NATIONAL BUREAU OF STANDARDS REPORT

5223

SUMMARY REPORT - VAPOR BARRIER INVESTIGATION

Thule and Sondrestrom Air Force Bases, Greenland

by

F. J. Powell, National Bureau of Standards

W. 0. McClune, Eastern Ocean District, C. E.

B. J. Small, LaPierre, Litchfield & Partners

(Alfred Hopkins & Associates)

R. J. Persson, Metcalf & Eddy

Report to

U. S. Army Engineer District

Eastern Ocean

New York 11, New York

NBS

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

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NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-30-4830 April 8, 1957 5223

SUMMARY REPORT - VAPOR BARRIER INVESTIGATION

Thule and Sondrestrom Air Force Bases, Greenland

F. J. Powell, National Bureau of Standards

W. O. McClune, Eastern Ocean District, C. E.

B. J. Small, LaPierre, Litchfield & Partners

(Alfred Hopkins & Associates)

R. J. Persson, Metcalf & Eddy

Report to

U. S. Army Engineer District

Eastern Ocean

New York 11, New York

by

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on October 9, 2015 .

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U S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
*
CONTENTS

I

.

INTRODUCTION

A. Purpose

B. Personnel

C. Itinerary

II. FINDINGS

A. General

B. Description of Buildings

C. Observations

D. Laboratory Tests and Inspection

E. Discussion of Principles, Observations, and

Laboratory Results

II. GENERAL CONCLUSIONS

TABLES

1. Water-Vapor Permeance Results for Inner-Wythe Material

2. Water-Vapor Permeance Results for Coated Inner-Wythe

Materials

.

FIGURES

1. Building Type No. 1, Thule Air Force Base

2. Building Type No. 2, Sondrestrom Air Force Base, BW-8

3. Aluminum Clad Prefabricated Insulated Panel

4. Aluminum Clad Prefabricated Insulated Panel

5. Aluminum Clad Prefabricated Insulated Panel

6. Panel Joint, Building Type No. 1, Exploded View

APPENDIX

"Moisture Condensation in Building Walls," by Harold W.

Woolley, National Bureau of Standards Building Materials

and Structures Report BMS63.

SUMMARY REPORT - VAPOR BARRIER INVESTIGATION

Thule and Sondrestrom Air Force Bases, Greenland

I

.

INTRODUCTION

A . Purpose

At the request of the Office of the District Engineer,

U. S. Army Engineer District, Eastern Ocean, an investigation

and inspection of various buildings located at two Air Force

Bases, Sondrestrom (BW-8) and Thule, Greenland, was completed

to study the cause of frost, ice, and water accumulation in the

interior construction of walls, floors, and ceilings of several

buildings. This report covers the observations, discussion,

results of laboratory tests, and conclusions of the investigating team. Individual reports containing recommendations for

remedial action to correct failures in the buildings observed,

or other existing buildings showing similar failures, will be

submitted by the four offices involved. It is also proposed

that this study be utilized as an aid to the development of

data for future design of Arctic construction.

B . Personnel

Personnel charged with carrying out the investigation

were as follows

F. J. Powell - National Bureau of Standards

W. 0. McClune - Eastern Ocean District, C. E.

B. J. Small - La Pierre, Litchfield & Partners

(Alfred Hopkins & Associates)

R. J. Persson - Metcalf & Eddy

C . Itinerary

1. Departed McGuire Air Force Base, 29 April 1956,

at 1600.

2. Sondrestrom (BW-8), five days.

3. Thule, three days.

4. Returned McGuire Air Force Base, 9 May 1956,

at 1630.

- 2 -

II. FINDINGS

A. General

Permanent prefabricated buildings were examined, such

as airmen’s and officers’ quarters, hangars, mess halls, and

communications buildings. Evidence of ice and water accumulation was found or reported observed in the cavities of most

buildings examined. The extent of the accumulations of moisture observed varied from traces to completely soaked or frozen

insulation. Water dripping or running into occupied spaces was

observed in several instances.

Several buildings of each of the two main types of construction described below, were inspected. Conferences and

interviews were held with field engineering personnel to discuss

the past performance and failures of the buildings and corrective

investigations undertaken to date. Several samples of representative vapor barriers and building sections were selected for

transmission to the National Bureau of Standards for water vapor

permeance and thermal conductivity tests.

Weather data for the first four months of 1956 was

procured from base weather stations for use in studying the

problem

.

B . Description of Buildings

General descriptions of the two main types of prefabricated buildings observed, designated as Nos. 1 and 2, are as follows

1. No. 1 (See Figure 1).

This is an aluminum clad structure consisting of an

assembly of prefabricated insulated panels of various sizes

composed of 1/4-inch interior grade plywood faced with 0.015-

inch 2S or 3S half-hard mill finish aluminum, cold bonded

adhesively to the plywood on the exterior and interior faces,

the cavity being filled completely with 3 1/2-inch thick glass

fiber batt insulation. The panels contain for their framework

3/4 x 3 l/2-inch vertical plywood stiles and 3/4-inch thick

overlays at the top and bottom for proper integration with the

structural frame. Where panels abut, solid wood splines,

13/32 x 1 27/32 inches are provided. In addition to being

bolted to the structural frame, within each panel at the top

and bottom a 3/4-inch pipe runs continuously around the structure. Joints are filled in the field with a specially prepared

low temperature resistant caulking compound.
,* . *
-3-

These panels are employed for exterior walls, and

flat and pitched roofs, as well as building liners, as in the

case of hangars. Floor panels are constructed of aluminum clad

plywood which has been previously secured to the floor joists.

The floor joist spaces are filled in the field with insulation

of the glass fiber batt type and subsequently floored over with

an asphaltic vapor barrier, 1 x 4-inch wood sleepers, b/8-inch

interior grade plywood, and finished with various materials

such as felt backed vinyl and vinyl-coated glass fiber floor

tiles. There are several panel types such as those incorporating

windows, doors, previously prepared penetrations for admission

of piping, corner members, filler pieces, eave members, joint

covers, etc. On most buildings, a vinyl coating approximately

0.040 inches thick has been sprayed on the exteriors of roofs

or of roofs and walls. In other instances, roof panel joints

are covered on the exterior with an asphaltic type mastic reinforced with burlap. In some instances, some joints are covered

on the interior with one-inch wide lead tape. This is applied

to both wall and roof panels.

2. No. 2 (See Figure 2).

This is a precast steel-reinforced concrete structure consisting of an assembly of prefabricated concrete panels

placed on a prefabricated steel-reinforced concrete structural

frame. The buildings are of one or two stories, with first

floor constructions of two basic types; (a) with floor raised

above grade and (b) slab on grade. Type (a) consists of prefabricated concrete panels covered with a minimum of two inches

of cellular glass insulation which was in turn protected by a concrete finish slab over which floor tile was applied. Type

(b) consists of a concrete base slab on grade covered with

insulation, concrete finish slab, and floor tile. External wall

cladding of the building consists of prefabricated waffle-like

steel-reinforced concrete panels on a four-foot four-inch module.

The inner wythe of the walls, separated from the external cladding by a two-inch air space, consists of a four-inch thick preformed steel-reinforced insulating panel on a two-foot module.

This panel has 1/2-inch fine aggregate concrete skins covering

a three-inch rigid core of a mixture of wood chips and portland

cement. The interior concrete face was finished with a cementwater wash and a varying number of coats of rubber base or leadin-oil paint. Weep holes to the outside are provided from the

bottom of the two-inch air space. Joints in the exterior cladding were packed with rammed mortar or gunnited. The joints of

the inner wythe of insulating panels are ram-mortared or gunnited

except for the top joint which is caulked. Window frames are

inserted with back-up pieces intended to seal the two-inch

cavity from the room, and are caulked inside and outside to

-4-

prevent infiltration of air. In two-story buildings, the second

floor consists of prefabricated reinforced concrete panels on

reinforced beam supports with a two-inch concrete floor finish

covered with floor tile. The ceilings are not painted or

finished. The flat roof consists of reinforced concrete panels

over which a vapor barrier and three inches of thermal insulation are placed. A five-ply built-up roof is applied on the top

surface of the insulation, and a mixture of asphalt and gravel

three inches thick is placed over the roll roofing to complete

the weather side.

C. Observations

1. Thule Air Force Base, Buildings Type No. 1.

a. Barracks 1-20.

This building was unoccupied and was being subjected to tests to determine the quantity of moisture penetrating the insulation. This was done by measuring the gain in

weight of the insulation and the weight of water consumed by

the humidifier at building conditions of 70 F and 40 percent

relative humidity. Measurements were being made on the windward, center, lee, and wash room portions of the building.

Visual observations were also being recorded.

The exterior roof joints of this building had been sealed with

a low temperature asphaltic compound.

The interior surfaces of the building were dry.

Caulking in the interior joints appeared continuous and the

metal surfaces were unbroken. Some joints could have been

assembled with more caulking while others had too much. Most

triple-pane windows had condensation on the interior surface

of the outer pane. Most test areas of insulation were dry to

the touch. Upon opening the test area in the flat ceiling of

the wash room, some water dripped from around the sealed edge;

the insulation itself appeared dry. A sling psychrometer

measurement of the relative humidity in the wash room was 50

percent at the time.

Numerous panels in the roof and sidewalls were

deformed or buckled, and considerable delamination was observed

between the outer aluminum skin and its adjacent plywood.

b. VIP Barracks 1-112.

This building had been re-roofed. The top surface of each of the horizontal aluminum skin panels was cut out

,

-13-

expos ing the insulation and an inclined roof of 2 x 6 rafters,

plywood sheathing and roll roofing fastened with nailing strips

was erected in cap fashion on the building. The between-roof

space, or plenum, varied in height from about 1 1/2 feet at one

long edge to three feet at the other, and was closed at the

edges with plywood. The plenum was divided into two equal parts

by a partition at mid-length of the building.

The plenum at the windward (east) end of the

building was ventilated by three-inch diameter ports, on

approximately eight-inch centers, high in the longitudinal

plywood sides of the plenum, designed to exclude blown snow.

The plenum on the west or lee end of the building was not

ventilated. This building was occupied and tests were being

conducted to determine the rate of moisture gain by the insulation and to the plenum.

The interior of the building was dry and there

were no recent reports of dripping from ceilings. Traces of

condensation were visible on the triple-pane glass windows.

Interior caulked joints were tight and appeared in good condition. In the unvented roof plenum, the plywood sheathing

appeared wet and the top surface of the exposed insulation contained frozen beads of water. The day was sunny and the undersurface of the plywood sheathing was above 32 F. The exposed

insulation looked and felt soaked with water down to the room

side aluminum-faced plywood, which was wet. In the vented end

of the plenum, signs of moisture on the plywood sheathing were

not visible, but the insulation was wet down to the inner or

room side face.

A second visit was made to this building on a windy colder day as compared With the still sunny day of the

first visit. The unventilated end of the attic had frost patterns on the underside of the sheathing and on protruding nails.

The insulation was wet to the touch for about one inch in depth

from the top side and appeared damp at the lower levels. The

ventilated end of the building did not exhibit any frost patterns on the sheathing but had traces on protruding nails and

the insulation felt damp throughout but not wet.

No reliable observation could be made as to the

degree of ventilation occurring through the ports in the east

plenum, equipped as they were with filters to exclude driven

snow.

- 6 -

c„ Airmen's Mess Hall, 43-1.

This building was similar in construction to

others visited except for its larger volume and a peaked instead

of a flat roof. Several insert-type specimens of the roof insulation were being observed for moisture gain.

The condition of this building was similar to

that of the barracks previously inspected. A joint high in the

peaked roof was dripping water one drop at a time. Personnel

in the building reported that this was the first sign of water

so far this season.

d. Readiness Hangar, 3B-1.

This large building has a peaked roof that had

been re-roofed. The top of each insulated panel was cut out,

exposing the insulation. A cap-roof consisting of 2 x 6-inch

nominal rafters covered with plywood sheathing was laid over

the sloping roof. Roll roofing had not yet been installed;

the joints in the plywood sheathing were covered with wood

nailing strips. A crack vent was left at each eave, and several

roof ventilators were mounted at the ridge to provide ventilation

through the space immediately above the insulation. Insulated

panels were being observed for moisture ingress by utilizing test

sections containing an adsorbent ,• and , in other cases, plain test

sections of insulation.

