condensation problems - Forest Products Laboratory

condensation problems - Forest Products Laboratory
U.S.D.A. FOREST SERVICE RESEARCH PAPER FPL 132 1972 Forest Products Laboratory, Forest Service, U.S. Department of Agriculture, Madison, Wisconsin
ABSTRACT Excessive moisture in some form is often the cause
of condensation problems in a house or other structure.
Perhaps the most aggravating and also the most easily
prevented are those caused by the movement of water
vapor through walls or ceilings. Such problems may
result in excessive maintenance costs, such as the
need for frequent repainting. However, properly in­
stalled vapor barriers, in conjunction with proper us e
of insulation, and adequate ventilation will avoid most
of such difficulties.
The results of many years of research by Forest
Products Laboratory and other scientists, together
with their field experience in solving condensation
problems, have provided much valuable information.
This publication contains recommendations based on
these data. In addition to good practices in the use of
vapor barriers,
and ventilation, other
construction details are also described and illustrated.
Such practices followed in the construction of a new
home can pay for themselves many times over in
reduced maintenance costs.
Vapor barriers of adequate resistance, properly
installed, are the most important factors in re­
ducing the movement of water vapor through the
walls and ceilings of a home. In cold weather,
without some means of eliminating this movement
that results from vapor pressure differences,
water vapor moving through the wall or ceiling
will condense on a cold surface. Such condensation
in the form of moisture or frost often causes
problems which result in increased maintenance
This publication shows how to control condensa­
tion and minimize problems by the proper use of
good vapor barrier materials and by adequate
practices. The modern house is well
insulated, has weather stripped windows, and is
much tighter than the houses our fathers lived in.
The vapor and air resistance of exterior wall
materials being used in modern construction
often trap condensed moisture in wall cavities.
In homes having heating units which do not aid in
ventilation of the living area, cold weather con­
problems may be amplified because of
humidity buildup.
Furthermore, many heating
units have humidifiers which increase the relative
humidities in the home. A combination of such
conditions makes it even more important that
condensation control be practiced in order to
reduce the problems associated with inadequate
Because wood is so important a part of
American homes, the research program of the
Forest Products Laboratory has included studies
related to proper use of wood-based materials.
In addition to the results of recent research,
this publication contains information from other
papers now out of print. Included are U.S. Forest
Products Laboratory reports originally prepared
by L. V. Teesdale:
FPL Report 1710, “Remedial Measures for
Building Condensation Difficulties.” (1947)
FPL Report 1196, “Condensation Problems in
Modern Buildings.” (1939)
Data are also included from “Condensation
Control in Dwelling Construction,” originally
prepared by M. E. Dunlap in 1949 for the Housing
and Home Finance Agency, Washington, D.C.
Some current publications of particular value
are listed in the “Selected Bibliography” at the
end of this report. Prominent here is the ASHRAE
“Handbook of Fundamentals, Heating, Refrigerat­
ing, Ventilating and Air Conditioning.”
A glossary of some condensation and housing
terms is also included at the end of the report
for the convenience of readers.
L. O. ANDERSON, Engineer
Forest Products Laboratory,
Forest Service
U.S. Department of Agriculture
Purpose and Scope
as a guide
This publication is intended to
for homeowners-not only as a means of preventing condensation problems, but also to better
understand their cause. It contains information
and recommendations for correct methods of
installing vapor barriers, thermal insulation, and
inlet and outlet ventilators in new homes. Methods
which can be used to correct moisture problems
in existing houses are also included.
The majority of the suggestions and recommendations are applicable to the typical woodframe house. This publication illustrates use of
typical materials and does not imply that other
materials of equal quality and arrangement cannot be used successfully. Principles involved
and procedures used to minimize the problem
are equally applicable to commercial and farm
buildings where conditions are similar to those
in a home.
Information is based both on research by
Laboratory and other scientists and engineers as
well as experiences provided by the building
industries and homeowners.
In the colder regions of the United States-notably where the January temperature averages
35° F. or lower--the first signs of spring may
include dark stains on house siding and blistered,
peeling paint. These often indicate a cold weather
condensation problem. The formation of icicles
or an ice dam at the cornice of a house after a
heavy snowfall indicates another type of moisture
problem that requires correction. In each instance
the culprit is condensation.
Condensation can be described as the change
in moisture from a vapor to a liquid state. In
homes not properly protected, condensation
caused by high humidities often results in Inconvenience and in many cases excessive maintenance
costs. Water vapor within the house, when unrestricted, can move through the wall or ceiling
during the heating season to some cold surface
where it condenses, collecting generally in the
form of ice or frost. During warm periods the
frost melts. Men conditions are severe, the
water from melting ice in the attic may drip to
the ceiling below and cause damage to the finish.
1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin.
loss. Moisture is attracted to cold surfaces and,
unless moisture movement is restricted, it will
condense ox form as frost ox ice on cold surfaces.
Unfortunately from the standpoint of condensation,
more efficient the insulation is in retarding
heat transfer. the colder the outer surfaces
become and the greater the attraction for moisture.
Inexpensive methods of preventing condensation
problems are available. They mainly involve the
proper use of vapor barriers and good ventilating
practices. Naturally it is simpler. more inexpensive, and more effective to employ these
during the construction of a house than to add
them to existing homes.
Moisture can also soak into the roof sheathing or
rafters and set up conditions which could lead to
decay. In walls, water from melting frost may
out between the siding laps and cause staining or soak into the siding and causepaint blistering and peeling.
Many of the wood and wood-base materials
used for sheathing and panel siding may swell
from this added moisture and result in bowing,
cupping, or buckling. Thermal insulation
also become wet and provide less resistance to
heat loss. Water held in the wall for long periods
can also cause decay in the studs or wood sheathing. Efflorescence may occur on brick or stone of
an exterior wall because of such condensation.
The cost of painting and redecorating, and
excessive maintenance and repair caused by
cold weather condensation can be easily reduced
or eliminated when proper construction details
are used.
