. In both cases the framing MC was in the 11 to 

12 percent range, which also verified the moisture-probe 

readings. There was also no visual indication of 

condensation, such as waterstains, in either of these two 

walls. 

None of the panels had moisture levels in the framing that 

would support decay; water in the insulation in panel 5S was 

visible evidence of conditions that could lead to decay if 

continued over a long time period. Also, as with other 

studies (Joy 1957), the effectiveness of the insulation was 

greatly reduced by the water. 

The overall results from the walls without penetrations in the 

vapor retarder are summarized as follows: 

1. No condensation was detected in any of the walls 

tested during winter. However, all of the walls had some 

type of warm-side vapor retarder. 

2. The framing in all but one panel was at 11 to 

12 percent MC throughout the summer. 

3. The wall panel with 6-inch studs had condensation in 

the insulation near the top through all of the summer months 

(June, July, and August) and the MC of framing was 15 to 

16 percent by the end of summer. 

4. Walls with wood-base sheathing had greater increase 

in MC on the south than on the north; this trend was not 

observed in walls with foam sheathing. 

Table 2.—Humidity as indicated by moisture content1 of wood probes In insulation and framing during Phase 1 (no penetrations). Elevated MC’s are considered only where at least three 

consecutive readings are in that range 

Panel No. 

Insulation-sheathing 

interface 

Sheathing-siding 

interface Panel No. 

Insulation Framing 

December January February December January February June July August June July August 

2N Low Low Low Low Low LOW 2S LOW Low Low Low Low Low 

3N Low Low Low Low Low LOW 3S Low Low Low Low Low Low 

4N Low Low Low Low Low Low 4S LOW Low Low Low Low Low 

5N Low Low Low Low Low Low 5S Condensation3 Condensation3 Condensation3 Low Low Moderate 

6N Low Low Low Low Low LOW 6S LOW Low Low Low Low Low 

7N Low Low Low Low Low Low 2

7S — — — — — — 

8N Low Low Low Low Low Low 8S LOW Low Low Low Low Low 

9N Low Low Low Low Low LOW 9S Low Moderate Moderate Low Low Low 

1

Low = <12 percent MC; moderate = 12 to 16 percent; high = 18 to 20 percent; condensation = >20 percent.

2

Data from panel 7S are not shown because construction damage allowed intrusion of outdoor air into the panel, thereby invalidating the data.

3

Range of humidity was not replicated on opposite facing walls for the same time period. The reasons for differences were nol resolved, and indicate the need for further study.

8 
Figure 8.—Moisture content of wood probes in panel 2N (polyethylene, R-13 glass fiber, fiberboard), December 1979 through 

February 1980; and in panel 2S, June through August 1980. (ML85 5042) 
10
Figure 10.—Moisture content of wood probes in panel 4N (asphalted paper, R-11 glass fiber, plywood), December 1979 through 

February 1980; and in panel 4S, June through August 1980. (ML85 5044) 
Figure 11.—Moisture content of wood probes in panel 5N (polyethylene, R-19 glass fiber, fiberboard), December 1979 through 

February 1980; and in panel 5S, June through August 1980. (ML85 5045) 
13
Figure 13.—Moisture content of wood probes in panel 7N (asphalted paper, R-11 glass fiber, polystyrene), December 1979 through 

February 1980. Data from panel 7S are not shown because construction damage allowed intrusion of outdoor air into the panel, 

thereby invalidating the data. (ML85 5047) 
15
16
Figure 16.—High and low outdoor temperatures 

during Phase 1, December 1979 through February 

1980 and June through August 1980. (ML85 5059) 

Phase 2—Outlet Penetrations 

Moisture in all north panels during December 1980 and 

January and February 1981 (Phase 2) is plotted in 

figures 17 through 24. High and low outdoor temperatures 

for Phase 2 plots are shown in figure 25. Moisture in all 

southfacing panels during June and July 1981 is also plotted 

in figures 17 through 24. Heating and cooling degree-days 

for these time periods are shown in table 1. A direct 

lightning strike near the end of July resulted in discontinuing 

the data collection. Complete replacement of the 

instrumentation would have been required for continued 

operation. 