The interior of this building was not inspected.

Observations made through removable areas of the cap-roof showed

the insulation to be dry in appearance and to the touch throughout its thickness. A definite current of air between the

rafters was perceptible, indicating an appreciable ventilating

effect. The relative humidity at the peak inside the building

was measured with a sling psychrometer . It was less than five

percent at a dry bulb temperature of 99 F. The condition of

the insulation was better in this roof than in any other

building roof examined. Presumably, this building was modified

with a cap-roof because of previous difficulty with wet roof

insulation

e. Other Barracks, Similar to 1-20.

The exterior of the roofs and some interior

rooms of occupied barracks similar to and in the near vicinity

of Building 1-20 were examined. The aluminum on the roof was

noticeably delaminated from the plywood on many panels. Walking

on the roof produced further delaminations. Crunching noises

were heard when a man shifted his weight to the center of a

-7-

panel, and this was presumed to be due to ice within the insulation. This effect was noticeable on about 50 percent of the

panels examined. Mastic covering the exterior joints was soft

and appeared in good condition. Dripping was reported in a room

in one of these barracks. The particular panel affected was

dished downward toward the room approximately two inches from

the normal ceiling level. The joints at the edge of this panel

appeared to be typical, and similar to other joints in the

building. By pressing upward on the sagged portion of the panel,

one could hear a soggy sloshing sound originating within the

panel. A small hole was cut with a knife through the aluminum

skin and plywood at the center of the panel. Some droplets of

water ran out and wet insulation was present. After this, the

hole and joints of this panel were patched with a pressuresensitive adhesive lead tape.

f. Communications Center, N-32.

This group of buildings is located on a mountain

peak in the vicinity of the air base. Buildings at this site

are exposed to high winds and weather of greater severity than

buildings on the air base. The roof joints of these buildings

were heavily covered with low temperature mastic. Most buildings

had flat roofs. Some delamination of aluminum from the outside

plywood was noticeable, and considerable buckling and deformation

accompanied the delamination. Within the buildings, several

spots were observed where dripping was occurring at joints,

particularly at the low end of connecting corridors. In one

instance, light was visible through a wall joint, but in most

cases, joints and aluminum cladding appeared tightly constructed .

2. Sondrestrom Air Force Base (BW-8), Building Type No. 2.

a. Airmen's Dormitory - Building 320.

A portion of the north wall of this two-story

building built on grade was damaged by a truck, affording an

opportunity for inspection of the interior of the wall. The

painted indoor concrete surface appeared pitted. Joints

appeared tight and continuous except for the joint at the

ceiling under the fascia board. This joint in some cases was

cracked and appeared open. The smooth reinforced dense concrete exterior cladding appeared well sealed against the

weather. Recent reports on this and other buildings of this

type state that during thaws water drained from the wall cavity

toward the interior causing a condition of water on the first

floor. This occurred during a period of foehn winds which

caused a sudden rise in outdoor air temperature from much below
,

*
- 8 -

freezing to above freezing (as much as 50 deg. F in eight hours).

Weep holes from the cavity to the outside were plugged with ice,

and sudden melting of ice within the cavities of the walls

caused water to drain onto the first floor of the building, since

insufficient thawing time elapsed to allow weep holes to melt

open. Manual opening of the weep holes dissipated relatively

large quantities of accumulated water, and sounds of what was

presumed to be falling ice in the wall cavity were audible.

b. Bachelor Officers Quarters, Building 321.

The construction of this building was identical

with the airmen’s dormitory except that it was constructed with

an open crawl space beneath. Frozen puddles of water were still

visible on the ground where water in the cavities was relieved

by opening weep holes and by drilling upward into the cavity from

the crawl space. The condition of the building in general was

comparable with that of the airmen’s dormitory. Field reports

indicate that this building also had a serious drainage of water

onto the floor from the wall cavity.

3. Sondrestrom Air Force Base (BW-8), Building Type No. 1.

a. Global Communications Center.

This center is located on a mountain top in the

vicinity of the air base. The larger main building was constructed with a peaked roof. The exterior of the building had

been sprayed with a vinyl plastic. The building as a whole

appeared in excellent condition as regards tightness and caulking

of interior joints. Operating personnel report no trouble from

leaks or drips. Several sections of the exterior vinyl coating

on the roof were peeled and had been blown off.

D . Laboratory Tests and Inspection

Representative panels or components of Building Types

Nos. 1 and 2 were inspected and water-vapor permeance and thermal

conductivity tests were completed in the laboratories of the

National Bureau of Standards.

1. Panel Inspection, Building Type No. 1.

Several representative pieces of aluminum-plywood

skin typical of those in service were requested and subsequent

to the inspection trip were forwarded from salvage to the

National Bureau of Standards for water-vapor permeance tests.

The results of these tests show that the permeance of the

samples, not including joints, was zero.
.•
-9-

Two full-size panels from buildings in service

were requested for detailed inspection. Photographs of these

panels are labeled Figures 3, 4, and 5. The location in

buildings from which these panels were removed is unknown at this

time, but it is believed the smaller panel was taken from a wall

section and the larger panel from the corner of a roof. Figures

3, 4, and 5 show the panels as viewed from the exterior side

with the insulation removed. These panels were examined approximately 1.5 months after removal from the buildings.

The smaller panel (Figures 3 and 4) was 16

inches wide and approximately eight feet high, and is probably

of 1951 construction. This panel was generally in poor condition; most nailed joints were loose and the interior of the

panels was wet and water-marked. The exterior skin was in good

bond with the 1/4-inch plywood except near the edges (see

Figure 3) where the aluminum is wrapped around the edge to form

part of the joint. Nails holding this wrapped edge in place

were loose and rusty and the thin strip of plywood down the

eight-foot length was warped and delaminated to the extent that

the three plies could be peeled with the fingers. The plywood

immediately under the exterior skin (left side of Figure 4) was

damp and wet to the touch; some delamination was visible on the

inner ply next to the insulation, especially near knot holes.

The 3/4-inch plywood overlay at the bottom of the panel was

water-marked and contained surface cracks. This piece was probably exposed to exterior weather conditions. One 3/4-inch plywood stile was sound, but the other near the male portion of the

tongue and goove joint was delaminated two plies deep in two

places (center of Figures 3 and 4). The thickness of this stile/- ranged from its original value of 3/4 inch to a maximum swelled

dimension of 1 1/2 inches, and the stile was wet completely

through its thickness and 3 5/8-inch width. The sound stile

showed moisture marking for 1 1/2 inches of its width as measured from the exterior side. The insulation was moist to

the touch for depths of 1/2 inch to two inches in 3 1/2 inches,

as measured from the exterior side. Several samples of the

glass fiber insulation were weighed, oven-dried, and re-weighed,

and the results showed overall moisture contents from 10 to 14.5

percent above dry weight at this time. The dry density was 3.1

pounds per cubic foot. Normally, insulation of this type could

have one or two percent hygroscopic moisture above dry weight.

The greatest concentration of moisture occurred near the bottom

of the panel. The warm side plywood was not delaminated, and

the bond between aluminum and plywood was tight and continuous.

This plywood was damp near the stiles, presumably from moisture

migrating from the stiles.

- 10-

The larger panel (Figure 5) was 18 inches wide

(insulated space), approximately eight feet high, and was probably of recent construction. This panel, generally, was in

good condition. Considerable moisture-staining and wet areas

were noted on the interior plywood. A small amount of delamination occurred on this sheet. Moisture concentrated near the

central 3/4-inch plywood divider (Figure 5) and at the edges of

the panel. The wood members of the remainder of the panel were

sound. The insulation was damp on the indoor surface and did

not appear so moist as in the smaller panel. Both interior and

exterior aluminum skins were well bonded to their respective

sheets of plywood except for the wet areas near the center of

the interior sheet; at this point, the aluminum was separated

from its backing sheet of plywood.

2. Water-Vapor Permeance Tests.

Water-vapor permeance measurements were conducted on

several samples of materials from Building Types 1 and 2. The

dry-cup method of testing was used, conforming to the requirements of ASTM E96-53T, "Measuring Water Vapor Transmission of

Materials in Sheet Form," Procedure E. The edges of each

specimen were sealed with microcrystalline wax over a metal

cup approximately four inches in diameter, providing an exposed

area of approximately 11.5 square inches. The cup contained

eight mesh anhydrous calcium chloride as a desiccant and the

atmosphere above the specimen was controlled at 100 ±2 F and

82 ±2 percent relative humidity. Weighings were taken periodically and the steady rate of weight gain indicated the rate of

water-vapor transfer, or permeance, of the specimen.

RESULTS

Building Type No. 1 - Four specimens of the aluminum skin,

cut from salvage panels removed from roofs, were measured and

all results showed a permeance of zero.

Building Type No. 2 - Specimens from the inner wythe of the

building included the painted interior concrete skin, wood-chip

and cement insulation, and exterior unpainted skin all removed

from Building No. 320 at Sondrestrom. Specimens of the outer

cladding of the building were not measured because a suitable

specimen could not be cut from between the steel reinforcing.

Additional specimens of inner-wythe concrete skins, procured

from stock material, were also measured. The inner-wythe concrete specimens were manufactured with two different surface

finishes. One side of each panel had a s teel-troweled finish

and the other a wood form cast finish. The reported coatings

of the painted specimen were as follows: one coat of cement
.
- 11 -

water wash for pore sealing, two coats of latex-base paint and

two coats of lead-in-oil paint. The permeance results of specimens measured are shown in Table 1.

Further dry-cup permeance tests were conducted on the above

specimens after various coatings were applied.

Coatings were selected on the basis of the following:

(a) alkali-resistant to prevent rapid failure, (b) low permeance based on past specification tests of selected coatings,

and (c) readily available and reproducible under a Federal

Specification. The original specimens of cast surfaced concrete,

from stock materials Nos. 7 and 8, were not suitable for retest

because the calcium chloride in the permeance cup went into

solution at the conclusion of the last tests, saturating these

specimens. Four additional specimens, cut from the same stock

as specimens 7 and 8, were used for applying coatings. These

specimens were assigned the numbers 7a, 8a, 10, and 11. All

coatings were brush applied and water-vapor permeance results

are shown in Table 2, Measurements were not made after the

first or primer coat of the first finish coat because the

coverage or hiding power of these coats was not satisfactory

when applied to porous surfaces. The estimated spreading rates

applied per coat are 400-500 ftVgal. for primers and 300-400

ftVgal. for finish coats. A permeance result for specimen

No. 11 was not obtained because of a leak that developed in the

test cell.

3. Thermal Conductivity Tests.

The thermal conductivity of the wood-chip and

cement insulating core of the inner wythe of Building Type No.

2 was measured in an eight-inch guarded hot-plate apparatus

conforming with the requirements of Federal Specification

LLL-F-321b and of ASTM C177-45, The specimen was air dried to

constant weight in an oven at 215 F immediately prior to the

conductivity measurement,. The conductivity of the specimen

was about 0.67 Btu/hr ft^(deg. F/in.) at 64 F mean temperature.

The density was 29.3 lb/ft^ after drying, which caused a loss

in weight of about 13 percent of the dry weight.

Measurements of the concrete skins of the inner

wythe, and of the concrete of the outer cladding, were not made

because of the difficulty of procuring suitable samples of

large enough size. On the basis of past measurements of many

concretes of various densities and aggregate sizes, it is

believed that the thermal conductivity of the concrete used

in these buildings may be estimated with sufficient accuracy.
*

.
- 12 -

The thermal conductivity of these concretes was estimated as about 6.2 Btu-in./hr ft 2 deg. F, assuming an average density

of 125 pounds per cubic foot.