Condensation problems are not new but changes
in house design materials. and construction methods since the midthirties have accentuated the
Additionally, the relative humidity
within newer houses is generally higher than in
those houses built many years ago. Old houses
were usually large with high ceilings, had windows
that were not weather-stripped, and their construction details allowed air loss and air infiltration. New types of weather stripping, storm sash,
and sheet materials for sheathing in new houses
provide tight and air-resistant construction. Thus,
homes have not only become generally smaller
with leas atmosphere to hold moisture but are
also tighter.
Estimates have been made that a typical family
of four converts 25 pounds of water into water
vapor per day, Unless this water vapor is
removed in some way (ventilation usually), it
will either increase the humidity os condense on
cold surfaces such as window glass. More serious,
construchowever, it can move in or through
tion, often condensing within the wall, roof, or
floor cavities. Heating system equipped with
also increase
winter air-conditioning systems
the humidity.
In colder climates, new houses have from 2 to
3-1/2 inches of insulation in the walls and 6 or
more inches in the ceilings. However, this
insulation causes the outer portions of
became of lower heat
for example, to be
Condensation will take place anytime the temperature drops below dewpoint (100 percent
saturation of the air with water vapor). Commonly,
under such conditions, some surface accessible
to the moisture in the air is cooler than the dewpoint and the moisture condenses on that surface.
Types of condensation
Two types of cold weather condensation normally occur within a house. They can be classed as
(a) visible and (b) concealed. During cold weather,
visible condensation is usually first noticed an
window glass but may also be discovered on cold
surfaces of closet walls and ceilings. Visible
condensation might also occur in attic spaces
rafters or roof boards near the cold cornice
area (fig. 1) or form as frost. Such condensation
or melting frost can result in excessive maintenance such as the need for refinishing of
sash and trim or even decay. Water from melting
frost in the attic can also damage ceilings below.
Another area where visible condensation can
occur is in crawl spaces under
This area usually differs from those on the interior
of the house and in the attic because the source
of the moisture is usually from the soil or from
warm moisture-laden air which enters through
Approximately 3 gallons.
FPL 132
foundation ventilators. Moisture vapor then condenses on the cooler surfaces in the crawl space
(fig. 2). Such conditions often occur during warm
periods in late spring.
Another factor in surface condensation is the
relative humidity of the air near the condensing
surface. When the relative humidity of the inside
atmosphere is increased, surface condensation
Figure 1.--Darkened areas on roof boards and rafters in attic area indicate stain that
stemmed from condensation.
This usually could be prevented by vapor barrier in the
ceiling and good attic ventilation.
Figure 2.--Surface condensation on floor joists in crawl space.
A vapor barrier ground
cover can prevent this because it restricts water vapor movement from the soil and
thus avoids high humidity of crawl space and subsequent surface condensation. M 105 308
Moisture Sources
which is produced in or
which enters a home, changes the relative humidity of the interior atmosphere, Ordinary household functions which generate a good share of
the total amount of water vapor include dishwashing, cooking, bathing, and laundry work, to
say nothing of human respiration and evaporation
from plants. Houses may also be equipped with
central winter air conditioners or room waterevaporators. Still another source of moisture
may be from unvented or poorly vented clothes
dryers. Several sources and their effect in adding
water vapor to the interior of the house are:
Figure 3.--Relative humidity at which
v i s i b l e condensation will appear on
inside surface at a room temperature
M 139 218
of 70°
Pounds of water
Floor mopping Plants
will occur at a higher outside temperature. This
is illustrated by figure
For example, when
inside temperature is 90°
surface condensation will occur on a single glass window (U = 1.13)
when outside temperature is about -10° F. and
inside relative humidity is 10 percent. When inside
relative humidity is 20 percent, condensation will
not occur on the single glass until outside temperature falls to about +7°
when a storm
window is added or insulated glass is used (U =
surface condensation will
occur until
the relative humidity has reached 38 percent
when the outdoor temperature is -10° F. The
above conditions apply only where
are tight and
is good circulation of air on
the inside surface of the window. Where drapes
restrict circulation of air, storm windows are
not tight, or lower temperatures are maintained
in such areas as bedrooms, condensation will
occur at a higher outside
Condensation in concealed areas, such as wall
spaces, is usually more harmful than visible
condensation. Such problems often are first
noticed by stains on the siding or by paint peeling
after the heating season. Water vapor moving
through permeable walls and ceilings is normally
responsible for such damage. Water vapor also
by constant outleakage
escapes from
through cracks and crevices. around doors and
windows, and
ventilation, but this moisturevapor loss is usually insufficient to eliminate
condensation problem.
FPL 132
1.8 for each plant in 24 hours
0.5 for each shower and
0.12 for each bath
3.0 per 100 square feet,
each washing
- 5.7 per day
Clothes (washing, steam
ironing, drying) - 30.7 per week
Crawl space.--Water vapor from the soil of
crawl-space houses does not normally affect the
occupied areas. However, without good construction practices or proper precautions it can be a
factor in causing problems in exterior walls
over the area as well as in the crawl space itself.
It is another source of moisture that must be
in providing protection.
Water from construction.--People moving into
a newly constructed house in the fall or early
winter sometimes experience temporary moisture
problems. Surface condensation on windows, damp
areas on cold closet walls where air movement
is restricted, and even stained siding--all indicate
an excessive amount of moisture. Such conditions
can often be traced to water used in the construction of a house.
Basement floors, concrete walls,
walls and ceilings require a tremendous amount
of water during their construction. While much
of this water has evaporated from the surface
after a month or so, the addition of heat aids in
driving off more moisture as these elements
reach moisture equilibrium with the surrounding
atmosphere. This moisture creates higher than
normal humidities, and increased vapor pressures
move the water vapor to colder areas in attics or
in the walls.
A concrete floor in the basement of a small
home contains more than 2,000 pounds of water
when it is poured. The concrete basement walls
pounds of
of this same home contain over
water. If the house is plastered, over 2,500
pounds of water are used. Thus, it is often a
common occurrence to have some moisture
problems when a house is just completed. These
are normally corrected after
first heating
season. However, several methods may be used
to reduce this excessive moisture. Perhaps the
simplest is to heat and ventilate a new house sa
that excessive moisture is dissipated outdoors
before the occupants move in
Miscellaneous.--There are other sources of
moisture, often unsuspected, which could be the
cause of condensation problems. One such source
can be a gas-fired furnace. It is desirable to
maintain flue-gas temperatures within the recommended limits throughout the appliance, in the
flue, the connecting vent, and other areas; otherwise excessive condensation problems
If all sources of excessive moisture have been
reasons for a conexhausted, in determining
densation problem, it is well to have the heating
unit examined by a competent heating engineer.
