United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory

Research

Paper

Condensation

Potential in High

Thermal Performance

FPL 433 Walls – Cold Winter

Climate

by G. 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 walls. To

observe actual moisture patterns and the potential for

condensation, a test structure was constructed near

Madison, Wis., 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.

Continuous vapor retarders effectively prevented

condensation; asphalted paper stapled between studs

was inadequate. The installation of an electrical outlet

changed the moisture profile and resulted in some

condensation in most panels. Moisture levels on the

back of siding in most Phase 2 panels have been

known to produce buckling in long sections of

hardboard siding. Although streaking occurred on the

siding of two types of Phase 1 panels and three Phase

2 types, and some condensation occurred in all types

of Phase 2 panels, there was no long-term

accumulation of free water in the structure. The

moisture content of framing remained below 12 percent

throughout the P-year study. There was no apparent

increase in condensation potential with the addition of

low-permeance foam sheathing in this study with

controlled indoor conditions.

This paper should be useful to building designers,

builders, and building code officials.

KEYWORDS: Condensation, Moisture control, Vapor

retarder, Air leakage, Wood-frame walls, Foam

sheathing.

Acknowledgments

The author acknowledges Doug Wheeler’s valuable

contributions in instrumenting the test structure and

providing electronics expertise throughout the P-year

study. Thanks also go to Terry Niedziela for his creative

solutions to problems during the constructon,

instrumentation, and installation of test panels.
United States

Department of

Agriculture

Forest Service

Forest

Products

Laboratory1

Research

Paper

FPL 433

May 1983

Condensation Potential

in High Thermal

Performance Walls–

Cold Winter Climate2

BY

G. E. SHERWOOD, Engineer

Introduction

The escalation of energy costs in recent years plus a

concern for reducing dependence on foreign oil has

resulted in the development of highly efficient thermal

insulation systems for wood-frame residential

construction. These systems include rigid foam wall

sheathing, foil-backed foam wall sheathing, or nominal

6-inch wall studs with 6-inch insulation batts.

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 wood-base sheathing materials.

Excessive moisture in wall cavities can have several

detrimental effects. It may decrease the effectiveness

of the cavity insulation (7)3. If the cavity remains wet for

extended periods coincident with warm temperatures in

the wall, wood structural components may decay. As

the moisture moves to the outer face of the wall it may

cause buckling or warping of siding or paint peeling (7).

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.

1Maintained at Madison, Wis., in cooperation with the University of

Wisconsin.

2

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

3

Italicized numbers in parentheses refer to literature cited at end of

report.

This paper presents the results of a study of moisture

patterns in a variety of wall construction types exposed

to weather conditions in a cold winter climate-i.e.,

Madison, Wis. This study is part of an ongoing program

of thermal/moisture research at the Forest Products

Laboratory (FPL). 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. Similar studies in a

hot, humid summer climate will be reported in a

subsequent report.

Background

The results of previous research at FPL on moisture

condensation in walls have been summarized (7).

General recommended practice in cold climates has

been to provide a vapor retarder on the inside face of

the wall with a perm rating of no more than one-fifth

the rating of the outside covering material. Where

closed-cell foam sheathing is used, this ratio can

usually be achieved by applying a polyethylene film

vapor retarder on the warm side of the wall. However,

the addition of unperforated foil backing to rigid foam

results in a near-zero permeance, so the 5-to-1 ratio of

cold-side to warm-side permeance is not achieved.

The fact that moisture reduces the thermal resistance

of insulating materials was established by Joy (7) in the

1950’s. A more recent study by Burch (3) showed that

for certain conditions, condensation occurred as a thin

film on the surface of sheathing 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.
Figure 1.–Experimental condensation-study structure at Valley View test site near Madison, Wis. (M 147 763-11)

Moisture also reduces the thermal resistance of wood

and wood products. A method for estimating that

reduction is presented in the Wood Handbook (70).

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 not probable because

decay fungi do not grow at temperatures below 40° F,

and the wall can dry out at higher temperatures (70).

The most visible problems are paint peeling or

blistering; expansion of the siding with increased

moisture content may cause paint peeling, and

moisture migration through the siding may blister the

paint.

