1978 

PAINT 

AS A VAPOR BARRIER 

FOR WALLS 

OF OLDER HOMES 

RESEARCH 

PAPER 

FPL 319 

FOREST PRODUCTS LABORATORY 

FOREST SERVICE 

U.S. DEPARTMENT OF AGRICULTURE 

MADISON, WIS. 
ABSTRACT 

Insulation added to walls of an older 

home without vapor barriers may cause 

moisture condensation problems. Such 

problems appear to be minimal in the 

Madison, Wis., climate so long as 

mechanical humidification is not used. 

However, where a relative humidity of 35 percent or higher is maintained in the older 

home, troublesome condensation in the 

walls is likely without remedial measures. 

And even in homes without peeling paint or 

other overt moisture problems, buildup of 

moisture in walls may reduce the effectiveness of insulation. 

To obtain information on the effectiveness of some commercially available 

paints as vapor barriers, three types of paint 

(interior semigloss acrylic latex, exterior 

acrylic latex, and exterior soya-alkyd resin) 

were applied to the plaster on selected wall 

sections. A relative humidity of 35 percent 

was maintained in the test structure through 

one winter, and moisture levels were observed in the wall cavity. All of these paints 

served as adequate vapor barriers (where applied in two coats) to keep moisture in walls 

at acceptable levels. Labeling commerical 

paints with perm ratings would help the consumer determine those most suitable for use 

as vapor barriers. 

This information should be useful to 

homeowners and home-improvement contractors. 
PAINT 

AS A VAPOR BARRIER 

FOR WALLS 

OF OLDER HOMES 

BY 

GERALD E. SHERWOOD, Engineer 

Forest Products Laboratory 1

Forest Service 

U.S. Department of Agriculture 

Introduction 

Insulation is being blown or foamed into 

the wall cavities of many homes built prior to 

1940 because it was usually not included at 

the time the house was built. The added insulation has the effect of moving the zone in 

which condensation occurs toward the inside face of the wall, which sometimes 

results in moisture condensing in the insulation. 

In many structures where insulation has 

been added without benefit of a vapor barrier, 

no damage from moisture buildup has 

become apparent. However, there is no way 

of knowing the exact conditions inside the 

wall cavity without removing the covering 

materials. In other structures, condensation 

in walls has resulted in serious problems of 

paint peeling or extractives from sheathing 

or siding running down the face of the exterior wall. 

Condensation problems are aggravated 

when mechanical humidification is used in 

the older home. Even where condensation 

presents no visible problems, research has 

shown that moisture does reduce the thermal efficiency of insulation (1).2 Also, when 

the moisture content (MC) of wood reaches 

high levels and temperatures are 35° F or 

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

University of Wisconsin. 

higher, decay can occur with consequent 

deterioration of the structure. 

It is quite difficult to stop condensation 

in walls of older homes by adding a membrane-type vapor barrier to an existing wall. 

(In new construction, such a vapor barrier is 

applied to the wall framing before the plaster 

or drywall is installed.) An alternative 

solution to the condensation problems 

described might be the use of paints 

resistant to vapor penetration for interior 

walls. In effect, the paint itself then might 

become a vapor barrier. The possibility of 

such an application of paint was first 

suggested by Joy, Queer, and Schreiner in 

1948, but no research was conducted to 

verify their suggestions (4). 

This paper reports the testing of three 

paint types that are commercially available to 

determine their effectiveness as vapor 

barriers. They were applied to stud walls built 

to simulate walls of older homes which lack 

vapor barriers. 

Background 

During the winter of 1974-1975. a 

simulated older structure with insulation added to the wall cavity was instrumented with 

moisture sensors (5). The variables included 

2 Numbers in parentheses refer to Literature Cited at the 

end of the paper. 
three types of wall construction and two indoor humidity conditions. Indoor relative 

humidity conditions resulting from normal 

household activities were simulated in part 

of the building. The remainder of the interior 

space was maintained at 35 percent relative 

humidity. Walls tested incorporated the 

usual plaster with oil-base paint, unmodified 

and with two types of remedial modification 

(two coats of aluminum paint, exterior 1-inch 

vents at top and bottom of stud spaces). Only 

plaster walls with oil-base paint were used as 

controls because that is typical of houses 

built prior to 1940. 

Test data showed that insulation added 

to walls of an older home subject to the 

climate at Madison, Wis., may not cause 

visible moisture problems where mechanical 

humidification is not used. However, where a 

relative humidity of 35 percent is maintained 

in the house, condensation in the walls is 

likely without remedial measures. Two coats 

of aluminum paint on the plaster were an effective vapor barrier and thus kept moisture 

in the walls at acceptable levels even where 

indoor relative humidity was maintained at 35 

percent. One-inch-diameter vents installed 

near the top and bottom of a stud space 

provided little help in keeping the wall cavity 

dry, and low temperatures resulting from air 

moving through the cavity indicated increased heat loss through the wall. The vents 

did keep moisture levels lower at the 

sheathing-siding interface, which may help 

prevent paint peeling. 