One factor which can influence combustion in a
heating unit is the amount of air required. Without an ample supply of oxygen, gas (or other types
of fuel) cannot be completely burned In the tight
modern house it is considered good practice, in
forced air heating systems. to reduce
humidity buildup by introducing low-moisture
outside air into cold air return ducts.
Figure 4.--Relation of normal humidity
within a house to outside temperature.
M 81281 F
maintain a high relative humidity in an older
house than a tight modern house. In houses where
winter air conditioners are used, automatic controls maintain the desired relative humidity.
The older, loosely constructed house has more
areas which allow the escape of moisture and
consequently it would be more difficult to maintain a particular relative humidity level within it.
Many materials used as interior
ceiling, such as plaster, dry
exposed walls
wall, wood paneling. and plywood, permit water
vapor to pass slowly through them during cold
weather. Temperatures of the sheathing or siding
on the outside of the wall are often low enough to
cause condensation of water vapor within the
cavities of a framed wall. When
humidity or vapor pressure within the house at
the surface of an unprotected wall is greater than
that within the wall, water vapor will migrate
through the plaster or other finish into the stud
space; there it will condense if it comes in contact with
below its dewpoint. Vapor
barriers are used to resist this movement of
water vapor or moisture in various areas of the
The amount of condensation that can develop
within a wall depends upon (a) the resistance of
the intervening materials to vapor transfusion,
Influences of Construction
on Condensation
There is a distinct relationship in all homes
between indoor relative humidity and outdoor
temperature. The humidity is generally high
indoors when outdoor temperatures are high and
decreases as outdoor temperatures
relationship is shown in figure 4 for (a) an old
house (loosely constructed), (b) an average house,
and (c) a modern house (tightly constructed). In
other words, because of construction details and
other factors it would be much more difficult to
differences in vapor pressure, and (c) time.
Plastered walls or ordinary dry walls have little
resistance to vapor movement. However, when the
surfaces are painted, the resistance is increased.
High indoor temperatures and relative humidities
result in high indoor vapor pressures. Low outdoor
vapor pressures always exist at low temperatures.
Thus, a combination of high inside temperatures
and humidities and low outside temperatures will
normally result in vapor movement into the wall
if no vapor barrier is present. Long periods of
severe weather will result in condensation problems. Though fewer homes are affected by condensation in mild winter weather, many problems
have been reported. Where information is available, it appears that the minimum relative humidities in the affected homes are 35 percent or
Vapor barrier requirements are sometimes
satisfied by one of the materials used in a construction. In addition to integral vapor barriers
which are a part of many types of insulation,
such materials as plastic-faced hardboard and
similar interior coverings may have sufficient
resistance when the permeability of the exterior
construction is not too low. The permeability of
the surface to such vapor movement is usually
expressed in “perms,” which are grains of water
vapor passing through a square foot of material
per hour per inch of mercury difference in vapor
pressure. A material with a low perm value is a
barrier, while one with a high perm value is a
The perm value of the cold side materials
greater than those on
should be several
the warm side. A ratio of 1 to 5 or greater from
inside to outside is sometimes used in selecting
materials and finish. When this is not possible
because of virtually impermeable outside construction (such as a built-up roof or resistant
exterior wall membranes), research has indicated
the need to
the space between the insulation and the outer covering. However, few
specific data are available on ventilation requirements for walls.
Areas of vapor barrier use,--Vapor barriers
are used in three general areas of the house to
minimize condensation or moisture problems:
Walls, ceilings, floors,--Vapor barriers used
on the warm side of all exposed walls, ceilings,
and floors greatly reduce movement of water
vapor to colder surfaces where harmful condensation can occur. For such uses it is good pracFPL 132
tice to select materials with perm values of 0.25
or less, table
Such vapor barriers can be a
part of the insulation or a separate film. Commonly
used materials are (a) asphalt-coated or laminated
papers, (b) kraft-backed aluminum foil, and (c)
plastic films such as polyethylene, and others,
Foil-backed gypsum board and various coatings
also serve as vapor barriers, table 1. Oil-base
or aluminum paints, or similar coatings are often
used in Rouses which did not have other vapor
barriers installed during their construction.
Concrete slabs.--Vapor barriers under concrete slabs resist the movement of moisture
through the concrete and into living areas. Such
vapor barriers should normally have a maximum
perm value of 0.50. But the material must also
have adequate resistance to the hazards of pouring
concrete. Thus, a satisfactory material must be
heavy enough to withstand such damage and at
the same time have an adequate perm value.
Heavy asphalt laminated papers, papers with
laminated films, roll roofing, heavy films such
as polyethylene and other materials are commonly
used as vapor barriers under slabs, table
Crawl space covers.--Vapor barriers in crawl
spaces prevent ground moisture from moving up
and condensing on wood members, figure 2. A
perm value of 1.0 ox less is considered satisfactory for such use, table
Asphalt laminated
paper. polyethylene, and similar materials are
commonly used. Strength and resistance of crawl
space covering to mechanical damage can be lower
than that for vapor barriers used under concrete
Moisture Content
An illustration of the effect of a vapor barrier
in an outside wall was brought out in an exposure
test conducted at the Forest Products Laboratory.
It had been found that an excellent method of
was by determeasuring condensation
mining the moisture accumulation in wood sheathing. A small test house contained sections of
sheathing installed in such a way that they could
be removed for moisture content determination
and observation. The wood sheathing was conditioned to 6 percent moisture content before installation. Readings were made at the top and bottom
of each panel, as more moisture collects in the
sheathing at the bottom of uninsulated walls than
at the top of stud spaces. In insulated walls, the
Figure 5.--Moisture content of wood sheathing at the top and bottom of wall panels in
the Laboratory test house.
The temperature within the house was maintained at 70°
and the relative humidity at 40 percent.