Previous studies (8, 9) have shown the increased

potential for condensation with high indoor humidities

when outdoor winter temperatures are low. As more

airtight houses result in higher indoor humidities, an

even greater potential for condensation may be

expected. Although laboratory tests have included

condensation studies, 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 changes. This can best be accomplished

by testing exposure structures in more than one

climate to include the effect of climate on moisture

patterns.

Materials and Methods

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,

Wis.; the other structure was erected near Gulfport,

Miss. 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

Madison. Results from the Gulfport building will be

discussed in a later report.

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

feet 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 were insulated

with R-384 glass fiber batts to limit heat transfer so the

walls would be the major element of heat loss from

each room.

Rooms are individually heated by a resistance-type

electric heater, and individually cooled by a

4

“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
Sheathing, insulation &

vapor barriers for walls

Fiberboard, R-13 batt,

6-mil polyethylene

Fiberboard, R-11 blanket,

asphalted paper

Plywood, R-11 blanket,

asphalted paper

Fiberboard (6” stud),

R-19 batt (compressed),

6-mil polyethylene

Extruded polystyrene foam,

R-13 batt, 6-mil polyethylene

Extruded polystyrene foam,

R-11 blanket, asphalted paper

Foil-backed, GF-reinforced

polyisocyanurate foam w/vent

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 of each wall panel. Note that both “R” values

and interface temperature are based on calculation methods shown in the ASHRAE Handbook of

Fundamentals (2). (ML83 5060)
Figure 3.–Cross section of experimental structure showing

construction details, (M83 5061)

window-type air-conditioner mounted in the floor.

Humidification is provided by a vaporizing-type

humidifier in each room during the heating season.

Humidity is not controlled during the air-conditioning

season. Heaters are controlled by wall thermostats to

maintain a temperature between 67° and 70° F. Airconditioners are set to cycle on at 79° F and off at 76°

F. Heating season relative humidity is maintained at

40 ± 5 percent, as compared to 35 percent for previous

studies of retrofit insulation (8, 9). High humidity was

desirable for this study to reveal any potential for

moisture problems. While 40 percent humidity is higher

than generally recommended, a limited survey of homes

built in Madison since the energy crisis began showed

it to be realistic. The current trend toward airtight

houses may produce more widespread occurrence of

high indoor humidity.

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.

Test Panels

For this study, test panels all 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

Figure 4.–Six-mil polyethylene film being applied as a

continous vapor retarder on an experimental

panel. Lead wires to moisture sensors and

thermocouples are brought through a small slit

thoroughly caulked to preserve the integrity of

the vapor retarder. (M 147 188-11)

glass fiber insulation was placed in each wall cavity.

One type of panel was framed with 2 by 6 studs at

24-inch spacing. The remaining panels were all framed

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

variables are the sheathing material and the vapor

retarder (fig. 2). Polystyrene sheathing was in 2-by

4-foot sections, while all other sheathings were 4-by

8-foot sections. Sheathing materials included: 1/2-inch

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

polystyrene foam, and 1-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.

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 foot and 7 feet above the floor,

moisture measurements were made at the sidingsheathing interface, at the sheathing-insulation

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

the adjacent stud. Sensors were also located in the top

plate, the sole plate, and between siding and sheathing

at the midheight of the wall. Lead wires from all these

data points were brought into the room through the

vapor retarder and gypsum board at two points (1 foot

and 7 feet above the floor). The punctures in the vapor

4
Figure 5.–Asphalted kraft paper vapor retarder stapled to

sides of studs. (M 147 191-11)

retarders were caulked around each wire individually

(fig. 4).

All test panels were without open punctures in the

gypsum board or vapor retarder for Phase l-the first

year-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.

As moisture-laden indoor air moves through and around

the outlet bypassing the vapor retarder, it creates a

potential for cold weather condensation. 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).

Data Acquisition

Moisture Content

Moisture conditions were measured at 189 locations

(fig. 6) using small wood sensors. The moisture content

(MC) of the wood sensor was converted to MC of the

members in which they were imbedded, based on the

relative humidity of the air in the immediate vicinity.

Relative humidity in the rooms, outdoors, and at

interfaces between two materials was also recorded.