Moisture levels in all the panels increased after the walls 

were penetrated by electrical outlets (table 3). Instead of 

being in the low range as during the previous winter, 

readings were in the moderate range through both the winter 

and summer. Panels 2N through 6N had no readings above 

the moderate level during the winter months. Panel 7N had 

short periods of high moisture in the sole plate. Since no 

other moisture probes in that panel had elevated moisture 

levels and condensation would be expected to occur in other 

parts of the wall before it would occur on the sole plate, it is 

expected that the sole plate was being influenced by an 

outside moisture source such as rain or outdoor humidity. 

The other panels that had elevated moisture levels were 

panels 8N and 9N. Rooms 8 and 9 both had RH’s of 60 to 

70 percent during the winter compared to about 40 percent 

in all other rooms. There was no moisture being added to 

any of the rooms, so the only moisture source was residual 

moisture from the previous summer. Most of the rooms dried 

out as they were heated, but Rooms 8 and 9 retained 

moisture. This may have been caused by the sheathing, 

which was essentially impermeable to water vapor. This 

possibility is supported by the study of the cold winter 

climate (Sherwood 1983) in which considerably less water 

was required to humidify Rooms 8 and 9 than to humidify 

the other rooms. The high humidity condition would not 

necessarily exist in an occupied building with ventilation from 

exhaust vents, opening doors, and leakage around windows. 

During the summer of 1981, most of the MC’s in all panels 

remained in the moderate range. Panel 2S had some MC’s 

in the high range that appeared to be from outside 

influences. The highest level was at the siding-sheathing 

interface. The MC of the stud near the bottom moved also 

slightly into the high range at times, but there was no 

increase of MC at other locations in the wall. Panels 3S and 

4S had no MC’s in the high level at any time. Panels 6S and 

9S both had several one-time readings of 20 percent, but 

there was no extended period of condensation in either 

panel. 

17 
The only panel with any extended period of condensation 

was, again, panel 5S. MC readings indicated that insulation 

near the bottom became wet in late June and remained wet 

through the recording period in late July. The stud near the 

bottom also showed MC’s of 20 percent for a period of 

5 days in July. Other moisture probes near the bottom of the 

wall cavity had readings in the high range. At the end of the 

summer, panel 5S was opened for inspection and the 

insulation was found to be wet. The MC of the framing was 

determined by a moisture meter and found to be about 

16 percent. 

The overall results from the walls with penetrations are 

summarized as follows: 

1. The MC at all points in all the panels increased about 1 

to 3 percent after the walls were penetrated by an electrical 

outlet. 

2. None of the walls had extended periods of 

condensation during the winter. 

3. Under test conditions with all rooms essentially sealed, 

RH in the rooms with foil-backed sheathing remained higher 

during winter than in other rooms. 

4. Because of the high room RH during winter, panels 

with foil-backed polyisocyanurate sheathing had high levels 

of moisture in the walls. 

5. The only panel having extended periods of 

condensation during the summer was the wall with b-inch 

studs. 

Examination of the Test Structure 

After completion of both phases of the test, the experimental 

wall panels were disassembled and examined for evidence 

of moisture and verification of test data. Panels were 

disassembled in place from the outside, beginning with 

siding and examining each interface of materials as the 

process continued. One observation was a lack of 

compression of the foam sheathings at the sole plate, which 

could have resulted in air leakage at that joint. The 

examination generally verified test data and provided 

evidence to support some conclusions. There was no fungal 

growth or other deterioration in the wood or wood-based 

materials in any of the test panels. There were adhesions of 

glass fiber to polyethylene in the fiberboard-sheathed walls 

and no such adhesions in plywood- or foam-sheathed walls. 

Since these adhesions are normally due to the presence of 

water, the low-permeability sheathings appeared to be 

effective in limiting moisture movement into the wall cavity 

during the air-conditioning season. Streaking on the 

polyethylene in fiberboard-sheathed walls and waterstains 

on sole plates were further evidence that some 

condensation had occurred for limited time periods and ran 

down to the sole plate. Even where plates were 

waterstained, there was no elevation of MC and no fungal 

growth, indicating that water was present for only brief 

periods. 

Table 3.—Humidity as indicated by moisture content1 of wood probes in insulation and framing during Phase 2 (with penetrations). Elevated MC’s are considered only where at least three 

consecutive readings are In that range 

Panel No. 

Insulation-sheathing 

interface 

Sheathing-siding 

interface Panel No. 