E . Discussion of Principles, Observations, and Laboratory

Results

1. Introduction.

In this sec ti on

,

a dis>cuss

several ways in wh i

h the observed moi

the var ious building cons true tions and

t ative es tim ates are made of rates of

several poss ible mec hanisms

This inf

in rea

hing conclusi ons a s to the imost

of the moisture accumulations observed

the preventive or curative measures of

ion i s give n of the

s ture could have ent ere

, whe re pos sible , quant

moist ure in gress by the

ormat ion is used to ass

prob able c ause or c aus

, and thus to po int to

most importance .

could have

The moisture observed in the various constructions

been the result of one or more of the following:

Materials wet or moisture laden when erected.

b. Inleakage of water or snow through exterior

joints or openings.

c. Vapor permeance through inside finish.

d. Vapor diffusion through still air in unsealed

interior joints.

e. Flow of moist indoor air into constructions

through leaks in interior finish.

f. The probable extent of moisture egress from the

interior of the wall construction to the outdoors, as a result

of the vapor permeance of the exterior finish, or of ventilation

with outdoor air, or of water drainage during thaws.

The considerations involved in estimating vapor

permeance and condensation in constructions are given in National

Bureau of Standards Report BMS-63, "Moisture Condensation in

Building Walls," attached hereto as an Appendix.

The entry of moisture into a construction by its

conveyance in a flow of relatively moisture-laden air into its

interior from the indoor space may be considerable, depending on

the rate of inflow of air, its moisture content, and the effective dewpoint maintained in the interior of the construction by

the existing temperature conditions. As an illustration, if

-13-

leaks at the top and bottom of a panel construction allowed a flow of one cfm of indoor air at 70 F and 30 percent relative

humidity (moisture content = 2.5 grain/cu ft) through the panel

interior, and the temperature of the inside surface of the

outer panel face were 0 F (moisture content of air at 0 F dewpoint = 0.4 grain/cu ft), the rate of accumulation of moisture

in the cavity would be 126 grains/hr or 0.43 Ib/day. The problem of estimating moisture ingress due to air inleakage is

therefore chiefly one of estimating the rate of air inleakage.

2. Sondrestrom Air Force Base (BW-8), Building Type No. 2.

Observations concerning Building Type No. 2 are discussed in relation to the factors (a through f) listed in the

introductory paragraphs.

a. Materials Initially Wet.

Observations were made on the walls of the concrete buildings only, since the roofs and floors have not

exhibited any visible moisture damage. The materials from a broken wall that was examined appeared dry to the touch.

If a wall of this kind were erected in a wet

condition, the moisture in its inner wythe would tend to migrate

to the outer wythe as a result of the indoor-outdoor temperature

difference in winter.

Only a rough estimate can be made of the possible

moisture content of this construction as erected. A sample of

the wood-chip concrete insulation had a moisture content of 13

percent of its oven-dry weight, when it was measured at the

National Bureau of Standards after shipment from BW-8. Assuming

a possible initial moisture content of 25 percent for this component, which weighed approximately 147 pounds per 2 x 10-foot

panel, and of 10 percent for the 1/2-inch concrete faces of the

inner wythe, which weighed approximately 200 pounds per panel,

the moisture available for migration would be less than 57

pounds per panel. This amount of water is equivalent to a layer

of ice 0.6-inch thick on the inner surface of the outer wythe.

It may be added that in the first winter of

exposure of an initially-wet wall, its excess moisture would

probably be removed from the inner wythe by migration, and

drained away through the weepholes during thaws. Thereafter,

normal hygroscopic moisture in the inner wythe would be equivalent to a much smaller ice-layer thickness in successive

winters

-14-

b. Exterior Leaks.

The exterior joints of wall panels appeared

tight and, based upon observation, it appears that little if

any moisture could have entered the joints from the exterior.

The whole surface of the outer cladding appeared well able to

resist the elements.

c. Vapor Permeance Through Inside of Finish of Wall.

The method of manufacture of these prefabricated

interior wythes yields one concrete skin with a form or cast

finish and the other with a steel- troweled finish. Since these

panels are reversible end-wise in construction, it is possible

to have either a cast or a s teel-troweled surface facing the

interior

Data on the vapor permeance of various samples

of materials from typical walls are given in Table 1, and of the

materials with various paint coatings in Table 2. The permeance

of unpainted concrete skins of the insulating wythe was approximately three perms when they had been s teel-troweled , and about

20 perms when they had a cast surface. It is evident that since

these panels are reversible f ace-for-f ace , the s teel-troweled

surface should be on the indoor side to reduce vapor permeance

into the insulation. The data for the various painted specimens show that the overall permeance of the wythe can be reduced

to less than one perm by suitable paint coatings.

An estimate of the possible amount of moisture

over a winter, using a permeance of one perm for the wythe

based on measurements of the materials for Building No. 320

given in Table 1, shows the total amount of water vapor transferred to be equivalent to a layer of ice 0.028-inch thick on

the inner surface of the outer wythe.

d. Vapor Diffusion Through Unsealed Interior Joints.

The inside vertical wall joints that were

visually inspected appeared tight. However, the existence of

vertical wall joint cracks in various buildings has been reported

but the extent of joint cracking is not known. The joints

between the floor and wall were hidden by floor tile and wood

molding and possible cracks were not readily visible. The joint

between the wall and the spandrel beam at the ceiling in some

cases revealed gaps about 1/16-inch in width.

If such a gap were continuous from the interior

of the room to the wall cavity, water vapor could diffuse through
•••

.
-15-

it and be carried

outer wythe where

by convection to the

it could condense.

inner surface of the

An estimate of the amount of moisture

accumulate over a winter by diffusion through a 1/16-i

two feet long, indicates the equivalent of a layer of

inch thick, if this moisture were distributed over the

one 2 x 10-foot prefabricated panel.

that could

nch gap,

ice 0.0004-

area of

e. Flow of Moist Indoor Air Into Construction

Through Leaks in Interior Finish.

If a passage for air flow between the room and

the interior space of the wall existed at both top and bottom,

stack action would cause a circulation of room air through the

wall interior, carrying moisture with it. Joint cracks were

observed at the ceiling-wall junction; the leakage of water

into the room at the floor during foehn winds indicates a passage also at the bottom of the wall. Thus, air circulation

between the room and the wall interior seems possible in at

least some panels. An additional contributing factor to air

circulation between the room and the wall cavity are the cracks

around the periphery of the window casing. (it should be noted

that one gap into a panel could also allow room air to enter

its interior if exterior wind caused a pressure differential

from the room to the wall interior, i.a., on the lee side of

the building .

)

A quantitative estimate of the amount of moisture transferred by circulation of moist air through openings

1/32 inch wide and two feet long at the top and bottom of the

inner wythe show an accumulated amount over a winter equivalent

to a layer of ice 1.33 inches in thickness distributed uniformly

over the area between the top and bottom cracks and deposited

on the inner face of the outer cladding. Cracks at both the

top and bottom probably are not continuous or may not exist for

every prefabricated panel and their frequency around the periphery is unknown for comparative purposes. If the rate of

vapor flow accomplished by convection under the above assumed

conditions were expressed in terms of equivalent vapor permeance

for a wall area between the top and bottom cracks, the value

calculated is 48 perms. Thus, moist air inleakage appears to

be potentially a much more important method of moisture entry

than vapor permeance through the panel itself.

A summary of the amounts of moisture estimated

as possibly accumulating as a layer of ice on the inner surface

of the cladding per winter is given below.
.

*

,
-16-

Cause

a) Initial moisture

b) Exterior leaks

c) Vapor permeation

d) Vapor diffusion

e) Convection

Equivalent thickness

of ice, inches

0.6

0.028

0.0004

1.33

Total 1.96

f. Egress of Moisture From the Cavity to the Outdoors by Vapor Permeation Through Exterior Cladding, Vengilation

with Outdoor Air, or Water Drainage Through Weepholes.

The vapor permeance of the two-inch concrete

exterior cladding was not measured. Data on concrete permeance

are very limited, especially for conditions of below-freezing

temperatures. Assuming the permeability of the cladding concrete to be about the same as that of the cast inner wythe skins

(10.4 perm-inches), the permeance of the cladding is taken as 5.2 perms. On this basis, vapor permeation to outdoors per

winter would be equivalent to an ice layer of 0.014 inch. This

is negligible compared to the total accumulation of 1.96 inches

of ice within the wall cavity per winter as given above.

Ifs'the cavity were to be ventilated with outdoor

air to remove th^s moisture, it is estimated that the necessary

ventilation to remove it, with outdoor air at -10 F and 55 percent relative humidity, would be on the order of 17.7 cfm per

story for each two linear feet of wall.

On the basis that the maximum winter accumulation of ice on the outer cladding is 1.96 inches thick, sudden

thawing due to a foehn wind would call for a water run-off

through the cavity weepholes for each interior panel of 22

gallons of water (89 percent of the cavity volume). This volume

of water could readily escape from open weepholes, especially

since thawing would not be rapid, but if weepholes were not open

from the start of the thaw, there would be danger of soaking the

inner wythe with the melt water until drainage occurred, leading

to a condition somewhat similar to that considered under the

heading of initial moisture in the construction.

3. Thule Air Force Base, Building Type No. 1.

Observations concerning Building Type No. 1 are discussed in relation to factors (a) through (f) listed in the

introductory paragraphs of this section.

-17-

a. Materials Initially Wet.

Field observations and previously reported observations at Thule indicate that water does not drip into the

interior from every roof panel. Observed badly-wetted areas

were more or less localized and not always at the lowest point

on the roof in any particular buildings, and some roofs of some

buildings did not drip at all. This distribution suggests that

the panels that are wet might have been erected containing considerable moisture while the others did not. Admission of moisture could conceivably have occurred during manufacture, storage,

transportation, and erection of some of the panels. One field

observation, on one flat roof and based on the sound of ice

crunching in roof panels when walked upon, indicated that about

50 percent of these roof panels contained some moisture. Other

reported observations of several buildings and of cored samples

removed from selected sections of the roof panels of buildings

indicate that some buildings did not exhibit any visible moisture, while others showed as high as 80 percent of the panels

examined contained moisture in varying amounts. Of the ten

buildings examined, only two showed high percentages of the

incidence of moisture.

If prefabricated panels of this type were

erected in a wet condition, the moisture would tend to migrate

to the outer face as a result of the indoor-outdoor temperature

difference in winter. The metal exterior skin constitutes a good vapor barrier and if the exterior joints were vapor tight,

the initial moisture would remain trapped in the panel.

Only a rough estimate can be made of a possible

moisture content of a panel as erected. For moisture contents

of 50 percent of the dry weight of the insulation and plywood,

the total amount of water in a two-foot by eight-foot panel

would be equivalent to a layer of ice 0.35-inch thick, distributed uniformly over the inner surface of the outdoor face.

b. Exterior Leaks.

A possible source of moisture trapped in roof

and communicating wall panels could originate from rain or

melting snow entering the exterior joints of the roof panels

immediately after erection and prior to the application of

mastics or other joint coverings. Lack of caulking during construction, contraction of the exterior portion of the panel

joints, and movement or settling of the building are reasonable

possibilities. Flat roofs allow puddling of water and one

could expect more moisture problems on this type than on

pitched roofs. Water entering pitched roofs could run on panel
.
-18-

interior surfaces from panel to pa nel from the ridge to the

eaves if it did not drain through unsealed interior j oints at

the ends of the panels.

An estimate of the amount o f leakage per panel

cannot be made with any accuracy. Use of weather pre cipitat ion

data would be misleading because of run-off from one panel

entering another, puddling in low spots, varying amou nts of

caulking

,

degree of joint opening, and the various ag es and

histories of the buildings.

c. Vapor Permeation Through Inside Finish of Walls

and Roofs

Laboratory tests on aluminum skins taken from

salvage panels indicate zero perms; hence, these specimens were

not capable of passing water vapor. If the interior aluminum

skins were penetrated or marred, a path for the transfer of

water vapor would exist. Field observations indicate that very

few large penetrations are present.

d. Vapor Diffusion Through Unsealed Interior Joints.

Most of the joints examined visually appeared

tight and continuous. In some cases caulking was sparse and in

others excessive. For most of the buildings observed, the

exterior joints were treated with a continuous thick mastic

which provided a seal impermeable to water vapor.

An estimate of the amount of moisture that could

accumulate in a 2 x 8-foot panel during a winter by diffusion

through a 1/32-inch gap in half the peripheral joint length

indicates the equivalent of a layer of ice . 0 . 0016- inch thick,

if this moisture were distributed uniformly over the outdoor

f ace

e. Flow of Moist Indoor Air Into Panels Through

Leaks in Interior Finslh.