Figure 6.–Moisture sensor locations in each test panel. Four

sensors are in framing; two are in the center of

insulation; two are at the insulation-sheathing

interface; and three are at the sheathing-siding

interface. (ML83 5062)

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 (4)). The sensors were calibrated in

humidity rooms to an accuracy of ± 2 percent MC over

a relative humidity range of 35 to 90 percent, which

corresponds to an MC in the wood sensor of 7.0 to 20.0

percent. Determination of MC beyond these limits was

less accurate because of difficulties in measuring

extreme ranges of resistance and because of 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

5
Figure 7.–Flow chart of instrumentation and data recording Figure 8–Instrumentation room showing data logger and

system. (ML83 5063) electric meters for individual rooms. (M 147 451-11)

to the logger, amplifiers were located as close to the

test wall, as close to each sensor as practical; their

output was connected to the data logger and calibrated

(fig. 7).

pressurization, heat and moisture leakage should have

been minimal because temperature and humidity

conditions were the same in all rooms. A heat balance

could not be calculated because of the air leakage at

doors.

The resistance readings are 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 in structural members or to

provide the relative humidity of ambient air conditions.

Data Recording

All of the moisture and temperature data were digitized

and recorded on cassette tape using a multichannel,

programmable data logger (fig. 8). Because of equipment malfunctions often caused by local storms, hand

readings were required much of the time. Data were

collected three times a week.

Temperature

Temperature measurements were made at each wood

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

and used for the temperature corrections.

Power and Water Usage

Power and water usage were measured weekly.

Because a corresponding monthly record could not be

obtained, a 3-week period in February of both 1980 and

1981 was selected to compare consumption for walls

with and without electrical outlets.

The original purpose of recording power input was to

evaluate the effect of moisture in insulation on rate of

heat transfer. Because there was no evidence of

moisture accumulation in the insulation, that effect

could not be observed. Pressurization of individual

rooms showed significant air leakage around doors

between rooms; these leaks could not have been sealed

without major redesign and construction. Without

Results

Phase 1–No Penetrations

During the summer of 1979, MC’s of probes in all parts

of most walls not penetrated by electrical outlets

remained constant between 8 and 12 percent. Probe

MC below 12 percent was considered low; 12 to 16

percent was moderate; 16 to 20 percent was high;

above 20 percent caused condensation. For brief time

periods the moisture level in several panels at a

specific data point rose for 1 to 3 days and returned to

normal; these were usually one-time occurrences for

which no explanation could be found. In certain cases

there was a repetitive pattern in slight moisture

changes. This appeared to be caused by day-night

cycling with the cooling of the siding at night. Moisture

6
content at the siding interface dropped to very dry

readings on warm, dry days, especially where siding

was exposed to the sun (6). If MC’s above or below 12

percent sustained for less than 3 days are considered

insignificant, all of the walls remained essentially dry

throughout the summer.

The first high moisture levels occurred in December

when temperatures dropped to consistently below

freezing levels; the most severe month was February

(figs. 9-16). A plot of daily low temperatures during

January, February, and March is shown in figure 17. In

all cases the northern exposure resulted in higher

measured moisture levels than did the southern

exposure, so discussion will be primarily directed to

the north-wall panels. Three panels (2N, 3N, and 5N)

had fiberboard sheathing.

Three factors affected moisture patterns: 1) type of

vapor retarder, 2) permeability of sheathing material,

and 3) temperature of sheathing material. Panel 2N

(polyethylene, R-13 glass fiber, fiberboard)5 remained

quite dry, with MC at the sheathing rising slightly but

not until February (fig. 9). Panel 3N (asphalted paper,

R-11 glass fiber, fiberboard), with similar construction

but with side-stapled asphalted kraft paper rather than

polyethylene vapor retarder, showed high MC’s by

December (fig. 10). Condensation occurred at the

sheathing-siding interface and later at the insulationsheathing interface. Brown streaking on the siding

(figs. 18 and 19) when warm weather followed a period

of extreme cold gave visual verification that significant

condensation did occur on the back of the siding.

Moisture levels also increased somewhat in panel 5N

(polyethylene, R-19 glass fiber, fiberboard) (fig. 12)

which was similar in construction to 2N except that it

had 2 by 6 studs and thicker batt insulation. The

thicker insulation resulted in lower sheathing

temperature and consequently higher MC as more

moisture condensed on the colder surface. However,

panel 5N did not exhibit any streaking because the

polyethylene vapor retarder limited the amount of

moisture entering its wall cavity.