Insulation Framing 

December January February December January February June July June July 

2N Moderate Moderate Moderate Moderate Moderate Moderate 2S Moderate Moderate Moderate High 

3N Moderate Moderate Moderate Moderate Moderate Moderate 3S Moderate Moderate Moderate Moderate 

4N Moderate Moderate Moderate Moderate Moderate Moderate 4S Moderate Moderate Moderate Moderate 

5N Moderate Moderate Moderate Moderate Moderate Moderate 5S Moderate Condensation3 Moderate Condensation3 

6N Moderate3 Moderate3 Moderate3 Moderate Moderate Moderate 6S Moderate3 Moderate3 Moderate Moderate 

7N Moderate Moderate Moderate Moderate Moderate Moderate 2

7S — — — — 

8N Moderate Moderate Moderate Moderate Moderate Moderate 8S Moderate Moderate Moderate Moderate 

9N High3 High3 High3 Moderate Moderate Moderate 9S High3 Moderate3 Moderate Moderate 

1

Low = <12 percent MC: moderate = 12 to 16 percent; high = 16 to 20 percent; condensation = >20 percent.

2

Data from panel 7S are not shown because construction damage allowed intrusion of outdoor air into the panel, thereby invalidating the data.

3

Range of humidity was not replicated on opposite-facing walls for the same time period. The reasons for differences were not resolved, and indicate the need for further study.

18 
Figure 17.—Moisture content of wood probes in panel 2N (polyethylene, R-13 glass fiber, fiberboard), December 1980 through 

February 1981; and in panel 2S, June and July 1981. (ML85 5050) 
20
21
22
23
Figure 22.—Moisture content of wood probes in panel 7N (asphalted paper, R-11 glass fiber, polystyrene), December 1980 through 

February 1981. Data from panel 7S are not shown because construction damage allowed intrusion of outdoor air into the panel, 

thereby invalidating the data. (ML85 5055) 
25
26
Findings 

Figure 25.—High and low outdoor temperatures 

during Phase 2, December 1980 through February 

1981 and June through July 1981. (ML85 5058) 

The following findings apply to the climate of Gulfport, MS, 

at controlled indoor conditions of 67 to 70 °F during winter 

and 76 to 79 °F during summer. Indoor RH did not go below 

40 percent during winter and was not controlled during 

summer. The test building was electrically heated so there 

were no pressure changes due to combustion air 

requirements. Air-conditioner fans operated only when the 

air conditioner was running; ceiling fans operated when 

either the heater or air conditioner was running. 

1. No condensation was detected in any of the walls 

during the first winter with no penetrations in the walls 

(Phase 1). 

2. The only wall with sustained condensation during the 

first summer with no penetrations in the walls (Phase 1) was 

the wall with 6-inch studs. 

3. The MC at all points in all walls increased from about 

11 percent to about 14 percent when the walls were 

penetrated by an electrical outlet (Phase 2). 

4. Although some walls had periods of elevated MC’s 

during the second winter with penetrations (Phase 2), there 

were no extended periods of condensation recorded. 

5. Rooms with foil-backed foam sheathing had winter 

RH’s of about 70 percent compared to 40 percent in other 

rooms. This occurred both with and without penetrations. 

6. The only room having extended periods of 

condensation during the second summer with penetrations 

(Phase 2) was the wall with 6-inch studs. 

7. Framing in the wall with 6-inch studs had MC’s of 

about 16 percent at the end of the summer in both Phase 1 

and Phase 2. 

8. Penetration of vapor retarders increased MC’s in walls 

both in winter and summer. 

9. Moisture was driven from hygroscopic sheathing in 

south walls resulting in greater moisture increase than in 

north walls. This difference did not exist between north and 

south walls where nonhygroscopic foam sheathing was 

used. 

10. Low-permeance sheathings reduced the movement of 

outdoor moisture into the wall cavity of air-conditioned 

buildings. 

11. There was no deterioration of any wood or wood 

products in any type of wall construction tested. 

27 
Conclusions Literature Cited 

The findings of this study are limited to specific indoor and 

outdoor conditions; they should, however, be relevant to the 

southeastern United States. Where summer temperatures 

are consistently higher, the summer condensation potential 

is greater. Where winter temperatures are consistently lower, 

the winter condensation is greater: however, the indoor 

humidity may be lower in colder climates, which would 

reduce the condensation potential. 