If caulking of the interior joints of the walls

or ceilings were faulty, paths for the circulation of the relatively moist room air into the joint space between panels would

be possible. Presumably, the vapor conveyed in the air would

condense in the colder parts of the joint spaces, whence it

might soak through or run through cracks into the panel space

itself. In some cases, the room air might circulate even through

the panel space, and condense on the cold surface of the outer

skins. In any case, such moisture in cold weather would tend to

migrate to and collect on the inner surface of the outer skin.
• .
19-

Only a rough estimate can be made of the possible

moisture transfer by convection. The most reliable information

concerning the total rate of moisture ingress with this type of

joint would be a laboratory test on roof, wall, and floor panels.

Laboratory observations of some panels removed from the walls

and ceiling of a building indicated more rapid deterioration of

the wood had occurred in the wall panel as compared to the

ceiling panel examined. An estimate for the joint in the walls

of a two-foot by eight-foot wall panel with openings through

the joints at the top and bottom, and with an estimated air

path in the joint space of 0.00125 sq ft in area, shows the

total amount of water transfened as vapor over a winter would

be equivalent to a layer of ice 0.24 inch thick distributed

uniformly over the area of the inner face of the outdoor surface, assuming that the moisture accumulation in the joint

space as ice or frost did not prevent continued air circulation.

However, for the low outdoor temperatures generally prevalent,

it seems probable that the deposited moisture would choke the

joint space and inhibit the continued flow of air. It is

j udged that the ingress of moi s ture due to convection in

ceiling and fl oor panels would be 1 ess than for wall pan els . A summary of the amount s of moisture es t imated

as poss ibly ac cumulating on the inner s urf a ce of the out er fac> as a layer of ice is given bel ow.

Eq uiva lent thicknes s Cause of ice, inche s

a) In itial moisture 0.35

b) Ex terior leaks Fortuitous

c ) Va por permeation 0

d) Va por diffusion 0.0016

e ) Convection 0.24

Tota 1 0.59

f

Egress of mois ture from the interior of the

c avity to 0 utd oors by permeati on through ex terior fin ish

ventila t ion of the cavity with outd oor air

,

or water dr a inage

The zero perme ance of the e xterior su rf a ce and

the res is ta.nee of the heavy ma stic cove ring the exter ior joint:

effectively prevents moisture transfer to the outdoors. For

buildings without mastic over the joints, some moisture transfer out of the joints probably takes place but is quickly canceled by the leakage of rain or melting snow.

- 20-

Venting the cavity with outdoor air cannot be readily

accomplished without structural changes and venting ports that

would exclude blown snow. The rates of ventilation necessary

to dry out the present construction and keep it dry without

seriously affecting the heat transfer rate are difficult to

predict, especially for roof panels.,,,

4. Remedial Considerations and Drying,

a. Sondrestrom Air Force Base (BW-8), Building Type

No. 2.

The moi

ice will probably drain

summer. If further acc

season would have the e Preventing the ingress

through leaking inside

rapid thawing during an

are to be avoided.

sture accumulated in the wall cavity as through exterior weepholes during the

umulation does not occur, a second winter

ffect of self-drying the inner wythe.

of moisture to the cavity from indoors

joints is imperative if the effects of

above-freezing temperature foehn wind

b. Thule Air Force Base, Building Type No. 1.

If the structural soundness of these buildings

is to be maintained, the wet insulation and wooden portions of

the panels should be dried or replaced. In view of the heavy

mastic over the exterior joints, a practicable and economic

way to expel moisture from these panels appears to be an

approach from the interior of the structure. Some possible

methods considered includes (1) cutting holes in the interior

surface of roof and wall panels for water drainage and resealing

with tape or a self tapping plug, (2) circulation of dry air in

the panel cavity, and (3) removal of one or both surfaces. The

first method does not dry out the interior of the panel and the

second method would probably take too much time. It appears

that a solution involves removing either the indoor or outdoor

surface and replacing or drying existing wet insulation.

5. Curative Measures.

Curative measures taken should provide for better

prdtection against leakage of indoor air into the construction

through the joints. All gaps and cracks should be filled to

prevent convection of indoor air to the building cavities.

For walls and roofs, ventilation or an exterior covering

arrangement that would allow escape of water vapor to the outdoors and at the same time provide protection from the elements

would provide additional protection against the effects of conT

dens ation

- 21 -

For future construction, the use of dry materials

and installation in a manner to prevent wetting should be

rigidly controlled. The design of Building Type No. 1 and the

walls of Building Type No. 2 should be changed to accommodate

provisions to prevent moisture accumulation in future cons true tion

III. GENERAL CONCLUSIONS

f A. Thule

1. The factors contributing to the accumulation of

moisture in the interior construction of these buildings are

believed to be residual moisture at the time of erection,

leakage from the exterior after construction, and condensation

of water vapor originating in the interior of the building.

2. Quantitative estimates indicate that residual

moisture and exterior leakage before sealing over joints probably provided the largest portion of the moisture presently

in the panels.

3. It is difficult if not impossible to determine

quantitatively the proportionate part of the total accumulation

contributed by each of the above factors. Quantitative estimates indicate the possibility of each factor, if taken

separately, being capable of contributing enough moisture to

produce a wetting or dripping from joints.

4. Any transfer and subsequent condensation of water

vapor from the interior occurred through the joints rather

than the panel faces.

5. Deleterious effects of moisture in panel cavities and

joints include (a) water and caulking falling from ceilings to

the interior space, (b) disintegration of bonding materials and

glues causing delamination of plywood and separation of aluminum skins resulting in the loss of structural soundness, and

(c) increased heating fuel consumption resulting from greater

heat loss caused by wet materials.

B . Sondres trom

1. The factors contributing to the accumulation of

moisture in the wall cavities are believed to be residual

moisture at the time -of construction and condensation of water

vapor transferred from the interior of the structure to the

wall cavity.

- 22 -

2. Quantitative estimates of each of the above factors

indicate that water-vapor transfer was probably the largest

contributing factor. This transfer was probably accomplished

by (a) convection of air and water vapor from the interior

through openings at the floor and ceiling levels, (b) diffusion

of moisture from the interior through holes or cracks to the

wall cavity, and (c) permeation of water vapor through the surface of the wall itself. The last two are estimated as small

compared to the amount possible by convection alone.

3. Laboratory measurements of the water vapor permeance

of the inner wythe skins indicated that the permeance of the

skin with a steel- troweled surface is about one-seventh of that

with a cast surface, and therefore the panels should be erected

with the steel- troweled surface facing the interior of the

building

4.

Damaging effects from excessive moisture include

(a) leakage of water from the bottom of the wall cavity to the

first floor caused by melting of ice or frost in the wall cavity

at a time when weepholes are frozen closed, (b) increased fuel

consumption for heating due to lower thermal resistance of the

wall because of wet materials, and (c) possible structural and

material damage from freezing and thawing.

Table 1

WATER-VAPOR PERMEANCE RESULTS FOR INNER-WYTHE MATERIALS

NBS

Spec . No . Material

Specimens

Thickness

of

Specimen Specimen,

Surface Inches

from Stock

Permeance

Grains/

hr ft 2

(in. Hg)

Permeability

Grains-inch/ Perms for

hr ft 2 Design

(in. Hg) Thickness

5 Concrete

Skin

SteelTroweled , Unpainted

0.61 3 . 1 1.9 3.8

6

If ft 0.63 2.9 1.8 3.6

avg . 3.7

7

ft Cast Side

Unpainted

0.36 24.9 9.0 18.0

8

ft ft

Specimens

0.70 16.7

from Building No.

11.7

320

23.4

avg . 20.7

3 Interior

Skin

Painted

on Cast

Side

—

0729 273 0.67 1.34

4

ft ft 0.32 2.0 0.64 1.28

4 3, f f t! 0.35 1.7 0 60 1 . 19

avg . 1.27

9 Wood-Chip

Cement

Core

i Specimen

Sanded

0.63 27 .17.4 5.8

1 Exterior

Skin

SteelTroweled , Unpainted0.59 3 . 5 2 . 1 4.2

2

f? tf 0.55 3 . 1 1.7 3.4

avg .3.8

*For inner wythe skins 1/2 ,v

, for wood chip-cement, 3"
I
Table 2

WATER-VAPOR PERMEANCE RESULTS FOR

Specimens from Stock, SteelINNER-WYTHE MATERIALS

-Troweled Finish

NBS

Spec Coatings Applied*

Permeance

grains/

hr ft^ (in.

Permeability

grains-inch/

Hg) hr ft^ (in. Hg)

1 No cement wash; 1

coat primer; 2 coats

enamel

0.52 0.31

2 Cement wash; 1 coat

primer; 2 coats

enamel

0.43 0.24

5 No cement wash; 2

coats latex base

paint

1 . 10 0.67

6 Cement wash; 2 coats 1.17 0.74

latex base paint

Specimens from Stock, Cast Finish#

7a Cement wash; 1 coat

primer; 1 coat enamel

undercoat; 2 coats

enamel

0.18 0 . 12

8a Cement wash; 1 coat

primer; 2 coats

enamel

0.57 0.34

10 Cement wash; 1 coat

primer; 2 coats

latex base

0.80 0.49

11 Cement wash; 2 coats

latex base

— — —

Painted Specimens from Building No. 320

3 Two coats enamel 0.58 0.17

4 Two coats flat oil 0.84 0.27

paint

4a Two coats latex base

Perms for

Design

Thickness

0.62

0.48

1.34

1.48

0.24

0.68

0.98

0.34

0.54

1 . 28 0.45 0.90

Table 2 (Continued)

Coatings applied to specimens of Table 1 (numbers and

thicknesses correspond in Table 2) as follows:

Cement Wash - 2 parts cement plus 1 part water by weight

Primer, alkalai resistant - Sherwin Williams No. 61 used

Enamel, gloss interior - Federal Specification No. TT-E-506c.

Undercoat, enamel - Fereral Specification No. TT-E-543

Oil paint, interior flat - Federal Specification No. TTP-51d.

Latex base, interior flat - Federal Specification No. TTP-29

^Thicknesses of these four specimens were: 7a, 0.64";

8a, 0.60"; 10, 0.61"; 11, 0.53".
,* .




i-j-erure i

.f'l crnTP <->

Mastic c£

-< <

CD

!Uw3

O)

Ll.

Pdnel

Jointj

BuOding

Type

No,

S,

Exploded Vi

ey/

U. S. DEPARTMENT OF COMMERCE

BUILDING

MATERIALS

AND

STRUCTURES

MWfat ksB***^5 REPORT BMS63

Moisture Condensation

Building Walls

HAROLD W. WOOLLEY

gggjj53^E3ggB

NATIONAL

BUREAU OF STANDARDS

The program of research on building materials and structures, carried on by the

National Bureau of Standards, was undertaken with the assistance of the Central Housing Committee, an informal organization of governmental agencies concerned with

housing construction and finance, which is cooperating in the investigations through a committee ot principal technicians.

CENTRAL HOUSING COMMITTEE

ON RESEARCH, DESIGN, AND CONSTRUCTION

A C. Shirk, Chairman.

United States Housing Authority.

Howard P.- Vermilya, Vice Chairman.

Federal Housing Administration.

Sterling R. March, Secretary.

Pierre Bloujke,

- Federal Home Loan Bank Board.

Hugh L. Drvden,

National Bureau of Standards.

Louis A. Simon,

Public Buildings Administration.

Luther M. Leisenring,

Construction Division (War).

Edward A. Poynton,

Office of Indian Affairs.

John S. Donovan,

Farm Security Administration.

George W. Trayer,

Forest Service (F. P, Laboratory).

Joseph M. DallaValle,

Public Health Service.

George E. Knox,

Yards and Docks (Navy)

William R. Talbott,

Veterans’ Administration.

Wallace Ashby,

Bureau of Agricultural Chemistry and Engineering

NATIONAL BUREAU OF STANDARDS

STAFF COMMITTEE ON ADMINISTRATION AND COORDINATION

Hugh L. Dry den, Chairman.

Mechanics and Sound.

Pi i aon H. Bates,

Clay and Silicate Products.

Hobart C. Dickinson,

Heat and Power.

Warren E. Emley,

Organic and Fibrous Materials.

Gustav E. F. Lundell,

Chemistry.