The remaining wall cavities had sheathings resistant to

water vapor movement. Panel 4N with plywood

sheathing (asphalted paper, R-11 glass fiber, plywood)

(fig. 11) had increased MC at the insulation-sheathing

interface, but probes did not indicate condensation.

The plywood-sheathed panel dried more quickly than

did the polystyrene-sheathed panel 7N (fig. 14) when

outdoor temperatures increased, possibly because it

was able to absorb and redistribute the moisture

present. Panel 4N, however, had no brown streaking on

its siding as wall cavity moisture could not readily

move through the plywood sheathing to the siding.

Panel 7N (asphalted paper, R-11 glass fiber,

polystyrene) had condensation on the sheathing from

mid-February through March and had very slight

streaking of the siding, probably because sufficient

‘Vapor retarder, insulation, and sheathing, respectively.

moisture could leak through to the siding at horizontal

joints in the 2-by 4-foot polystyrene sheathing. In early

April the panel was opened to check on the probe that

was reading saturated; the wall was found to be

completely dry. The probe had malfunctioned due to

fungal growth on the surface, an indication that an RH

of at least 85 percent had existed for some period of

time.

Wall cavities with both polyethylene vapor retarder and

low-permeance foam sheathing had no indication of

condensation. Moisture content at the insulationsheathing interface in the wall with polystyrene

sheathing (6N) (fig. 13) did rise significantly by

December and remained high until March. Both walls

with foil-backed foam (8N, 9N) had increases in MC at

the insulation-sheathing interface only during periods

of extreme cold in January and February (figs. 15 and

16).

Moisture content levels during the three winter months

at the insulation-sheathing interface and at the

sheathing-siding interface are presented in table 1. The

moderate range may be high enough to cause problems

in certain cases, and the high range is definitely high

enough to cause expansion and potential buckling of

thin materials.

The overall results from the Phase 1 walls, with no

penetrations, are summarized as follows:

1. No condensation was detected in walls having a

continuous 6-mil polyethylene vapor retarder,

whereas walls with fiberboard or polystyrene

sheathing and a vapor retarder of asphalted paper

stapled between studs with no overlap showed

visual evidence of condensation.

2. Plywood-sheathed panels had no indication of

condensation even though a vapor retarder of

asphalted paper stapled between studs was used.

3. Moisture levels at the insulation-sheathing interface

in the wall cavity were highest in walls with the

lowest sheathing temperature-i.e., walls with the

lowest “R” value for sheathing.

4. Where large amounts of water vapor entered wall

cavities, it passed through permeable sheathing

and condensed on the siding, resulting in

streaking. However, sheathings with low perm

ratings and an effective vapor retarder generally

resulted in small amounts of water vapor that

entered and remained in the cavity.

5. In no case did MC gains remain in liquid form for

more than about 6 weeks, and MC of framing

never exceeded 12 percent.

Phase 2–Outlet Penetrations

Based on the experience of the first summer when all

moisture readings were constantly between 8 and 12

percent, the test building was not conditioned and no

data were taken during the second (Phase 2) summer.

Conditioning of the rooms was resumed with the

1980-1981 (Phase 2) heating season.

7
Figure 9.–Moisture content of wood probes in panel 2N (polyethylene, R-13 glass fiber, fiberboard) and corresponding temperature

on the warm face of sheathing, January through March 1980. The only probe showing MC exceeding 10 percent was

at the insulation-sheathing interface. (ML83 5064)

Figure 10.–Moisture content of wood in panel 3N (asphalted paper, R-11 glass fiber, fiberboard) and corresponding temperature

on the warm face of sheathing, January through March 1980. (ML83 5065)
Figure 11.–Moisture content of wood in panel 4N (asphalted paper, R-11 glass fiber, plywood) and corresponding temperature

on the warm face of sheathing, January through March 1980. (ML83 5066)