M 139 213
would result in even lower moisture pickup by the
sheathing. This indicates the need for a vapor
barrier on exposed walls even when no insulation
is used in the stud spaces.
higher moisture contents are found to be at the
top of the stud spaces.
Figure 5 shows the average moisture contents
of the test house sheathing from October to June
in three sets of typical wall panels. The walls were
of conventional wood-frame construction with
gypsum lath and plaster, wood sheathing,
felt sheathing paper, and bevel wood siding with
two coats of exterior paint. Three panels had no
insulation and no vapor barrier in the stud space
(A panels); three had fill-type insulation with no
vapor barrier (B panels); and three had fill-type
insulation with a vapor barrier
panels). Three
panels of each variable were used to obtain average
moisture contents. The perm value for the vapor
barrier was about 0.21 and for
about 2.5.
The sheathing in panels reached an average
moisture content of about 12 percent by December.
However, the sheathing near the bottom of the wall
reached values well over 20 percent in the next
2 months. Sheathing in B panels reached an
moisture content as high as 47 percent and remained above 20 percent for over 4 months. C
panels. with insulation and vapor barrier, performed well as the sheathing did not reach 18
percent moisture content. It is likely that a better
or less
vapor barrier with a perm value of
Materials used on the exterior of walls, such
as sheathing, sheathing paper, siding, and paint
coatings, have an influence on the escape of water
vapor which might reach the stud spaces. It is
desirable to use materials and coatings in this
location that will allow moisture to escape rather
than hold it in. Exterior cover materials should
than interior covering
be far more
unless the stud space is ventilated.
Some of the materials normally used on the
outside of a house, and which can be considered
as having a low resistance to vapor movements,
Sheathing paper such as red rosin or low
asphalt content felt (breathing types).
Wood siding, shingles, or shakes with stained
Wood sheathing or sheathing grade plywood
sheathing (except those assembled with
exterior glue).
FPL 132
Table 1.--Permeance and permeabiIity of materials to water vapor
Insulation board sheathing with light asphalt
coating or light impregnation.
Gypsum board sheathing.
Masonry veneer (without coatings).
equipment. Furthermore, during the summer,
thermal insulation provides greater summer
comfort by slowing the movement of heat into
interior areas. This will also result in a lower
air-conditioning load when such equipment is used.
of insulation.--Normally, any combination of building materials provides some insulation, especially those used in wood-frame construction. While plywood, conventional boards,
siding, and other wood-base materials are usually
much better insulators than many nonwood structural elements, the use of these materials alone
is usually not sufficient in preventing excessive
heat loss. Thus, some type of thermal insulation
should be provided in walls, ceilings, and also in
floors of houses with unheated crawl-spaces.
However, it is not enough to say that a wall is
insulated because it contains some material sold
for insulation; it is the quality and insulating
value of the assembly that counts.
The relative value of thermal insulations is
based on their heat conductivity, represented by
the symbol k. This is defined as the amount of
heat in British thermal units that will pass in 1
hour through 1 square foot of material 1 inch
thick per 1° F. temperature difference between
faces of the material. Flexible insulation in
blanket and batt form is often marked with the
total resistance value represented by
R. R is the resistance to heat flow and the
reciprocal of conductance. Thus, knowing the k
value of an insulation. the total R value for it can
be found by the formula:
Materials which can be considered to have
moderate to high resistance to the escape of
water vapor are as follows:
Wood siding with low-perm paint finish (2
coats of white-lead base paint),
Exterior grade or paper-overlaid plywood with
paint coatings.
Metal or plastic sidings.
Aluminum foil, plastic films, or asphaltlaminated sheathing papers (nonbreathing
Any wood-base panel siding with high quality
nonbreathing paint or film finish,
The perm values for some of these low,
moderate. and highly permeable materials and
coatings are listed in table
It is not always possible, because of design,
material selection, and other reasons, to have a
combination of exterior materials which allow
the escape of water vapor. However, this can be
compensated to a great extent by the proper use
of a vapor barrier with a very low perm value.
movement.--Thermal insulation has a
influence on the need for vapor barriers.
The inner face of the wall sheathing in an
insulated wall, for example, is colder than the
sheathing face in an uninsulated wall and consequently has a greater attraction to moisture.
Thus there is greater need for a vapor barrier
in an insulated wall than in an uninsulated wall.
Heat is transferred through a wall or other
building component by (a) conduction, (b) con3
vection, and (c) radiation. Heat loss through a
wall may be by all three methods.
In the winter, thermal insulation reduces the
rate of flow of heat from the home and thereby
reduces the amount of fuel required. This provides
more comfortable living quarters than an uninsulated house and uses smaller, lower cost heating
R =
x thickness of the insulation in inches
Such values are used in determining the total U
or overall heat transmission coefficient.
The most common insulations used in the
construction of houses are as follows:
Fill insulation.--Fill insulation consists of
granular fibrous materials used in
intended to be poured or blown into place. It is
commonly used in attic spaces above living areas
and can also be poured or blown into wall cavities
of existing houses. A vapor barrier of some type
should be used under these conditions. Fill
insulating materials include vermiculite, rock
3 Lewis, Wayne C. Thermal Insulation from Wood for BuiIdings: Effects of Moisture and Its Control.
U.S.D.A. Forest Service Research Paper FPL 86, Forest Products Laboratory, July 1968.
FPL 132
attic spaces aids in keeping the air moving and
preventing the accumulation of frost or condensation on roof boards in cold areas. “Dead” air
pockets in the attic can normally be prevented
by good distribution of inlet ventilators in the
soffit areas. However, there is still a need for
vapor barriers in
ceiling; ventilation alone.
when insulation is used. does not prevent condensation problems. A good vapor barrier is especially needed under the insulation in a flat roof
where ventilation can normally be provided only
in the overhang.
Crawl space moisture, which results in high
moisture content of the wood members, can be
almost entirely eliminated by a vapor barrier
over the soil. When such protection is used,
need for ventilation is usually reduced to only
10 percent of that required when a soil cover is
not present.