Adda ms S. McAllister,

Codes and Specifications.

Henry S. Rawdon,

Metallurgy.

The Forest Products Laboratory of the Forest Service is cooperating with both

committees on investigations of wood constructions.

(For lint of BMS publications and directions for purchasing, sec cover page in]

UNITED STATES DEPARTMENT OF COMMERCE • Harry L. Hopkins, Secretary

NATIONAL BUREAU OF STANDARDS • Lyman J. Briggs, Director

UNITED

BUILDING MATERIALS

and STRUCTURES

REPORT BMS63

Moisture Condensation in Building Walls

by

HAROLD W. WOOLLEY

ISSUED DECEMBER 14, 1940

The National Bureau ol Standards is a fact-finding organization;

it does not “approve” any particular material or method of construction. The technical findings in this series of reports are to be construed accordingly.

STATES GOVERNMENT PRINTING OFFICE • WASHINGTON • 1 94°

FOR SALE BY THE SUPERINTENDENT OF DOCUMENTS, WASHINGTON, D. C. PRICE IO CENTS
••

.
Foreword

This paper was prepared as a result of numerous requests for information on moisture condensation in insulated walls, which will occur under certain conditions. It discusses these conditions in a general way and makes available to architects, builders,

and others information concerning factors which control the humidity in walls. Available data relative to certain types of wall have been collected so that estimates can be made concerning the probability of moisture condensation in walls of dwellings

if the factors governing such condensation, including design, structural materials, construction details, and moisture conditions inside and outside of the building are known.

The Forest Products Laboratory has made extensive studies of this problem in relation to frame construction, and their reports, given in the list of references at the end

of this paper, may be consulted for specific experimental data.

Lyman J. Briggs, Director.

[ill

Moisture Condensation in Building Walls

BY HAROLD W. WOOLLEY

C O N T E N T S

Page

Foreword u

I. Introduction 1 II. General principles involved in condensation. 1

Page

III. Means of preventing condensation 11 IV. Conclusions- ... _______ 13

V. References 13

ABSTRACT

Moisture condensation in insulated walls will occur under certain conditions. 'I'lds paper discusses these conditions in a general way and makes available to architects, builders, and others, information concerning factors which control humidity in walls. These

factors are presented so as to make possible estimates

of the probability of moisture condensation in walls of dwellings if the factors governing it such as design, structural materials, construction details, and moisture

conditions inside and outside the building, are known.

I. INTRODUCTION

In recent years difficulties resulting from

condensation of moisture, particularly within

insulated walls of dwelling houses, have re- ceived considerable attention. When such

condensation occurs to a sufficient degree,

damage to the building structure or decorations may result. No definite statistics are available to indicate to what extent condensation is producing damaging results. However, a

large number of eases of condensation iiave

been reported, particularly in new houses in the colder parts of the country. Condensation

problems are most frequent in houses of modern

tight construction, with weather stripping,

storm sash, insulation, and the newer types of

heating systems that provide some means of

humidification.

The principles involved in the condensation

of moisture in building walls, though comparatively simple, are not understood by the aver- age layman and not always by architects and

builders. A general discussion of the principles of moisture condensation is therefore in- cluded in this paper, which may be helpful to

both the buying public and the builder. The

discussion is based not so much on definite

tests made at the National Bureau of Standards

as on well-established facts derived from various

sources.

II. GENERAL PRINCIPLES INVOLVED

IN CONDENSATION

Atmospheric air always contains moisture.

Generally the moisture is not seen because it is in the form of an invisible vapor. It, can easily

be made visible as liquid water by cooling the

air. Thus in the summer water condenses as dew on the outside of a pitcher of cold water.

If the condensation occurs below the freezing

point, it is expected to be in the form of frost. The freezing coils of a refrigerator acquire such

a deposit continually. Condensation of moisture is so common an occurrence that the principles governing it should be known to everyone.A definite volume of air held at a fixed temperature can contain permanently no more than

a definite amount of water in the form of vapor.

This limiting quantity of water per given

volume is termed “moisture content at saturation.” If the air contains a greater proportion

of moisture than this at the particular temperature, the water will start condensing on the

surfaces of the container or even on the dust

particles in the air which then fall out in a

fine mist. The ratio of the actual moisture

content to the saturation moisture content for the particular temperature is termed “relative

humidity.” It is customarily expressed in percent.

2010) 1 ° -40
I
The concentration of water vapor may also be stated by giving its pressure. If water

vapor is present, part of the atmospheric

pressure is maintained by the water vapor and

the remainder of the pressure by the other constituents of the atmosphere. At a particular

temperature and at saturation the water vapor

exerts a definite pressure. Data on pressure

and density of saturated water vapor will be

found in table 1. If the pressure of the water

vapor at a particular temperature is less than

the tabulated value the air is not saturated.

The ratio of the actual pressure of the water

vapor to the saturation pressure of water vapor

for the particular temperature is also termed

relative humidity. Its value when defined

thus is essentially the same as that given

previously. Throughout this paper, use is made of the last-named “relative humidity.”

Table 1 . —Pressure and density of water vapor

at sat unit ion

(Saturation vapor pressure for water, p, in pounds per square inch. Sat- uration vapor density for water, d, in grains » per cubic foot]

Tempera- ture P d Temperature P . d

°F

-15 0.008230 0. 2182 °F

25 0. 06406 1 . 558 -14 .009704 .2303 26 . 06710 1. 629 -13 .009201 .2430 27 . 07034 1. 704 -12 .009725 .2562 28 . 07368 1.781 -11 . 010276 . 2702 29 . 07717 1.862 -10 . 010855 .2845 30 . 08080 1.946 -9 . 011464 .3001 31 . 08458 2. 033 -8 .012101 . 3158 32 . 08856 2. 124 -7 012775 .3327 33 . 09230 2. 203 —

. 013483 . 3505 34 . 09610 2. 288 -6 . 014226 . 3602 35 . 1000 2. 376 -4 .015006 .3883 36 . 1041 2. 469 -3 .015825 . 4086 37 . 1083 2. 563 -2 .016685 . 4299 38 . 1120 2. 660 -1 . 017586 . 4521 39 . 1171 2. 760

0 . 018533 . 4750 40 . 1217 2. 803

1 . 01953 . 500 41 . 1265 2. 970

2 . 02057 .525 42 1315 3. 081

3 02166 . 552 43 . 1367 3. 196

4 .02281 . .580 44 . 1420 H. HI

6 02401 .609 46 . 1475 H. 436

6 . 02527 .640 46 . 1532 3. 562

7 .02658 .672 47 . 1591 3. 692

8 . 02796 . 706 48 . 1052 3. 826

9 02941 .740 49 . 1715 H. 964

10 . 03092 .776 50 . 1780 4. 106

11 . 03251 . 814 51 . 1848 4. 255

12 .03418 .854 52 . 1918 4.407

13 . 03590 .8% 53 . 1989 4. 561

14 . 03772 .939 54 . 2063 4. 722

16 . 03961 . 0o4 55 . 2140 4. 889

16 . 04160 1 031 56 . 2219 5.060

17 . 04369 1. 081 57 .2300 5. 234

18 . 04586 1 132 58 . 2384 5. 415

19 04812 1. 185 59 . 2471 5. 602

20 . 05048 1. 242 j 60 . 2561 $. 796

21 . 05206 1. 299 61 . 2664 ‘6.993

22 . 05555 1. 361 62 . 2749 6. 1%

23 . 05826 1.423 I 63 . 2848 6. 407

24 06111 1.489 1 64 . 2949 6.622

Table 1. Pressure and density of water vapor

at saturation—Continued

Temperature P d Tempera- ture

1 p !

1

i

d

°F °F

i i

i 65 0. 3054 6. 845 78 I 0. 4744 10. 38

66 3162 7.074 79 . 1903 10. 71 67 3273 7. 308

68 . 3388 7.571 80 . 5067 11.04 69 , 3506 7.798 81 . 5236 11 . 39

82 . 5409 11.76 70 . 3628 8. 055 83 , . 5688 12. 1

71 . 3754 8. 319 84 . 5772 12. 49 72 . 3883 8. 588

73 4016 8. 867 85 . 5960 12.87

74 .1153 9. 153 86 . 6153 13. 27

87 I . 6352 [ 13. 67 75 . 4295 9. 448 88 . 6555 ; 14.08 70 . 4440 9. 749 89 . 6765 1 14. 51 77 .4590 10. 06

» 7,000 grains equals 1 pound. When a given mixture of air and water vapor

is cooled without loss of moisture, a temperature is eventually reached where the air becomes

saturated with water vapor and condensation

can occur. The temperature then existing is called the dew point. At any given dew point,

water vapor is always exerting a pressure corre- sponding thereto.

The air in the hollow spaces or pores in building walls always contains water vapor. Under

certain conditions the temperature of portions

of the wall may be below the dew point of the

air in contact with them, and condensation will occur. While at first it might appear that any water

not in vapor form in the wall would be ordinary

liquid water, the facts are not quite so simple.

Many building materials are able to absorb

water to a moderate extent without undergoing great changes. Wood will absorb water

up to from 20 to 30 percent by weight at rela- tive humidities near 100 percent. Beyond

some such point further water absorbed is retained as free liquid water. Other similar

materials differ in the moisture content at which

further absorbed water is only mechanically

held.

Water has several effects on building materials. As is very well known, some materials,

such as wood, expand with increasing moisture

content. If conditions of greatly varying

humidity occur in different parts of the cross section of a single framing member there will be some tendency for the wood to warp. High

humidity favors decay of wood. Some otluu

materials are adversely affected by continued
.
exposure to liquid water. Water promotes

corrosion of metals, and even some mineral

insulating materials show permanent change in the course* of time when in contact with water. The insulating value of materials in general is greatly decreased by the presence of free water. Droplets of water tend to bridge gaps between

separate solid portions of ordinary insulating

materials and increase the transfer of heat.

The distillation of moisture from place to place

in the wall can also increase heat transfer since

heat must be added to evaporate the moisture

and then is released again in the colder region

where the moisture may happen to condense.

Normally this effect will be important mainly

when the temperature indoors is increasing.

Moisture condensed in the walls may not

remain in its first location. It is even possible

for it to migrate and succeed in making the

plaster and wallpaper damp and discolored.

When that symptom appears in late winter or

spring, and unless the trouble is caused by leaks,

it is very probable that condensation in the

wall is responsible. In such a case the following explanation should be to the point.

If different gases mix without stirring, the

movement of the one gas into the other follows a simple law of diffusion. If one gas is of rela- tively low concentration, its rate of diffusion

into the other is closely proportional to its pressure difference per unit distance. The law

is found to hold fairly well for the diffusion of

moisture, even through many solid materials.

There are exceptions, but in this discussion

they will be largely ignored. Data in regard

to diffusion of water vapor through various

materials will he found in table 2. In tins has

been used as a unit

Table 2. —Permeability of various materials to water

Material

vapor

Thick- j ness j

1

Permeability

to moisture (P)

Va|>or

resistance

(1IP)

BABBITT [S] »

| Grains st/ ft hr (lb/sq in.) Inches ! st/ ft hr ( lb/sq in.) Grains Fiherboard 0. 492 60. 6 0.0168 Fiborhonrd, 1 surface asphalt,

roiled . 492 8.0 . 125 Fiber board, 1 surface asphalt, dipped ... .63 1 17.3 .0678

Table 2. —Permeability oj various materials to water vapor Continued

Material Thickness

Permeability

to moisture (P)

Vapor

resistance (UP) BABBITT |8J

Continued

Fiherboard, laminated, 2 Grains sqfl hr (lb/sq in.) samples cemented together Inches sq ft hr (lb/sq in.)

2. 74 Grains with asphalt .985 . 365 Fiherboard, laminated, 6 layers with 5 layers of as- phalt. . . . _ ... . 527 0. 23 4. 35 Fiherboard . . . . 1.06 37.