Although aluminum paint provided an 

adequate vapor barrier, it is difficult to mix 

and apply, and the house must be completely 

redecorated. A readily available premixed

paint that would result in a finished surface 

could be used to much better advantage. To 

determine the effectiveness of premixed 

paints as vapor barriers, selected paints 

were applied to the walls of the simulated 

older house used in previous tests and 

moisture measurements were made in the 

walls through a winter season. 

Construction and 

Test Conditions 

A 16- by 24-foot building was constructed near Madison, Wis., in 1964 to be 

used for the study of moisture distribution in 

walls of new construction (3) (fig. 1). It was 

later adapted for the study of moisture in a 

simulated older home conducted during the 

winter of 1974-1975. The building had been 

constructed with conventional floor and roof 

framing, but vertical support was provided by 

nominal 4- by 4-inch posts at 8-foot spacing 

to permit the use of seven 8- by 8-foot 

removable wall panels. Each of the seven 

wall panels was divided into four spaces with 

nominal 2- by 4-inch studs, resulting in 28 

test spaces (fig. 2, table 1). Only five of the 

wall panels were used in the tests presented 

here. A nominal 1- by 6-inch board coated 

with aluminum paint was placed adjacent to 

each stud and extended to the outside to 

prevent moisture transfer between spaces

and the sheathing or siding covering each 

space. 

Sheathing was of 1- by 8-inch boards 

spaced slightly to simulate older construction in which the boards often shrink 

and leave a slight gap. A 15-pound asphaltimpregnated sheathing paper was applied 

over the sheathing boards, and 1⁄2 - by 6-inch 

beveled siding was added. Siding was given 

two coats of oil-base paint. The interior was 

finished with gypsum lath and plaster, and 

given two coats of an interior oil-base paint. 

An older house would have wood lath, but 

both the wood lath and gypsum lath have 

very high permeability, so this substitution 

should have no effect on moisture transfer 

through the walls. Each space was insulated 

with 3-1⁄2-inch-thick glass fiber batts. Identical construction was used for all spaces in 

this study. 

Variables for the study included orientation of test walls (north or south), interior 

paint, and the presence or absence of exterior ventilation for the wall cavity. For this 

study the lower vent was plugged on half of 

the ventilated cavities to observe the effect 

of vents at the top only. 

The building was heated to 72°F ± 2° by 

electric heaters and humidified to 35 ± 5 percent relative humidity by a commercial 

humidifier. Location of wall panels with 

various treatments is identified in table 1. In 

addition to panels with aluminum paint and 

ventilated cavities, the plaster on some wall 

panels was painted with readily available 

commercial paints known to have good 

2 
Figure 1. - - The U.S. Forest Products Laboratory test house located on the exposure site near 
 

Madison, Wis. (M 144 205-3) 
 

Figure 2. - - Arrangement of panels in FPL 
 

test house. (M 144 897) 
 

3 
 
Table 1.--Location and variables for monitored test species 

Variables Location 

Panel 

code1 

Space 

No preventative measure S-1 

N-1 

1 

1 

Interior semigloss acrylic latex (one coat) 

(two coats) 

S-1 

S-1 

3 

2 

Exterior acrylic latex (one coat) 

(two coats) 

(two coats) 

S-2 

S-1 

N-1 

1 

4 

3 

Exterior soya-alkyd resin (one coat) 

(two coats) 

(two coats) 

S-2 

S-2 

N-1 

3 

2 

4 

Aluminum paint (two coats) 

(two coats) 

S-3 

N-3 

3,4 

1,2 

Vents (top only) 

(top only) 

(top and bottom) 

(top and bottom) 

S-3 

N-3 

S-3 

N-3 

1 

3 

2 

4 

1 To locate the panels and spaces, see fig. 2. 

Table 2.--Weather conditions during test period1 

Month Low High Degree Normal degree 

temperature temperature days days 

°F °F 

November 1976 -6 62 1,102 909 

December 1976 -17 50 1,602 1,336 

January 1977 -22 29 1,898 1,494 

February 1977 -19 54 1,188 1,252 

March 1977 11 70 772 1,079 

April 1977 20 87 409 591 

Total for test period 6,971 6,661 

Data from the Madison Weather Service Office, National Oceanic and Atmospheric Administration, U.S. Department of 

Commerce. 

4 

1 
moisture-excluding effectiveness. The three 

types of paint selected were exterior soya- alkyd, exterior flat acrylic latex, and interior 

acrylic latex semigloss enamel. 