During warm damp periods in early summer,
moisture often condenses on basement walls or
around the perimeter of the floor in concrete
slab houses. Soil temperatures in the northern
part of the United States remain quite low until
summer, and surface temperatures of the floor
or wall are often below dewpoint. When the
concrete reaches normal temperature and the
atmosphere changes, such problems are normally
glass wool, and wood fiber.
Flexible insulation. - - Flexible insulation is
supplied in blankets or batt form for use between
framing members spaced 16 or 24 inches on
center, Blanket insulation
often used in walls
has a vapor barrier on one face and a light
covering on the other. Batt insulation usually has
a vapor barrier on one face with
vary from
exposed on the other. Thickness
1 to 3-1/2 inches for the blanket and 4 to 6 inches
for the batt insulation. Materials include cotton
or wood fibers and mineral or glass wool,
A friction-type batt, usually supplied
coverings. is designed to
lightly between
frame members. Separate vapor barriers are
required when this type insulation is used.
Rigid insulation.--Rigid insulation consists of
board, expanded plastic board. rigid
glass board. and similar materials. Insulating
board in thicknesses of 1/2 and 25/32 inch is
commonly used for sheathing. It is also used as
above deck insulation in 2- and 3-inch thicknesses.
Expanded plastic insulations are 1/2 to 2
inches thick and are commonly used for perimeter
insulation around concrete slabs, as plaster base
over concrete block, and for roof deck insulation.
are those that reflect heat to a high degree.
Perhaps the most common is aluminum foil or
foil coatings on other materials. These reflective
surfaces must face an air space of at least 3/4
inch to be fully effective. A paper-reinforced
sheet with aluminum foil on one or two sides,
foil-backed gypsum board, and foldout accordion
sections are the most common form of this type
of insulation.
Condensation Control Zones
of condensation through the use of
vapor barriers and ventilation should be practiced
regardless of the amount of insulation used.
Normally, winter condensation problems occur
in those parts of the United States where the
or lower.
average January temperature is 35°
Figure 6 illustrates this condensation zone. The
northern half of the condensation zone has a lower
average winter temperature and, of
severe conditions than the southern portion Areas
outside this zone, such as the southeast and
coastal areas and the
states, seldom
have condensation
problems. Vapor barriers
should be installed at the time of construction in
all new houses built within the condensation zone
outlined in figure 6 and proper ventilation pro-
Ventilation used in proper amounts and locations
is a recognized means of controlling condensation
Inlet and outlet ventilators in attic
spaces, ventilation of rafter spaces in flat roofs
and crawl space ventilation aid in preventing
condensation in these areas, By introducing fresh
air into living quarters during the winter, some
humid air is forced out of the house while the
incoming air has a low water vapor content. Well
installed vapor barriers may increase the need
for ventilation because little of the moisture generated can get out. RH actually builds up.
The use of both inlet and outlet ventilators in
Figure 6.--Winter condensation
is 35° F. or lower.
occur where
cedures should be followed. These will insure
control over normal condensation problems.
Location of Vapor Barriers
A good general rule to keep in mind when
installing vapor barriers in a house is as follows:
“Place the vapor barrier as close as possible
to the interior or warm surface of all exposed
floors.” This normally means
walls, ceilings,
placing the vapor barrier (separately or as apart
of the insulation) on (a) the inside edge of the
studs just under the rock lath or dry wall
finish, (b) on the under side of the ceiling joists
of a one-story house or the second floor ceiling
joists of a two-story house, and (c) between the
subfloor and finish floor (or just under the subFPL 132
for January
M 139 490
floor of a house with an unheated crawlspace in
addition to the one placed on the ground). The
insulation, of course, is normally placed between
studs or other frame members
the outside of
the vapor barrier. The exception is the insulation
used in concrete floor slabs where a barrier is
used under the insulation to protect it from ground
Placement of vapor barriers and insulation in
one-story houses are shown in figure 7 (flat roof
and concrete floor slab) and figure 8 (pitched
roof and crawl space), Figure 9 shows barriers
and insulation in a 1-1/2-story house with full
basement, Figure 10 depicts a two-story house
with full basement. Other combinations of slabs,
crawl spaces, and basements in houses with 1,
1-1/2, or 2 stories, should follow the same
general recommendations. Detailed descriptions
12 Figure 7.--Location o f vapor barriers and
insulation in concrete slab and flatdeck roof.
M 139 236
10.--Location of vapor barriers
and insulation in full two-story house
with basement.
M 139 226
in the use of vapor barriers will be covered in
the following sections.
Concrete Slabs
Figure 8.--Location of vapor barriers
and insulation in crawl space of
another one-story house. M 139 237
Figure 9.--Location of vapor barriers and
insulation in 1-1/2-story house with
M 139 234
A house constructed over a concrete slab must
be protected from soil moisture which may enter
the slab. Protection is normally provided by a
vapor barrier, which completely isolates the
concrete and perimeter insulation from the soil.
Thermal insulation of some type is required
around the house perimeter in the colder climates,
not only to reduce heat loss but also to minimize
condensation on the colder concrete surfaces.
Some type of rigid insulation impervious to
moisture absorption should be used. Expanded
plastic insulation such as polystyrene is commonly
One method of installing this insulation is shown
in figure 11. Another method consists of placing
it vertically along the inside of the foundation wall.
Both methods require insulation at the slab notch
of the wall. In moderate climates the minimum
R value of the insulation should be at least 2.0;
in colder climates where temperatures reach
-20° F., an R value of at least 3.0 is recommended.
If the insulation is placed vertically, it should
extend a minimum of 12 inches below the outside
finish grade. In the colder climates a minimum
24-inch width or depth should be used.
13 In late spring
early summer, periods of
nigh humidity my
surface condensation
on exposed concrete slabs or on coverings such
as resilient tile before the concrete has reached
temperatures, A fully insulated slab or
a wood floor installed over wood furring strips
minimizes if not eliminates such problems.
Because the vapor barriers slow the curing
process of the concrete; find steel troweling of
the surface is somewhat delayed. Do not punch
holes through the barrier to hasten the curing
unheated crawl space this usually
moisture. In
consists of a vapor barrier over the soil, together
with foundation ventilators. In heated crawl
Spaces, a vapor barrier and perimeter insulation
is used but foundation ventilators are eliminated.