0. 0279 Fiherboard, same reduced in thickness ... 9.803 43. 4 . 0239 Do . 599 50. 4 .0177 Do . 405 74.5 .0134 Do . 201 133. 3 . 0075 Wood, spruce .563 3. 48 . 287 Do . 480 4. 03 .248 Do . 405 3.94 . 254 Do . 323 4.93 .203 Do . 232 7.24 . 138 Do . . 161 10. 35 .097 Wood, pine .... .80 1.88 .532 Do . 645 2. 52 .397 Do .496 3. 45 . 290 Do . 315 5. 55 . 180 Do . 109 9. 65 . 104 Wood (pine) A. . . 508 6.47 . 155 Wood (pine) A, 1 coat of A1

paint. . 3. 42 . 292 Wood (pine) A, 2 coats of A]

paint. .. 0.92 1.09 Wood (pine) A, 3 coats of A!

paint . . 71 1. 41 Wood (pino) B . 508 6.68 0. 150 W

r ood (pine) B, 1 coat of A1

paint . 3.85 . 260 Wood (pine) B, 2 coats of A1

paint ... . ... 1.95 .512 Wood (pine) B, 3 coats of A1

paint 1. .53 . 654 Kraft paper, 1 sheet .00394 168. .00595 Kraft paper, 2 sheets 107. . 00935

Fraf! paper, 3 sheets 80. 0125 Kraft paper, 4 sheets... 63. 6 .0157 Kraft paper, 6 sheets. .... 53. 5 .0187 Kraft paper, 5 sheets. . 65. 3 . 0153 Kraft paper, 5 sheets 61.

0162 Kraft paper, 7 sheets . 45. 5 . 0220 Kraft paper, 7 sheets . . 38 3 0261

38. 3 .0261 Kraft paper, 8 sheets . 33. 1 .0302 Black vulcanized rubber, hardness 40 .0791 0. 185 5.4 Plasticized rubber hydrochloride . 00158 .382 2. 62 30-30-30 paper A .0071 1.83 0.546 39-30-30 paper B . 0071 1.79 . 5.58 Duplex Scutan 6-6, asphalt between 2 sheets of kraft .0071 0.946| 1.06 Scutan 0-14 (kraft infused with asphalt on 1 surface) A .0071 8. 0 0. 116 Scutan 0-14 B .0071 15. 97 . 0626 Scutan 14 (kraft infused witli asphalt on surfaces) A .0071 13. 9 .0719 Scutan 14 1! .0071 16. S .0633 Black building paper, black shiny [sapor infused with

asphalt .0173 II. 370 2.66 Asphalt felt, 15-lb. felt building paper with soft dull appearance ...... .0319 13.5 0. 0741 Pressed cork board A .. ... .905 4.75 211 Pressed cork hoard B . 985 5. 42 . 184 Plaster 1. 34 27. 1 . 0369 Plasterboard, plaster between

sheets of heavy paper 0.37 70. 2 .0142 Masonite Prcsdwood, torn- pored . 13 9. 76 . 102 Masonite Prcsdwood . 13 21. 7 .0461 Masonite Prcsdwood, 6 thick- nesses . Masonite Prcsdwood, 7 thick- 6.26 . 16

nesses 4.9 .204
»
Table 2. —Permeability of various materials to water vapor—Continued

l able 2. —Permeability of various materials to water vapor— Continued

Material Thick- Permeability

to moisture Vapor

resistance Material Thick- Permeability

fo moisture Vapor

resistance (P) (1 IP) (P) 0 ip) TEESDALE (11] HERRMANN [4]

Inches

Foil-surfaced reflective insu- I lation, double-faced I Roll roofing— smooth, 40 to 65

Ib/roll 108 sq ft . Asphalt impregnated and | surface-coated sheathing

paper, glossy, 50 lb and 35

lb/500 .sq ft Duplex or laminated papers,

30-30-30 Duplex or laminated papers, 30-60-30 Duplex papers, reinforced Duplex paper, coated with

metal oxides Insulation backup paper,

treated

Grains sq ft hr ( lb/sq in.) Grains sq ft hr (lb/sq in.) sqft hr (lb/sq in.) Grains Inches sq ft hr (lb/sq in.) Grains

0. 172 to 0. 263 5. 82 to 3. 8 Hydrocarbon wax. b 1 0. 000052 1.9 XIO'

Thiokol _ >• 1 .60014 7. 1 X10 3

. 263 to . 348 3.8 to 2. 87 Gutta percha. b 1 00035 2. 80X10 3 Hard rubber » 1 . 00035 2. 86X10 3

f .433 to 1.57 2.31 to 0. 637 Para gutta .....

b 1 . 00012 2.38X10 3 \ . 348 to 4. 19 2. 87 to .239 Polystyrene •> i . 00087 1. 15X10 3 Asphalt sealing compound

•> i .00087 1. 15X10 3 Phenol fiber _ b 1 . 00148 676

2. 80 to 5. 24 0. 367 to . 191 Soft vulcanized rubber !> 1 .00157 637 Benzyl cellulose » 1 . 00226 442

1.05 to 1.75 .952 to .572 Bakelite b 1 . 0035 286

1. 396 to 4. 19 . 716 to . 239 Waterproof cellulose film b 1 .062 16. 1 Cellulose acetate _ _ b 1 . 12 8.3

1. 06 to 2. 63 .952 to .381

1.76 to 6. 97 . 572 to . 144 MILLER (7] Gypsum lath with Al-foil backing

Plaster, wood lath

Plaster, 3 coats of lead and oil. Plaster, 3 coats of flat wall

paint

Plaster, 2 coats of A1 paint

Plaster, fiberboard or gypsum

lath

Slater’s felt Plywood, ]4-in., Douglas fir, soy bean glue, plain Plywood, 2 coats of asphalt

paint .. Plywood, 2 coats of A1 paint.. Plywood, H in., 5-ply Douglas fir. Plywood, !4 in., 3-ply Douglas fir, artificial resin glue __ Plywood, y> in., 5-ply Douglas fir, artificial resin glue... Insulating lath and sheathing, board type

Insulating sheathing, surface- coated ... Compressed fiber board, 3it

in Insulating cork blocks, 1 in... Blanket insulation between

coated papers, M and 1 in... Mineral wool, unprotected,

4 in

0. 173 to 0. 785 5. 78 to 1. 27 22.4 0. 0446

7.6 to 7. 84 0. 133 to . 127

8. 72 . 115

2. 35 . 425

40.2 to 41.9 . 0249 to .0239

10. 5 to 52. 4 . 0952 to .0191

8. 72 to 13.. 115 to .0764

0. 87 1. 16

2. 63 0.38

6. 43 to 5.59 . 184 to . 179

8. 72 to 13. 1 . 115 to .0761

5. 59 to 6.85 . 179 to . 146

52.3 to 69. 8 .0191 to .0143

6. 17 to 8.88

10.3 .097

12.6 .0794

3.90 to 4.07 .256 to .246

69. 2 .0169 FOREST PRODUCTS LABORATORY

Kraft paper __ _ 112. Plastered wall, no paint, plasterboard lath .. 41. 7 Plastered wall, no paint, wood lath . . . 22. 2 Slater’s felt, best type 10. 1 Duplex paper 2. 78 Plastered wall, 2 coats of A1

paint, wood lath 2. 43 Asphalt-coated paper, 361b/500

sq ft roll 2.08 Asphalt-coated paper, 501b/500

sq ft roll 1.04 Metal-coated, paper - - 0. 174

0. 00893

.024

.045

.099

.36

.411

.481

.962

5. 75

Plaster base and plaster, %

in __ ___ 30 Of. 033 Vapor barrier (Kimberly

Clark Corp. data) . _ 1.65 . 61 Fir sheathing, in Waterproof paper

6 100

167

. 01 Pino lap siding ... .. Paint film ___ . _ — 10

7

. 1

. H

26.6 . 0392

2.2 .454 MARTLEY [2]

Wood, Scot pine, per inch 21. 4 0. 0467 WRAY

Wood, western yellow pine,

Y\ in . ...

1 1.8 0. 556

» Figures in brackets indicate the literature references at the end of

this paper.

k Data recalculated on basis of 1-inch thickness.

of permeability to moisture the number of

grains per hour passing through 1 sq ft of the

actual thickness specified when the vapor pres- sures at the two surfaces differ by 1 lb/sq in. The reciprocal of this permeability has also been listed and has been called the vapor resist- ance. The number of grains of moisture pass- ing through 1 sq ft in 1 hour with 1 -lb/sq in. pressure difference =permeability = 1 /vapor re- sistance.

If winter air is brought into a house from the

INTERNATIONAL CRITICAL TABLES

Still air, 3*H in 70. 9 0.0141

Still air, 1 in 257 . 00389

outdoors and is heated without addition of moisture, the resulting warm air will contain

the same proportion of moisture as before, and

WRAY (3]

coat of A1 paint on wood,

Bakelite resin varnish

1. 22 0. 82 coat of Al paint on wood,

trlyeerol phthalatc varnish 1. 22 .82 coats of ordinary paint, lin- seed oil 3. 48 .287 coat of Al paint on wood,

ester gum varnish 2. 26 . 446

therefore the pressure of the contained water

vapor will be the same, but t he relative humidity

will be lower than that of the air outside. This

is true because the warm air 1 can contain more

moisture than it could when cold. If the diffu-

• Strictly, the space contains tho moisture.

sion of the water vapor through the walls is proportional to the vapor-pressure difference

between inside and outside, there will be no net passage of moisture, since this difference is zero. In such a case there would be no tendency

for air in any part of the wall to have a relative

humidity greater than the value outdoors. In

fact all parts would be subject to lower rela- tive humidities, since the air would be warmer

than the air out of doors.

Actually, in an occupied house moisture is always being added to the air, so that the vapor

pressure is higher inside the house than outside.

The water vapor penetrates the plaster or other

interior finish and passes on through the rest

of the wall to the outside. Accordingly, the

vapor pressure within the walls will be higher

than that outside, although less than that within

the interior of the house. The manner in which

the vapor pressure falls off from the high value

inside the house to the low value outside the

house, together with the temperature distribution across the wall, determines the relative

humidity at all points within the wall. In an ordinary frame wall condensation is more likely to occur on the inner surface of the

sheathing than elsewhere, except when sheathing papers highly resistant to water vapor are employed, in which case condensation may first appear on the sheathing paper. In other types

of hollow walls condensation tends to occur in corresponding regions.

Differences in vapor pressure through dif- ferent parts of the wall from inside to outside

distribute themselves in proportion to the vapor

resistance of the respective parts. That is, the

fraction of the total vapor-pressure drop from

inside to outside occurring between the air inti.

/

UJa]

UJQ

UJ

o'D

h-<

LUa

5

UJ

fFiuuue 1. —Nature of the temperature, corresponding saturation vapor pressure,

in an uninsulated brick wall. and actual vapor pressure distributions

Wall composed of a/plaster on metal lath; B, air space between furring strips; and C, brick, exposed to 70° F and 30-porccnt relative humidity

indoors and 10° F and 80-percent relative humidity outdoors.
IIII—I
doors and some chosen point within the wall is the same as that fraction of the total vapor

resistance of the wall occurring between the

air indoors and the chosen point. In this it is analogous to heat flow, in which

differences in temperature through different

parts of the wall from inside to outside are pro- portional to the thermal resistance of these

parts. In other words, the fraction of the total temperature drop from inside to outside in- cluded between the indoor air and the chosen

point is the same as that fraction of the total resistance to heat flow included between the

indoor air and the same chosen point.

From these facts one can calculate the vapor

pressure and temperature at any point in the

wall and estimate whether the calculated vapor

pressure is safely below saturation vapor pres- sure for the calculated temperature. As an

explanation of this the following examples are

given.

1. A brick wall composed of metal lath and

plaster, air space between furring strips, com- mon and face brick is exposed to 70° F and 30-

percent relative humidity indoors and 10° F

and 80-percent relative humidity outdoors. A

section of this wall is pictured in figure 1 with

curves showing the nature of vapor-pressure

and temperature distributions within the wall. To determine whether condensation will occur on the inner surface of the brick, data on vapor

resistance and thermal resistance of the various

parts of the wall are examined.2 The following vapor resistances are taken,

starting with the indoor side of the wall

Air to surface of plaster _ not over 0.001 Metal lath and plaster (data for wood lath and

plaster) .045 Air space not over .014 Brick (doubling the figure for 4-in. masonry) .9 Surface of face brick to outside air Negligible.