Weather conditions for the period

covered by this study (November 1976 

through April 1977) are shown in table 2. 

These data indicate that the period covered 

by the study was slightly colder than usual 

for a winter in Madison, Wis. Degree days for 

the period were about 5 percent above the 

mean. 

Instrumentation and 

Measurements 

The method used for measuring

moisture within the walls is detailed in the 

following paragraph. 

Moisture Content 

The moisture content (MC) of small 

wood sensors was measured at three 

locations in each space (fig. 3). Actual installations are shown in figures 4, 5, and 6. A 

total of 60 locations were measured for wood 

MC with small wood-type sensors but MC of 

actual components was not determined. The 

relative humidity within the rooms was 

monitored using a sling psychrometer. 

A sensor capable of remote reading was 

required because most of the locations for 

moisture measurement were relatively inaccessible. The system used employed a 

calibrated wood sensor element and a commercial moisture meter. Construction and 

details of the operation of this sensor are 

given by Duff (2). The probes were calibrated 

in humidity rooms to an accuracy of ±2 per. 

cent MC over a relative humidity range of 35 

to 90 percent which was considered sufficiently accurate for this study. This 

corresponds to an MC in the wood probe of 7 

to 20 percent. Determination of MC beyond 

these limits was less accurate due to difficulties in measuring extreme ranges of 

resistance. Also, beads of condensed water 

were often present on probe surfaces at 

probe readings of 20 percent or higher. 

Temperature Measurement 

The MC as indicated by the probe

requires correction- only for temperature 

changes of 10° F or more. Inside room temperature was used for correcting the 

moisture probes located immediately adjacent to the interior wall lining. Outside air 

temperature was used to correct the probes 

immediately beneath the siding. For 

moisture probes within the wall between the 

insulation and sheathing, actual thermocouple measurements were made for 

each probe. Placement of a probe and its 

associated thermocouple for temperature is 

shown in figure 5. 

All temperatures were measured using 

Type T (copper-Constantan) thermocouples 

and an ice bath reference. 

Data Recording and Conversion 

The system for monitoring MC consisted of the moisture probes connected 

through a scanner to a commercial moisture 

meter, digitized through a voltmeter, and 

read manually. Thermocouple readings were 

also digitized through a voltmeter and read 

manually. 

Raw data from each moisture sensor 

were first converted to MC readings via a 

calibration curve, and then corrected to actual MC for the temperature associated with 

the specific probe. The last step corrected 

the probe readings for species of wood used 

in the probe – giving a final, true MC. The 

corrected data were then keypunched for 

computer plotting. 

Readings were taken at about 2 p.m. 

twice a week from November 8, 1976, to April 

29, 1977. 

Results and Discussion 

Data from moisture readings are report­ ed through the critical range of moisture 

levels. Values of wood MC greater than 20 

percent were observed in the probes. However, because percentage readings over this 

level are of questionable accuracy, values 

above 20 percent were not plotted. Probe 

readings above 20 percent generally indicat­ ed the presence of beads of condensed 

water on surfaces and thus a risk of decay if 

sustained. The structure is in danger at any 

sustained probe MC over 20 percent when 

temperatures are 35°F or higher, so that the 

amount by which the 20 percent reading is 

exceeded is not highly relevant. However, 

the length of time the probe reading remains 

5 
Figure 3. - - Cross section of stud space 

showing location of moisture probes 

and thermocouples. Probes were located at both one and seven feet from the 

floor. (M 144 899) 

Figure 5. - - Moisture probe and thermocouple attached to interior surface of 

sheathing board. (M 144 309) 

above 20 percent is important and is recorded in the data. The presence of free water 

also could increase the rate of heat loss if 

distributed through the insulation, and 

sustained free water creates a potential for 

paint peeling on siding.

Representative plots for walls with no 

remedial treatment, for each type of paint, 

and for both vented systems are presented 

in figures 7, 8, and 9. Plots shown are from 

north walls where condensation is most 

severe. A discussion of these figures is given 

in the sections which follow. 

The MC in walls with no remedial measures (fig. 7) shows that the moisture level 

at the sheathing-siding interface exceeded 

20 percent during most of the winter. Moist­ ure content at the plaster-insulation interface 

Figure 4. - - Moisture probe attached to sur- was constant at 7 percent except near the face of rock lath. (M 144 310) end of the winter when there was a rise in 

6 
Figure 6. - - Moisture probe located immediately beneath siding 

(M 142 642-9) 

MC at that location. This rise coincided with 

a drop in MC at the siding indicating a reverse 

moisture drive. Throughout the season, MC 

at the siding-sheathing interface remained 

higher than in walls having paints applied for 

moisture-excluding effectiveness. 