Unheated crawl space.--To provide complete
protection from condensation problems,
conventional unheated crawl space usually contains
(a) foundation ventilators, (b) a ground cover
(vapor barrier), and (c) thermal insulation between the floor joists. Foundation ventilators are
normally located near the top of the masonry wall.
In concrete block foundations, the ventilator is
often made in a size to replace a full block,
figure 12.
Crawl Spaces
Enclosed crawl spaces require some protection
to prevent problems caused by excessive soil
FPL 132
M 139 235
The amount of ventilation required for a crawl
space is based on the total area of the house in
square feet and the presence of a vapor barrier
soil cover. Table 2 lists the recommended net
ventilating areas for crawl space with or without
vapor barriers.
The flow of air through a ventilator is restricted
by the louvers.
by the presence of screening
This reduction varies with the size of the screening or mesh and by the type of louvers used.
Louvers are sloped about 45° to shed rain when
used in a vertical position. Table 3 outlines the
amount by which the total calculated net area of
the ventilators must be increased to compensate
for screens and thickness of the louvers.
In placing the vapor barrier over the crawlspace soil, adjoining edges should be lapped
slightly and ends turned up on the foundation wall
(fig. 13). To prevent movement of the barrier,
it is good practice to weight down laps and edges
with bricks or other small masonry sections.
An unheated crawl space in cold climates offers
insufficient protection to supply and disposal
M 139 233
pipes during winter months, It is common practice to use a large vitrified or similar tile to
enclose the water and sewer lines in the crawl
space. Insulation is then placed within the tile to
the floor level.
Insulating batts, with an attached vapor barrier,
are normally located between the floor joists.
They can be fastened by placing the tabs over
the edge of the joists before the subfloor is
installed when the cover (vapor barrier) is strong
enough to support the insulation batt (fig. 13).
However, there is often a hazard of the insulation
becoming wet before the subfloor is installed and
house enclosed, Thus, it is advisable to use
one of the following alternate methods:
Friction-type batt insulation is made to
tightly between joists and may be installed from
the crawl space as shown in figure 14,A. It is
good practice to use small “dabs” of mastic
adhesive to insure that it remains in place against
the subfloor. When the vapor barrier is not a part
of the insulation, a separate film should be placed
between the subfloor and the finish floor.
15 When standard batt or blanket insulation containing an integral vapor barrier is not installed
from above before the subfloor is applied, several
alternate methods can be used. If the vapor barrier
and enclosing paper wrap is strong enough it can
be installed with a mastic adhesive in the same
manner as the friction type (fig. 14,A). Floor
TabI e 3.--Ventilating area increase required if louvers
and screening are used in crawl spaces and
Table 2.--Crawl-spaceventilation
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13.--Vapor barrier
M 139 225
Figure 14.--Installation of vapor barriers and insulation in floor (unheated
A, Friction-type batts; B, wire mesh support; C, wood strip support.
insulation may also be supported by a wire mesh
held in place by wood strips
14,B). A third
method used to install the insulation from the
crawl space consists of using small wood strips
applied across the joist space (fig. 14,C). The
strips are cut slightly longer than the width of
the apace and sprung in place so that they bear
against the bottom of the insulation.
When only a small amount of insulation is
required between the joists because of moderate
climates, several other insulating materials can
be used. One such material is reflective insulation which usually consists of a kraft paper with
crawl space):
M 139 221
aluminum foil on each face. The reflective face
inch away from the
must be placed at least
underside of the subfloor or other facing to be
fully effective (fig. 15,A). Multiple or expanded
reflective insulation might also be used. A thin
blanket insulation can also be used between the
joists as shown in figre 15,B. This is installed
in much the same manner as thicker insulations
shown in figure 13 or 14. When vapor barriers
are a part of the flexible insulation and
installed, no additional vapor barrier is ordinarily
17 Figure
15.--lnstallation of vapor barriers and insulation
insuIation; and B, thin blanket insulation.
Heated crawl space.--One method of heating
which is sometimes used for crawl space houses,
utilizes the crawl space as a plenum chamber.
Warm air is forced into the crawl space, which
shallower than those normally used
without heat, and through wall-floor registers,
around the outer walls, into the rooms above.
When such a system is used, insulation is placed
along the perimeter walls as shown in figure 16.
Flexible insulation, with the vapor barrier facing
the interior, is used between joists; at the top
of the foundation wall. A rigid insulation such as
expanded polystyrene is placed
the inside
of the wall, extending below the groundline to
reduce heat loss. Insulation may be held in place
FPL 132
over crawl
ReflecM 139 229
with an approved mastic adhesive. To protect the
insulation from moisture and to prevent moisture
entry into the crawl space from the soil, a vapor
barrier is used over the insulation below the
groundline, figure 16. Seam of the ground cover
should be lapped and held in place with bricks
or other bits of masonry. Some builders pour a
thin concrete slab over the vapor barrier. The
crawl space of such construction is seldom
In crawl space houses, as well as other types,
the finish grade outside the house should be
sloped to drain water away from the foundation
18 Figure
Finished Basement Rooms
Finished rooms in basement areas with fully
or partly exposed walls should be treated much
the same as a framed wall with respect to the
use of vapor barriers and insulation (fig. 17).
When a full masonry wall is involved, several
factors should be considered: (a) When drainage
in the area is poor and soil is wet, drain tile
should be installed on the outside of the footing
for removing excess water; (b) in addition to an
exterior wall coating, a waterproof coating should
also be applied to the interior surface of the
masonry to insure a dry wall; and (c) a vapor
barrier should be used under the concrete floor
slab to protect untreated wood sleepers or other
materials from becoming wet.
M 139 239
Purring strips (2- by 2- or 2- by 3-inch members) used on the wall provide (a) space for the
blanket insulation with the attached vapor barrier
and (b) nailing surfaces for interior finish, figure
17. One- or 1-1/2-inch thicknesses of frictiontype insulation with a vapor barrier of plastic
film such as 4-mil polyethylene or other materials
might also be used for the walls,
Other materials which are used over masonry
walls consist of rigid insulation such as expanded
polystyrene. These are installedwith a thin slurry
of cement mortar and the wall completed with a
plaster finish. The expanded plastic insulations
normally have moderate resistance to vapor
movement and require no other vapor barrier.