Figure 12.–Moisture content of wood probes in panel 5N (polyethylene, R-19 glass fiber, fiberboard) and corresponding

temperature on the warm face of sheathing, January through March 1980. (ML83 5067)
Figure 13.–Moisture content of wood probes in panel 6N (polyethylene, R-13 glass fiber, polystyrene) and corresponding

temperature on the warm face of sheathing, January through March 1980. (ML83 5068)

Figure 14.–Moisture content of wood probes in panel 7N (asphalted paper, R-11 glass fiber, polystyrene) and corresponding

temperature on the warm face of sheathing, January through March 1980. (ML83 5069)
Figure 15.–Moisture content of wood probes in panel 8N (polyethylene, R-13 glass fiber, vented foil-backed isocyanurate) and

corresponding temperature on the warm face of sheathing, January through March 1980. The only probe showing

MC exceeding 10 percent was at the insulation-sheathing interface (top). (ML83 5070)

Figure 16.–Moisture content of wood probes in panel 9N (polyethylene, R-13 glass fiber, foil-backed isocyanurate) and

corresponding temperature on the warm face of sheathing, January through March 1980. (ML83 5071)
Figure 17.–Daily low temperatures for January through

March 1980. (ML83 5072)

Figure 18.–Severe staining on panel 3N (north side) due to

condensation on the back of siding.

(M 147 763-2)

Moisture in all north panels during January, February,

and March 1981 is plotted in figures 20 through 27.

Temperatures at the insulation-sheathing interface are

also shown for each panel. Daily low temperature

during January, February, and March is plotted in

figure 28.

By January condensation had occurred in some of the

panels on the sheathing directly behind the electrical

outlet and remained there through the coldest part of

the winter. The three walls with R-11 insulation were

exceptions in that there was no condensation behind

outlets. One explanation may be that the less-dense

glass fiber insulation allowed greater convection air

circulation within the wall cavity, which permitted water

vapor to distribute over more wall area rather than to

condense locally behind the electrical outlet. Streaking

occurred only on the fiberboard-sheathed panels 2N,

3N, and 5N.

Figure 19.–Slight staining on panel 3S (south side). (M 147 763-9)

Panel 9N (polyethylene, R-13 glass fiber, foil-backed

foam) (fig. 27) had condensation behind the outlet for

only about 2 weeks. It dried out in early February.

The remaining four north wall panels had

condensation remaining long enough that fungus on

the surface caused some moisture sensors to

malfunction and appear to be saturated. in early April

those sensors were removed and all walls were

observed to be completely dry. There was also no

staining or other visual evidence of previous excessive

moisture in the cavity. Panels 8N (fig. 26) and 9N (fig.

27) had R-8 sheathing, which resulted in the highest

sheathing temperature of all north-facing panels. Panel

8N was vented at the top plate. Under north wind

conditions, the north wall sheathing temperature was

observed to be colder in the top vented wall with

penetrated vapor retarder. Greater air movement from

the room through the cavity is likely with a vented top

plate. This possibility is supported by the fact that Test

Room 8 did require more water input than Test Room 9

to maintain 40 percent relative humidity.

Panels 4N (asphalted paper, R-11 glass fiber, plywood)

and 5N (polyethylene, R-19 glass fiber, fiberboard) (figs.

22 and 23) also had extended periods when

condensation was apparently present on the sheathing

near the top of the wall. Because these two types of

walls indicate the lowest sheathing temperatures,

condensation could occur at lower cavity humidity

conditions than in any other wall tested. The warmest

and coldest sheathing temperatures for January,

February, and March 1981 (fig. 29) illustrated a range of

up to 20° F temperature difference at this critical

interface for the various types of construction.

Condensation occurred on the back of the siding in all

panels with fiberboard sheathing. It remained through

12
Figure 20.–Moisture content of wood probes in panel 2N (polyethylene, R-13 glass fiber, fiberboard) and corresponding

temperature on the warm face of sheathing, January through March 1981. (ML83 5073)

Figure 21.–Moisture content of wood probes in panel 3N (asphalted paper, R-11 glass fiber, fiberboard) and corresponding

temperature on the warm face of sheathing, January through March 1981. (ML83 5074)
14
Figure 24.–Moisture content of wood probes in panel 6N (polyethylene, R-13 glass fiber, polystyrene) and corresponding

temperature on the warm face of sheathing, January through March 1981. (ML83 5077)