The sum of the first three is 0.060, and the

total for the entire wall is 0.960. The vaporresistance fraction is therefore equal to 0.060/

.960, or about 0.0625. This means that about

onc-sixteentli of the vapor resistance of the wall

is comprised between the air of the room and

the inner surface of the brick.

1 Units of vapor resistance and thermal resistance have been omitted

in much of the discussion following. The units are hr sq ft (lb/sq in.)/ grain, and hr sq ft deg F/btu.

One may similarly examine the data on thermal resistance, or insulating value, for the

separate parts of the wall, as given in table 3. The following arc taken as thermal resistances:

Table 3. - Thermal resistance of. various materials

Materials Thermal

resistance

•

Yellow pine or fir, 2 %2 in. thick

hr sq ft deg F

lit u 0.98 Yellow nine lap siding . . .78 Wood lath and plaster _ .40 Metal lath and plaster. ... _ .. .23 Mineral wool, per in ... 3. 70 Stucco. . ... .. . ... ... 0.08

Brick, common 4-in .80 Brick, face 4-in . . .. . 44 Fiberboard, per in. . ... . _ __ .. 3.0 Plasterboard H-in 0. 35

Surface film, still air. .01 Surface film, 15 mph . . 17 Surface film, rough stucco, 15 mph . 11 Air space more than in. across not lined with re- .91

flector

Air to surface of plaster 0. 61 Metal lath and plaster . 23

1%-in. air space . 91 Common brick . 80 Face brick . 44

Surface of face brick to outside air .17

The sum of the first three thermal resistances is 1.75, and the total thermal resistance of the

wall is 3.16. The thermal-resistance fraction

is therefore equal to 3.75/3.16, or 0.554. This means that about 55 percent of the thermal

resistance of the wall is comprised between the

air of the room and the inner surface of the

brick.

These fractions will now be applied. For the

water vapor there is indoors a vapor pressure of 30 percent of 0.3628 lb/sq in., or 0.109 lb/sq in. Outdoors there is a vapor pressure of 80 percent

of 0.03092 lb/sq in., or 0.02474 lb/sq in. The

drop in vapor pressure through the wall is 0.109

lb/sq in. minus 0.025 lb/sq in., or 0.084 lb/sq in. Multiplying this drop by the vapor-resistance

fraction 0.0625, one obtains 0.00525 lb/sq in. to be subtracted from the 0.109 lb/sq in. indoors.

The result, 0.104 lb/sq in., is the vapor pressure

adjacent to the inner surface of the brick—cor- responding to a “dew-point” temperature of 36° F. The same procedure may be continued in regard to temperature. The temperature in16
. .

.
doors is 70° K. The temperature outdoors is 10° F. The drop in temperature through the

wall is tlx'ii (i()°. Multiplying this drop by the

thermal-resistance fraction, 0.554, one gets

33.24° as the drop in temperature between the

air of tin' room and the inner surface of the

brick. Subtracting the 33.24° from 70°, the

indoor temperature, one obtains 36.76° as the

temperature of the inner surface of the brick

which is higher than the above-noted “dewpoint” tern perature

This temperature and the vapor pressure for the same point are compared with the figures

given in table 1 for saturation pressures. One.

finds for the. temperature calculated a saturation

pressure, of 0.107 lb/sq in. Since the actual

vapor pressure as calculated (0.104 lb/sq in.) is less than this, it is not indicated that condensation will occur on the inner surface of the brick

in this particular wall for the particular conditions of temperature and relative humidity

quoted.

2. As a second example the same wall is considered, except that for the air space will hi* substituted a fill of insulating material adding

negligible vapor resistance. The following

values of vapor resistances and thermal resist- ances are based on tables 2 and 3:

Vapor Thermal

resistance resistance Air to surface of plaster 0. 001 0. 01 Lath and plaster .045 .23

Insulation (1%-in. mineral wool) .014 0.01 Brick .... .9 1.24

Surface to air .0 0. 17 The vapor-resistance fraction is as before,

0.0625, and the thermal-resistance fraction is 6.85/8.26, or 0.83. The use of these, with the

total vapor pressure and temperature drops

calculated before, gives for the inner surface of the brick a temperature of 20.2° F, and, as before, a vapor pressure' of water of 0.104 lb/sq.

in. and a corresponding “dew-point” temperature of 36° F. Table 1 indicates that for the

temperature 20.2° F a vapor pressure of only

0.05098 lb/sq in. can occur without condensation. It is thus seen that under these conditions of temperature and humidity water will be condensing against the inner surface of the

brick-—the temperature being below the abovenoted “dew-point” value.

Similar calculations may easily be made for any wall, if data are available for its constituent

parts. As table 2 shows, data are not abundant

on all construction materials. For types of wall construction which have been considered,

the results are somewhat similar to those just

obtained for the brick wall. In practically all hollow walls, the possibility of condensation on

the cold side of the hollow space is the critical point, since condensation is generally more

likely to occur there than in neighboring regions.

A summary of calculations for different walls

for several indoor humidities and outdoor

temperatures is given in table 4. In interpreting the results given here, it is nec- essary to recall the warning that since the data were obviously very uncertain the results are

also very uncertain.

If the figures are to be used in building design, an ample factor of safety should be included,

although the figures seem likely to be in error

in such a direction that a factor of safety is already present. An important reason for this

is that in all this discussion the air leakage into

the wall from the outside has been disregarded.

Its effect is to reduce the tendency for condensation, but by an amount which naturally cannot he estimated without definite data as to the

amount of air leakage occurring.

Most of table 4 should be clear from the dis- cussion already given. To clarify the method

for calculating the last three columns, the

following example gives a calculation of the

vapor resistance which must be added to the

warm side of the wall to just barely prevent

condensation. The case considered is that of the brick wall with 1% in. of insulation and

exposed to 30° F and 80-percent relative

humidity outside and 70° F and 40-percent

relative humidity indoors. It is assumed that

the vapor resistance' of lath and plaster and in- sulation in the woill is 0.06 hr sq ft (lb/sq in.)/

grain and the vapor resistance of the remaining

outer part of the wall is 0.9 hr sq ft (!b/sq in.)/

grain. The temperature at the inner surface

of the brick, according to the previous explanation is found to be 70° F — 0.83 (70° F 30°

F) = 70° F—0.83 (40° F) = 70° F-33.20 F=

36.8° F.

[ 7 ]
>
Table 1. Illustrative calculations concerning vapor barriers

Wiill type

f rhornml n*sisl- imrt*

Inner

pnrl lot nl

TMornml-resis timer

fraction

(to chosen

point

:

( 'old side

of instilat ion, or hollow

space, if present

Vapor resistance k

. Inner

part

'1'otal

Ynporresist

-

ance

tract ion

(to chosen

point)

Necessary increase in inner vapor

resist ance »• to prevent condensation

a t t he chosen point

30° F. 80", ; io° f. ho*; io° f.ko\

outside; outside; outside; 70° F. 10 '

,

70° F. ‘MV,, 70°F.20',

inside i inside inside

Frame wall:

I ninsulated 3H in. of insulation

1 in. of insulation in middle of space

2 in. of insulation in middle of space

It. 41

6. 53

11). 23

3. 85

lf>. 34

8. 40

12. 10 Frame and stucco:

I ninsulated

3'S in. of insulation . -

1 in. of insulat ion

2 in. of insulation

1.92

14. 41

0. 53

10. 23

3. 09

15. 58

7. 70

11.40

Brick veneer: Uninsulated

3

r, s in. of insulation

.

1 in. of insulation

2 in. of insulation

Brick, solid 8 in., furred, metal lath, and

plaster: Uninsulated

1^6 in. of insulation - -- ---

1.92 14.41

0. 53

10. 23

3. 87

10. 30

8. 48

12. 18

1. 75 3. 10

6. 85 8. 20

» hr sq ft de^r F/Btu.

b hr sq ft (lb/in. 2 )/grain. The vapor pressure at this inner brick surface

must not be higher than the saturation vapor

pressure for 36.8° F, as given by. table I. namely 0.1075 lb/sq in. The vapor pressure on the outside of the wall is found to be 0.80

times 0.0808 lb/sq in., or 0.06464 lb/sq in. 'Pile vapor pressure indoors is 0.40 times 0.3028,

or 0.1451 lb/sq in. The drop in vapor pres- sure across the outer part of the wall is 0.1075

lb/sq in. minus 0.0646 lb/sq in., or 0.0429

lb/sq in.

r Phe drop in vapor pressure across the inner

side of the wall is 0.1451 lb/sq in. minus 0.1075

lb/sq in, or 0.0376 lb/sq in. Since the vapor

resistance of the various parts are proportional

to the corresponding vapor pressure drops, the

vapor resistance on the inner side needs to be

0.0376/0.0429 times as great as the vapor re- sistance of the outer side. This gives 0.0376/-

0.0429 times 0.9 hr sq ft (lb/sq in.)/grain =0.79

hr sq ft (lb/sq in.)/grain as the inner vapor

resistance desired. Since this wall was supposed to have an inner vapor resistance of 0.060 hr sq ft (lb/sq in.) /grain originally, it is

0. B0 0.06 0. 460 0. 13 0 0 0

.88 .06 . 160 . 13 .47 1.28 2. .77 .06 . 460 . 13 14 0. 48 0. .84 .06 . 160 . 13 . 32 .90 1.

. 62 . 06 . 305 . 107 0 0 0.

. 93 . 06 . 305 . 197 . 43 1 . 24 2

.85 . 06 . 305 . 197 . 19 0. 58 1. .90 .06 . 305 . 197 . 32 .92 1 .

. 50 .06 .71 .0845 0 0 0 .88 .06 . 71 . 0845 . 80 2. 11 3. .77 .06 .71 . 0845 . 27 0. 82 1 . .84 ,0H .71 .0845 .56 1. 49 2.

. 554 .06 .96 . 0620 0 0 0. .83 . 06 .96 . 0625 .73 1.02 3.

necessary to add a barrier having as vapor re- sistance the difference of these, or 0.73 hr sq fl (lb/sq in.)/grain. One might now search table 2

for possible barriers having vapor resistances of

t his magnitude or greater and having negligible

thermal resistance.

The graphs in figures 2, 3, and 4 may be convenient for rapid judgment as to the adequacy

of a given moisture barrier for a given wall. The outdoor relative humidity is here assumed

to he 80 percent and the indoor temperature

70° F for each case. These are considered more

or less typical of winter conditions occurring

in many parts of the country with snow throughout the winter, if thermal-resistance fractions

and vapor-resistance fractions are laid out along their corresponding axes, the point thus

located by intersection of the horizontal and

vertical lines indicates the indoor relative

humidity at which trouble might occur under

the conditions quoted for the separate charts,

namely for outdoor temperatures of 30°, 10°,

and — 10° F, respectively.
.
1.0

.9

z .0 o

h~V .7

o'

LL

LLl .6 u

z

< .5

t-

<4

i/)

LLl .4 Dd

-J .3 <5

cd .2

llI X

!— . 1

/

O .1 .2 .3 .4 .5 .6 7 a .9 1.0 VAPOR RESISTANCE FRACTION

Figure 2. Indoor relative h it nudity able to cause condensation at a given point in a wall, as determined by the thermal-resistance fruction and the vapor-resistance fraction.

(Fraction of thermal resistance on warm side of the point and fraction of vapor resistance on warm side of the point in question.) Turves 30%; H, 40%; C, />, 01)%; and /., 70% indoor relative humidity.

Conditions: 30° F outdoors,* 70° F indoors; and 80-percent relative humidity outdoors. For example, any combination of thermalresistunce fraction and vapor-rosislance frac- tion represented by a point on or below the 40-

percent line represents a type of wall in which

condensation will not occur unless the indoor

humidity exceeds 40 percent The region below the 40-percent line therefore represents

combinations ol thermal and vapor resistances

iu which relative humidities up to 40 percent are

safe. The region above the line represents

combinations in which humidities as high as 40

percent should be avoided.