When a vapor barrier has not been used under
the concrete slab, it is good practice to place
19 Figure
17.--Installing vapor
floor and wall
some type over the slab itself before applying
the sleepers. One such system for unprotected
in-place slabs involves the use of treated 1- by
4-inch sleepers fastened to the slab with a mastic.
This is followed by the vapor barrier and further
by second sets of 1- by 4-inch sleepers placed
over and nailed to the first set. Subfloor and
finish floor are then applied over the sleepers.
When the outside finish grade is near the level
of the basement floor, it is usually good practice
to use perimeter insulation around the exposed FPL 132
20 of
edges (fig. 17). To prevent heat loss and minimize escape of water vapor, blanket or batt insulation with attached vapor barriers should be used around the perimeter of the floor framing above the foundation wall (fig. 17). Place the insulation between the joists or along stringer joists with the vapor barrier facing the basement side. The vapor barrier should fit tightly against the joists and subfloor. Figure
Blanket insulation. - - Flexible insulation in
blanket or batt form is normally manufactured
with a vapor barrier. These vapor barriers contain tabs at each side, which are stapled to the
frame members. To minimize vapor loss and possible condensation problems, the best method of
attaching consists of stapling the tabs over the edge
of the studs (fig. 18). However, many contractors
do not follow this procedure because it is more
difficult and may cause some problems in nailing
of the rock lath or dry wall to the studs. Consequently, in many cases, the tabs are fastened
to the inner faces of the studs. This usually
results in some openings along the edge of the
vapor barrier and, of course, a chance for vapor
to escape and cause problems. When insulation
is placed in this manner, it is well to use a vapor
barrier over the entire wall. This method is
21 At junctions of interior partitions with exterior
walls, care should be taken to cover this intersection with some type of vapor barrier. For best
protection, insulating the space between the
doubled exterior wall studs and the application of
a vapor barrier should be done before the corner
post is assembled (fig. 18). However, the vapor
barrier should at least cover the stud intersections
at each side of the partition wall.
described in the next section.
Another factor in the use of flexible insulation
having an integral vapor barrier is the protection
required around window and door openings. Where
the vapor barrier on the insulation does not cover
doubled studs and header areas, additional vapor
barrier materials should be used for protection
(fig. 18). Most well-informed contractors include
such details in the application of their insulation.
F i g u r e 1 9 . - - l n s t a l I i n g vapor b a r r i e r o v e r f r i c t i o n - t y p e i n s u l a t i o n ( e n v e l o p i n g ) .
FPL 132
M 139 223
Friction-type insulation.--Some of the newer
insulation forms, such as the friction-type without covers, have resulted in the development of
a new process of installing insulation and vapor
barriers so as to practically eliminate condensation problems in the walls. An unfaced frictiontype insulation batt is ordinarily supplied without
a vapor barrier, is semi-rigid, and made to fit
tightly between frame members spaced 16 or
24 inches on center. “Enveloping” is a process
of installing a vapor barrier over the entire wall
(fig. 19). This type vapor barrier often consists
of 4-mil or thicker polyethylene or similar
material used in 8-foot-wide rolls. After insulation has been placed, rough wiring or duct work
finished, and window frames installed, the vapor
barrier is placed over the entire wall, stapling
when necessary to hold it in place. Window and
door headers, top and bottom plates, and other
framing are completely covered (fig. 19). After
rock lath plaster base or dry-wall finish is
installed, the vapor barrier can be trimmed around
window openings.
Reflective insulations,--Reflective insulations
ordinarily consist of either a kraft sheet faced
on two sides with aluminum foil, figure 20,A, or
Figure 20.--lnstalIing reflective
B, multiple reflective insulation.
two sides:
M 139 222
the multiple-reflective “accordion” type, figure
20,B. Both are made to use between studs or
joists. To be effective, it is important in using
such insulation that there is at least a 3/4-inch
space between the reflective surface and the wall,
floor, or ceiling surface. When a reflective
insulation is used, it is good practice to use a
vapor barrier over the studs or joists. The
barrier should be placed over the frame members
just under the dry wall or plaster base (fig. 20,A).
Gypsum board commonly used as a dry wall
finish can be obtained with an aluminum foil on
the inside face which serves as a vapor barrier.
When such material is used, the need for a
separate vapor barrier is eliminated.
FPL 132
Two-story house.--One of the areas of a twostory house where the requirement of a vapor
barrier and insulation is often overlooked is at
the perimeter area of the second floor floor joists.
The space between the joists at the header and
along the stringer joists should be protected by
sections of batt insulation which contain a vapor
barrier (fig. 21). The sections should fit tightly
so that both the vapor barriers and the insulation
fill the joist spaces.
A two-story house is sometimes designed so
that part of the second floor projects beyond the
first. This projection varies but is often about
12 inches. In such designs, the projections should
be insulated and vapor barriers installed as shown
barriers in walls and joist space of two-story house
M 139
in figure 22.
Insulation and vapor barriers in exposed second
floor walls (fig. 21) should be installed in the
same manner as for walls of single-story houses.
This might include: (a) standard blanket insulation
with its integral vapor barrier; (b) friction-type
insulation with separate vapor barrier (fig. 19);
or (c) reflective insulation with the protective
vapor barrier (fig. 20).
be taken when placing the insulating batt to allow
an airway for attic ventilation at the junction of
the rafter and exterior wall.
Insulation in the knee wall can consist of blanket
or batt-type insulation with integral vapor barrier
or with separately applied vapor barriers, as
described for first and second floor walls.
or blanket insulation is commonly used
between the rafters at the sloping portion of the
heated room, figure 23. As in the application of
all insulations, the vapor barrier should face the
inner or warm side of the roof or wall. An airway
should always be allowed between the top of the
insulation and the roof sheathing at each rafter
space. This should be at least 1-inch clear space
without obstructions such as might occur with
solid blocking. This will allow movement of air
in the area behind the knee wall to the attic area
above the second floor rooms (fig. 9).
Ceilings and Attics
Figure 22.--lnsulation and vapor barrier
at second floor projection.