Walls With Paint Applied As 

A Remedial Measure 

The MC in four wall sections – each 

with a different type of paint, applied in two 

coats to reduce permeability – is shown in 

figure 8. All of these walls had an MC at 

the siding-sheathing interface well below 20 

percent during the test period with one exception. The wall section with interior 

semigloss acrylic latex had an MC above 20 

percent for a brief period during the coldest 

weather. 

Moisture content at the plaster was constantly at 7 percent until early spring, when it 

increased slightly in all sections. This was 

due to a reverse in the moisture drive as outside temperatures increased. The increased 

MC at the plaster did not occur as early in the 

season as it did where no remedial measures 

were used; the reason is that other components of the wall were at lower MC than in 

the wall with no remedial measures. 

Vented Cavities 

An earlier study of vented cavities (5)

with vents near both the top and bottom 

showed the draft of cold air through the vented space resulted in high MC in the walls. 

This was again demonstrated in this study 

(fig. 9). Here the lower vent was plugged in 

one-half of the vented cavities to observe the 

effect of letting moisture escape at the top 

without having a draft through the entire 

cavity. Figure 9 shows that the MC was lower 

in the wall section with one vent than in the 

section with two vents. However, even one 

vent resulted in higher MC than walls with 

selected paints as remedial measures. One 

vent gave an improvement over the section 

with no remedial measure. 

Findings 

The following findings relate only to the 

climatic conditions of Madison, Wis. Where 

winters are more severe, condensation 

problems will be greater. For milder winters, 

condensation problems will be less. A 20 percent MC in the wood probes is cited as 

critical because beads of condensed water 

are generally present on surfaces; wood is 

7 
Figure 7. - - Moisture conditions through a typical wall section which incorporates no remedial 

measures. (M 146 537) 

considered in danger of decay when such 

free water accumulates and temperatures are 

35°F or higher, and thus the structure is in 

danger.

sections having aluminum paint. 

2. Vents near the top of a stud only are 

more effective in keeping the wall cavity dry 

than vents at both top and bottom. However, 

1. With one minor exception (noted 

earlier), two coats of interior semigloss 

acrylic latex, exterior acrylic latex, or exterior 

soya-alkyd resin generally all kept MC of 

walls below 20 percent through the entire 

winter season even though 35 percent relative humidity was maintained indoors. Moisture content was comparable to that in wall 

the single vent is still not as effective as 

selected paints applied to the plaster in 

keeping wall cavities dry. 

3. Wall cavities with no remedial measures 

did have MC above 20 percent where 35 percent relative humidity was maintained indoors. 

8 
 
Figure 8. - - Moisture conditions through typical wall sections with paint applied to piaster for 

vapor resistance. (M 146 535) 

9 
Figure 9.--Moisture conditions through typical wall sections with 1-inch diameter vents in wall 

(M 146 536) 

Conclusions 

Adding insulation to walls of an older 

house with no vapor. barrier subject to the 

climate of Madison, Wis., is likely to result in 

critical levels of condensation in the walls 

where mechanical humidification is used to 

maintain 35 percent relative humidity or 

higher. 

Two coats of interior semigloss acrylic 

latex, exterior acrylic latex, exterior soyaalkyd resin, or aluminum paint can be effective in keeping the moisture level of wood in 

the wall cavity below 20 percent even where 

35 percent relative humidity is maintained. A 

I-inch-diameter vent at the top of the wall 

cavity is less effective than using these paints 

to keep the wall dry, but more effective 

than using vents at both top and bottom of 

the wall cavity. 

It would be helpful to those considering 

insulating walls of existing homes if commercially available paints were labeled with 

perm ratings. 

10 
Literature Cited 

1. Adams, Ludwig. 

1974. 	 Thermal conductivity of wet insulations. ASHRAE J. 16(10):61-62. 

2. Duff, J.E. 

1966. A probe for accurate determination of moisture content of wood products in use. U.S. Dep. Agric. For. 

Serv. Res. Note FPL-0142. For. Prod. 

Lab., Madison, Wis. 

3. Duff, J. E. 

1968. 	 Moisture distribution in woodframe walls in winter. For. Prod. J. 

18(1):60-64. 

4. Joy, F. R., Queer, E. R., and Schreiner, R. E. 

1948. Water vapor transfer through 

building materials. Penn. State College, Engineering Exp. Stn. Bull. 

No. 61, University Park, Penn. 

5. Sherwood, G. E. 

1977. Moisture conditions in walls and 

ceilings of a simulated older home 

during winter. U.S. Dep. Agric. For. 

Serv. Res. Pap. FPL 290. For. Prod. 

Lab., Madison, Wis. 

U.S. GOVERNMENT PRINTlNG OFFlCE: 1980–651-111/40 

11