Thus omi may consider the case of a wall the

thermal-resistance fraction of which is 0.50 and

the vapor-resistance fraction 0.13. Figure 2 shows that a relative humidity of about 54

percent or more indoors could lead to condensation when the outdoor temperature is 30° F. Figure 3 indicates about 38 percent for a temperature of 10° F, and figure 4 indicates about

20 percent, for an outdoor temperature of —10°

b. These results would doubtless be considerably altered if the ventilation of the wall by

air leakage could be allowed for. Figures 2, 3, and 4 can be used for determining the moisture-condensation conditions

at a selected spot in insulated walls of many

types, through which heat is transferred by

conduction or its equivalent, provided the heattransfer resistance and the vapor-transfer re- sistance fractions arc known. It cannot be

directly utilized in connection with walls having appreciable ventilating spaces. When studying any thermal-conduction type

of wall, full consideration must be given to the

actual space distribution within the wall of both the thermal and the vapor-transfer re- sistances. Critical points are to be looked for where there is space concentration of vaportransfer resistance on the cold side of any space

concentration of thermal resistance.

Without regard in any respect to the actual

space distribution of the thermal and vaportransfer resistances, if the space distribution of

[ 9 ]
.

'
VAPOR RESISTANCE FRACTION

Figure 3. Indoor relative humidity able to cause condensation at a given point in a wall, as determined by the thermal-resistance fraction and the vapor-resistance fraction.

(Fraction of thermal reisistance on warm side of the point and fraction of vapor resistance on warm side of the point in question.) t'urves A, 10%; K, 20%; V, 30%, It, 10%; and /•’, fi0% indoor relative humidity.

Conditions: 10° F outdoors; 7(,° F indoors; and KO-percont relative humidity outdoors.

the thermal resistance is the same as (or approximately the same as) the space distribution

of the vapor-transfer resistance, the wall acts exactly (or approximately) as though it were composed of homogeneous insulating material.

In certain cases the walls may well be treated

as though made up of separate layers of homogeneous insulating materials. When this condition is reached (or approximated) one cannot

determine by mere superficial examination just

where and when condensation may take place

the below-dew-point temperatures may be

reached within the mass of the material, al- lhough not at either edge.

Consider, for example, the ease where the wall

may properly be regarded as a single uniform

thick sheet having at its warm surface air of one temperature and relative humidity and at the cold surface air of another temperature and

relative humidity. There is in all parts of the

wall the same rate of passage of moisture if no

condensation is occurring and a steady state

has been established. The vapor-pressure drop

is then uniformly distributed across the material in (he same manner as is the temperature

drop. This may be illustrated by figure 5 in which the temperature is indicated at uniform

intervals along a horizontal axis. This may be

considered the direction across the layer of material having uniform spacing of insulation

of uniform thermal resistance or at least uniform sections of the total thermal resistance* of the material. The two surfaces of the material

are then located on this scale by their corre- sponding temperatures. Plotted vertically are the saturation pressures for water vapor cor- responding to all temperatures along the horizontal axis. If the actual vapor pressure's at the two surfaces are now plotted for their positions as indicated on the temperature scale, for example A and B, the vapor pressure at any point between will be indicated by the

distance vertically upward to tin' straight line between them. As examples, three cases have
.
O J .2 .5 .4- .5 .6 .7 .6 .9 1.0 VAPOR RESISTANCE FRACTION

FiramK 4. - Indoor relative humidity able to cause condensation at a given point in a wall , as determined by the thermal-resistance fraction and the vapor-resistance fraction.

(Fraction of thermal resistance on warm side of the point, and fraction of vapor resistance on warm side of the point in question.) Curves A, 5%; B, 10%; C, 20%; I), 30%; and K, 40% indoor relative humidity.

Conditions: —10° F outdoors', 70° F indoors; and 80-percent relative humidity outdoors.

been shown, originating from having different,

vapor pressures at the point A corresponding

to 25-, 50-, and 80-percent relative humidities. In each case the vapor pressure at the point

C with temperature 40° F is indicated on the

vapor-pressure scale by the line upward from C

to the appropriate slanting line. Thus the

slanting line through D illustrates a case with

50-percent relative humidity at A in which the

vapor pressure at C is below the saturation

value, although the apparent tangency with the

saturation pressure curve near 20° F indicates

that condensation is in danger of occurring in a very limited region of the part at 20° F. The

slanting line through D' lies above the saturation curve lor much of the wall and in particular

for the point C, indicating that for a relative

humidity of 80 percent at A condensation is to

lie expected throughout a considerable portion

of the wall. The third slanting line, passing

through D"

,

lies entirely below the saturationpressure curve and indicates that condensation

will occur at no point in the slab when the

relative humidity at A is 25 percent. These

calculations, which are approximately correct,

may be useful in indicating possibilities of con- densation within the material of a wall with

similar spatial distribution of the heat and vaportransfer resistances, where seemingly there is no

specific critical condensation point.

111. MEANS OF PREVENTING

CONDENSATION

The methods by which condensation of

moisture in insulated walls may be avoided are becoming fairly well known. A little consideration of the principles already explained here

shows that possible remedies will include:

1. Lowering the indoor relative humidity,

either by lowering the rate at which water vapor

is added to the air in the house or by increasing

the ventilation of the house interior to the

outdoors.

11
t
.5

-20 -IO O iO 20 30 40 50 60 70 80

DEGREES F Figure 5.-—Method of 'predicting vapor pressure conditions in a wall composed of a single slab of permeable material.

80-percent outside humidity (at B)\ O', line for 80-percent humidity inside (at A)', I), line for 50- percent humidity inside (at A)', 1)", line for 25-percent humidity inside (at A); straight lines, actual vapor pressures; and curve, saturation vapor pressutes.

2. Increasing the vapor resistance on the

warm side of the insulation.

3. Lowering the vapor resistance on the cold

side of the insulation by venting to the outside

air or by using less vapor-resistant outer wall

materials without diminishing protection against

driving rains.

These will be considered in order.

1. The indoor relative humidity is likely to

lie kept at sufficiently moderate values to avoid

condensation on windows most of the time.

When use is made of single-glazed windows, the mean relative humidity is then likely to be so low that trouble from condensation within walls

will not occur. However, when use is made of

double-glazed windows, or storm windows,

there may be condensation within the wall

without condensation on the glass, either inside or outside.

2. The addition of a vapor barrier on the

warm side of the wall insulation is an effective

procedure. High humidities frequently occur

temporarily, even in houses which would

scarcely be described as humidified. If the

inside relative humidity is kept high the vapor

barrier is of value because it furnishes the only

remaining way of effectively reducing the rate

at which moisture enters the wall. Naturally,

if a vapor barrier prevents part of the loss of

moisture through the walls, it will also reduce

the rate at which moisture must be supplied to maintain any chosen relative humidity. Any

water evaporated is, of course, removed largely

by normal ventilation.

3.

The provision for adequate ventilation from

the interior of the wail to the outdoors or the use

of sheathing paper with smaller vapor resistance can be helpful. Unless the ventilation to out- doors is abundant, there is a tendency for the

beneficial effect of ventilation of the wall to be

limited to the vicinity of the openings to the

outside. The air space between the insulation

and sheathing should be so proportioned as to permit air circulation and thus allow water

vapor to soak natural outlets. It is, of course, obvious that the wall must be left capable of

12
.
shedding the rains, or even more water may

enter from the outside than would have

accumulated by condensation from the inside. The calculation of the resistance necessary

in a vapor harrier in order to prevent con- densation m a particular wall has already been

discussed, and some results are given in table 4.

It will be noted that for conditions which do

not seem unusually severe the resistance which

must In* addl'd is such that the vapor resistance on the warm side of the insulation becomes

several times as great as that on the cold side. The metal foils and some of the asphalt papers

an* effective vapor barriers.

Some of the flexible blanket and rigid board

types of insulating materials are now provided

with self-contained vapor barriers. When used on the warm side of the insulation a barrier is advantageous; on the cold side it is disadvantageous. When a moisture-repelling protective

sheet is used on the cold side, it is essential to have a sheet of considerably greater- usually

several times greater—vapor resistance on the

warm side. The blanket type of insulation can be in- stalled near the center of certain types of wall

space so as to leave a continuous air space for ventilation on the cold side. For old houses,

in which it is difficult to install barrier sheets

next to the insulation, the use of two coats of aluminum paint on the inside wall surface has

been recommended by some investigators.

In insulated attics the situation is quite

different. For insulation on the attic floor,

ventilation with attic window louvres may take

care of many cases so as to avoid condensation

of moisture on the under side of the roof, but

in some cases vapor barriers are needed in addition to ventilation. If the insulation is placed

close to the roof, it becomes much more diffi- eull to ventilate the space between, and re- course to effective vapor barriers becomes

especially necessary. An additional factor is that some modern roofs, such as the metal and

built-up types, are made of materials with very

high vapor resistance. For these it becomes

much harder to get a vapor barrier on the

warm side with vapor resistance several times

as great as that of the roof. In such cases attempts to provide both adequate ventilation

and vapor barriers would appear necessary.

Further discussions of the problem of condensation in insulated walls will be found in a number of the publications referred to at the

end of the paper.

IV. CONCLUSIONS

The available data on the moisture per- meability of various building materials and the more modern types of structures arc too meager and discordant to permit any more

than a very rough estimate of the conditions

under which condensation will take place in insulated walls. It is possible, however, in the

case of many types of wall to calculate for any

given condition of humidity and temperature

the ratio between the moisture permeabilities

of the interior and exterior portions of the wall

which is necessary to prevent condensation.

For exterior and interior conditions of temperature and humidity which do not seem impossibly severe, it is necessary in a well-insulated

wall to have the vapor resistance of the warm

side of the wall several times as great as that on the cold side in order to prevent the possibility of condensation. The extent to which

air leakage into a wall from the outside is significantly advantageous is unknown, but

any such leakage will decrease the tendency for condensation.

V. REFERENCES

[1] S. T. C. Stillwell, The movement of moisture with

reference to timber seasoning. Forest Products

Research, Dept. Sci. lad. Research, Great

Britain 'tech. Papers No. 1 (1926).

[2| .) . F. Martley, Moisture movement through wood;

the steady state. Forest Products Research,

Dept. Sci. lad. Research, Great Britain Tech. Papers No. 2 (1926).

[3] R. I. Wray anil A. It. Van Vorst, Permeability of paint films to moisture. lad. Fag. ('hem. 25,

842 (August 1933).

[4] R. L. Taylor, D. B. Herrmann, and A. R. Kemp,

Diffusion of water through insulating materials. lad. Eng. Cliom. 28, 1255 (November 1936).

[5] F. B. Rowley, A. B. Algren, and C. F. bund, Condensation within walls. Heating, Piping, Air Conditioning 10, 49 (January 1938).

[6] ,1. D. Babbitt, The permeability of building materials

to water vapor. Heating, Piping, Air Conditioning 10, 751 (November 1938).

[7] L. G. Miller, Calculating vapor and heat transfer through walls. Heating and Ventilating 35, No.

11. 56 (November 1938).

[ 13 ]
,

‘
[8] J. I). Babbitt, The diffusion of uniter vapour through

various building materials. Canadian .). Research 17, 15 (February 1939).

(9] L. V. Teesdale. Resistance of materials to vapor trans- mission. Heating, Piping, Air Conditioning 11 , 213 (April 1939).

1 10] F It Row lev, A. It. Algren, and C. I

1). Lund, Condensation of moisture and its relation to building

const ruction and operation. Heating, Piping, Air Conditioning 11 , 4! (January 1939).

[11) L. V. Teesdale, Ilow to overcome condensation in building walls and attics. Heating and Ventilating 30 , No. 4, 30 (April 1939).

[12) L. V. Teesdale, Condensation in walls and attics. HI 157. U. S. Department of Agr., Forest

Service, Forest Products Laboratory, Madison,

Wis.

[13) L. V. Teesdale, Condensation problems in farm

buildings, lit 180. II. S. Department of Agr., Forest Service, Forest Products Laboratory,

Madison, Wis.

[14) L. V. Teesdale, Condensation problems in modern

buildings. HI 190. L. S. Department of Agr., Forest Service, Forest Products Laboratory,

Madison, Wis.

[15| J. D. Babbitt, The per mcabilily of building papers

to water vapour. Canadian J. Research 18 , 90 (May 1940).

[10] J. D. Babbitt, Observations on the permeability of hygroscopic materials to water vapour. I Observations at relative humidities less than 75%.

Canadian .1. Research 18 , 105 (June 1940). Washington, August. 9, 1 940

.

[ H]
.
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,*

>
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