All of the walls performed satisfactorily during the winters 

both with and without penetrations. Winter condensation was 

not a hazard in any of the wall constructions tested. 

Penetrating walls with electrical outlets results in elevating 

MC’s in all walls from about 11 percent to about 14 percent. 

Where summer condensation occurs over an extended time, 

MC of framing members may rise as high as 16 percent. 

Even where extended periods of condensation occur, walls 

dry out in the fall as temperatures drop. The complete 

absence of fungal growth in any of the wall panels indicates 

that the potential for deterioration of wood or wood products 

is minor. 

Because the walls with plywood or foam sheathing had no 

evidence of high moisture levels during the summer, and 

fiberboard-sheathed panels had evidence of condensation 

on the vapor retarder, some resistance to diffusion of water 

vapor from the outside face of the wall appears to be an 

advantage. A nonhygroscopic sheathing also appears to be 

an advantage where summer sun shines on the wall. If 

sheathing is hygroscopic, moisture is driven from the 

sheathing into the wall cavity, allowing the dry sheathing to 

take on more moisture from outdoor air when the sun is not 

shining. The cycle can then be repeated, resulting in a very 

humid condition in the wall cavity. The type of vapor retarder 

had no observable effect on moisture in walls during 

summer. 

These conclusions apply only to conventional construction 

and the indoor conditions stated. Higher humidities may 

occur due to construction moisture, extremely tight 

construction, or major indoor sources such as numerous 

house plants, unvented clothes driers, etc. Also, some 

manufactured houses may be constructed in a manner that 

greatly limits air movement through the wall cavity, and thus 

moisture patterns may be different. 

Anderson, L. O.; Sherwood, G. E. Condensation

problems in your house: Prevention and solution. Agric. Inf.

Bull. 373. Madison, WI: U.S. Department of Agriculture,

Forest Service, Forest Products Laboratory; 1974.

American Society of Heating, Refrigerating, and

Air-Conditioning Engineers, Inc. ASHRAE handbook of

fundamentals. New York: ASHRAE; 1977.

Burch, D. M.; Treado, S. J. A technique for protecting

retrofitted woodframe walls from condensation damage.

ASHRAE Transactions, Vol. 84, Part I. New York: ASHRAE;

1978.

Duff, J. E. A probe for accurate determination of moisture

content of wood products in use. Res. Note FPL-0142.

Madison, WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory; 1966.

Duff, J. E. The effect of air conditioning on the moisture

conditions in wood walls. Res. Pap. SE-78. Asheville, NC:

U.S. Department of Agriculture, Forest Service,

Southeastern Forest Experiment Station; 1971.

Joy, F. A. Thermal conductivity of insulation containing

moisture. Symposium on thermal conductivity measurements

and applications of thermal insulations. ASTM No. 21.

Philadelphia, PA: ASTM; 1957.

Sherwood, G. E. Condensation potential in high thermal

performance walls-cold winter climate. Res. Pap. FPL 433.

Madison, WI: U.S. Department of Agriculture, Forest

Service, Forest Products Laboratory; 1983.

U.S. Department of Agriculture, Forest Service,

Forest Products Laboratory. Wood handbook-wood as

an engineering material. Agric. Handb. 72. Washington, DC:

Government Printing Office, Superintendent of Documents;

rev. 1974.

2.0-7/85 

28 
The Forest Products 

Laboratory (USDA Forest 

Service) has served as the 

national center for wood 

utilization research since 

1910. The Laboratory, on the 

University of WisconsinMadison campus, has 

achieved worldwide 

recognition for its 

contribution to the knowledge 

and better use of wood. 

Early research at the 

Laboratory helped establish 

U.S. industries that produce 

pulp and paper, lumber, 

structural beams, plywood, 

particleboard and wood 

furniture, and other wood 

products. Studies now in 

progress provide a basis for 

more effective management 

and use of our timber 

resource by answering critical 

questions on its basic 

characteristics and on its 

conversion for use In a variety 

of consumer applications. 

Unanswered questions remain 

and new ones will arise 

because of changes in the 

timber resource and 

increased use of wood 

products. As we approach the 

21st Century, scientists at the 

Forest Products Laboratory 

will continue to meet the 

challenge posed by these 

questions.