Figure 25.–Moisture content of probes in panel 7N (asphalted paper, R-11 glass fiber, polystyrene) and corresponding

temperature on the warm face of sheathing, January through March 1981. (ML83 5078)
Figure 26.–Moisture content of wood probes in panel 8N (polyethylene, R-13 glass fiber, vented foil-backed isocyanurate) and

corresponding temperature on the warm face of sheathing, January through March 1981. (ML83 5079)

Figure 27.–Moisture content of wood probes in panel 9N (polyethylene, R-13 glass fiber, foil-backed isocyanurate) and

corresponding temperature on the warm face of sheathing, January through March 1981. (ML83 5080)
Figure 28.–Daily low temperatures for January through

March 1981. (ML83 5081)

Figure 29.–Warmest and coldest north-side sheathing

temperatures (panels 5N and 9N) for January

through March 1981. (ML83 5082)

the end of the winter in most cases. There was slight

streaking of siding on panels 2N, 3N, and 5N, the

panels with fiberboard sheathing. For short periods of

time there was condensation on the back of siding in

panels 7S (asphalted paper, R-11 glass fiber,

polystyrene) and 7N. These panels had polystyrene

foam sheathing with horizontal joints at 2-foot spacing,

which permitted some moisture to pass through to the

siding.

The MC of wood probes at the insulation-sheathing

interface and the sheathing-siding interface is

summarized in table 2. All constructions showed high

enough MC’s near the siding to create a potential for

buckling of long strips of hardboard siding.

The overall results from walls with an electrical outlet

penetrating the vapor retarder are summarized as

follows:

1. All north walls with R-13 and R-19 batt insulation

had localized condensation on the sheathing

surface behind the electrical outlet in January.

This condensation appeared to remain in all but

one panel through February and March.

2. No localized condensation occurred behind electrical

outlets in walls with low-density R-11 blanket

insulation, presumably because of internal air

circulation by convection.

3. Condensation on sheathing at higher locations in

the walls was observed only in walls with very low

sheathing temperature.

4. Condensation occurred for long periods on the back

of the siding of all north walls with fiberboard

sheathing and briefly on two walls with

polystyrene sheathing having horizontal joints at

2-foot spacing. No siding condensation was

observed on north walls sheathed with foil-backed

foam.

5. All panels had high-enough moisture levels on the

back of the siding to produce buckling in long

strips of hardboard siding.

6. All data points showed MC below 11 percent by early

April and no rise in MC of framing at any time.

Power and Water Usage

Because power and water usage was measured weekly

and a record corresponding exactly to a month could

not be obtained, a 3-week period in February was

selected from both 1980 and 1981 to compare

consumption for walls without and with an electrical

outlet (table 3). Both power and water consumption

were higher in 1981 even though heating degree-days

were much less for both the period shown and the

entire winter (table 4). Although data are not available

to quantify the effects of solar gain and wind on power

consumption, the change in kilowatt-hours per degreeday is a good indication of increased power

consumption due to two electrical outlets in a wall

section 8 feet wide (two 4-foot sections). Based on

kilowatt-hours per degree-day, the power requirement

was 35 to 54 percent (average 41 pct) higher with the

electrical outlets. Rooms with the lowest R-value walls

showed the greatest increase in power consumption.

Water usage for humidification for the same time

period increased by 10 to 72 percent (average 42 pct).

This additional water escaped primarily by indoor air

movement through the electrical outlet and wall cavity.

This air movement also contributed to heat loss, by

loss of both the heated air and latent heat in the water

vapor.

Findings

The following findings apply to the climate of Madison,

Wis. (7,800 heating degree-days per year), at controlled

indoor conditions of 67°-70° F and 40 ± 5 percent

relative humidity. The test building was electrically

heated so there were no pressure changes due to

combustion air requirements or blower operation.

1. No condensation occurred in walls with a

continuous polyethylene vapor retarder,

regardless of type of sheathing.

2. North walls with fiberboard or polystyrene

sheathing and only asphalted paper backing on

glass fiber insulation (no punctures) stapled

between studs had condensation on the

sheathing for a limited time (no more than 6

weeks).

17
3. Where condensation occurred in walls with

fiberboard sheathing it initially formed on the

back of siding and later on the sheathing. Some

moisture also passed through horizontal joints in

polystyrene sheathing and condensed on siding.

4. A cold-side vapor retarder, such as the glue joint in

plywood sheathing, reduced the hazard of

condensation at the sheathing-siding interface

without unduly increasing the cavity MC.

5. Condensation formed on the sheathing behind

electrical outlets in all north-facing walls with

batt insulation of R-13 and R-19. No localized

condensation formed behind outlets in walls with

R-11 blanket insulation.

6. Condensation formed on sheathing near the top of

walls with electrical outlets only where sheathing

temperatures were quite low.

7. After electrical outlets were added all panels had

high enough moisture levels on the back of the

siding to create a potential for buckling of long

strips of hardboard siding.

8. For both years and all construction, all data points

showed MC to be below 11 percent by early April.

9. MC of framing did not increase significantly at any

time during the 2-year study.

10. The addition of two electrical outlets in each room

resulted in an average increase of about 40

percent in both heating energy and water

consumption for comparable time periods.

Conclusions

The findings of this study are limited to specific indoor

and outdoor conditions; they should, however, be

applicable to much of the upper midwest and northeast

of the United States. The winter condensation potential

18

would be less in warmer climates, but greater where

winters are more severe than those in Madison, Wis.,

were during 1980 and 1981.

Asphalted paper backing on insulation stapled between

studs does not provide adequate vapor retarder

protection to prevent condensation in the wall cavity or

streaking of the siding where a permeable sheathing is

used. A continuous 6-mil polyethylene vapor retarder

can control condensation in insulated walls even where

low-permeance sheathing is used. Puncturing the vapor

retarder, as with an electrical outlet, can completely

change moisture patterns in the wall and result in

condensation on the sheathing behind the electrical

outlet. Punctured polyethylene performed no better

than asphalt-coated paper.

In all of the types of construction observed both with

and without outlets, condensation in the wall cavity

forms on the back of siding or on the back surface of

the sheathing and does not wet the bulk of the cavity

insulation. There is no rise in MC of any framing

materials. All wood in walls remains below 12 percent

MC when temperatures are high enough for fungi to

grow, from April to December. Low-permeance foam

sheathings present no greater cold-weather

condensation hazard than the other types of sheathing

studied. Vent strips at the top of walls with high-“R”,

low-permeance sheathing result in greater air leakage

with no apparent benefit in moisture control.

While conditions that would promote decay in wood

framing do not appear to be a danger, moisture levels

can be high enough in all panels to produce significant

dimensional changes in thin panel products or long

strips of siding.
Fungal growth and consequent malfunction of some

moisture sensors indicate water was present for some

period of time when temperatures were high enough to

promote fungal growth. If this water could not escape

from the wall, a definite decay hazard would exist.

However, as observed in this study, in the conventional

site-built wood frame wall, moisture does escape

without adversely affecting wood framing.

These conclusions apply only to conventional

construction and indoor conditions of 70° F, 40 percent

relative humidity. Higher humidities may occur due to

construction moisture, extremely tight construction, or

major indoor moisture 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.

19
U.S. Forest Products Laboratory

Condensation potential in high thermal performance walls-cold

weather climate, by G. E. Sherwood, Madison, Wis., FPL 1983.

20 p. (USDA For. Serv. Research Paper FPL 433).

Increased use of high “R”-value, low-permeance sheathings and other

changes in construction practices have caused concern over moisture

patterns in walls. To observe actual moisture patterns and the potential for

condensation, a test structure was constructed near Madison, Wis., for

exposure of eight types of insulated wall panels at controlled indoor

conditions and typical outdoor weather conditions, for 2 years. Continuous

vapor retarders effectively prevented condensation; asphalted paper stapled

between studs was inadequate. Installation of an electrical outlet changed

the moisture profile and resulted in some condensation in most panels. The

moisture content of the framing remained under 12 percent throughout the

study and there was no long-term accumulation of free water in the structure.

Keywords: Condensation, moisture control, vapor retarder, air leakage,

wood-frame walls, foam sheathing.

2.0-20-5/83
U.S. GOVERNMENT PRINTING OFFICE: 1982-554-030/ 37