M 139
Knee walls.--In 1-1/2-story houses containing
bedrooms and other occupied rooms on the second
floor, it is common practice to include knee walls.
These are partial walls which extend from the
floor to the rafters (fig. 23). Their height usually
varies between 4 and 6 feet. Such areas must
normally contain vapor barriers and insulation
in the following areas: (a) In the first floor
ceiling area, (b) at the knee wall, and (c) between
the rafters. Insulation batts with the vapor barrier
facing down should be placed between joists from
the outside wall plate to the knee wall. The
insulation should also fill the entire joist space
directly under the knee wall (fig. 23). Care should
Vapor barriers and insulation.--Insulation in
ceiling areas normally consists of the batt or
fill-type. However, to provide for good condensation control, a vapor barrier should always be
provided (fig. 24,A). When an insulation batt is
supplied with a vapor barrier on one face, no
additional protection is normally required. Place
the batts with barrier side down, so that they fit
tightly between ceiling joists. Batts with the vapor
barrier attached can also be stapled to the bottom
edge of the joists before the ceiling finish is
applied. At the junction of the outside walls and
rafters, a space should always be left below the
roof boards to provide a ventilating airway (fig.
Ventilation.--Ventilation of attic spaces and
roof areas is important in minimizing water vapor
buildup. However, while good ventilation is important, there is still a need for vapor barriers
in ceiling areas. This is especially true of the
flat or low-slope roof where only a 1- to 3-inch
space above the insulation might be available for
In houses with attic spaces, the use of both inlet
and outlet ventilation is recommended. Placing
inlet ventilators in soffit or frieze-board areas
of the cornice and outlet ventilators as near the
ridge-line as possible will assure air movement
through a “stack” effect. This is due to the
difference in height between inlet and outlet
25 Figure 23.--lnstalling vapor
ventilators and normally assures air movement
even on windless days or nights.
Recommended ventilating areas.--The minimum amount of attic or roof space ventilation
required is determined by the total ceiling area.
These ratios are shown in figures 25, 26, and 27
for various types of roofs. The use of both inlet
and outlet ventilators is recommended whenever
possible. The total net area of ventilators is
found by application of the ratios shown in figures
25, 26, and 27. The total area of the ventilators
can be found by using the data in table 3. Divide
this total area by the number of ventilators used
to find the recommended square-foot area of each.
For example, a gable roof similar to figure
25,B with inlet and outlet ventilators has a minimum required total inlet and outlet ratio of
1/900 of the ceiling area. If the ceiling area of
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M 139 232
the house is 1,350 square feet, each net inlet and
outlet ventilating area should be 1,350 divided by
900 or 1-1/2 square feet.
If ventilators are protected with No. 16-mesh
insect screen and plain metal louvers, table 3,
the gross area is 2 x 1-1/2 or 3 square feet. When
one outlet ventilator is used at each gable end,
each should have a gross area of 1-1/2 square
feet (3 divided by 2). When distributing the soffit
inlet ventilators to three on each side, for a small
house (total of 6), each ventilator should have a
gross area of 0.5 square feet. For long houses,
use 6 or more on each side.
Inlet ventilators.--Inlet ventilators in the soffit
may consist of several designs. It is goodpractice
to distribute them as much as possible toprevent
“dead” air pockets in the attic where moisture
26 Figure
ceiIing insulation and
insulation; and B, airway for ventilation.
Vapor barrier and M 139 231 A, Louvers in end walls; B, louvers in end
Figure 25.--Ventilating areas of gable roofs:
additional openings at eaves; C, louvers at end walls with additional openings
at eaves and dormers. Cross section of C shows free opening for air movement between
roof boards and ceiling insulation of attic room.
M 87625 F
27 Figure 26.--Ventilating areas of hip roofs: A, inlet openings beneath
vent near peak; B, inlet openings beneath eaves and ridge outlets.
and outlet
M 87626 F
roofs: A, Ventilator openings under overhanging
Figure 27.--Ventilating area of
eaves where ceiIing and roof joists are combined; B, for roof with a parapet where roof
and ceiling joists are separate; C, for roof with a parapet where roof and ceiling
joists are combined.
M 87627 F
FPL 132
28 Figure 28.--Inlet ventilators
D, singIe ventiIator.
might collect. A continuous screened slot, figure
28,A, satisfies this requirement. Small screened
openings might also be used, figure 28,B. Continuous slots or individual ventilators between
roof members should be used for flat-roof houses
where roof members serve as both rafters and
C, perforated;
M 139 214
ceiling joists. Locate the openings away from the
wall line to minimize the possible entry of winddriven snow. A soffit consisting of perforated
hardboard, figure 28,C, can also be used to
advantage but holes should be no larger than
29 Figure
(for open cornice).
Figure 30.--Gable outlet
FPL 132
1/8 inch in diameter. Small metal frames with
screened openings are also available and may be
used in soffit areas, figure 28,D. For open cornice
design, the use of a frieze board with screen
ventilating slots would be satisfactory, figure 29.
Perforated hardboard might also be used for this
purpose. The recommended minimum inlet ventilating ratios shown infigures 25, 26, and 27 should
be followed in determining total net ventilating
areas for both inlet and outlet ventilators.
Outlet ventilators.--Outlet ventilators to be most effective should be located as close to the highest portion of the ridge as possible. They may be placed in the upper wall section of a gable-
M 139 230
ventilators: A,
C soffit
30 gable
M 139 228
roofed house in various forms as shown in
figure 30,A and B. In wide gable-end overhangs
with ladder framing, a number of screened openings can be located in the soffit area of the lookouts (fig. 30,C). Ventilating openings to the attic
space should not be restricted by blocking, Outlet
ventilators on gable or hip roofs might also consist of some type of roof ventilator (fig. 31,A and
B). Hip roofs can utilize a ventilating gable
C, modified hip ventilator,
(modified hip) (fig. 31, C). Protection from blowing
snow must be considered, which often restricts
the use of a continuous ridge vent. Locate the
single roof ventilators (fig. 31,A and B) along the
ridge toward the rear of the house so they are
not visible from the front. Outlet ventilators
might also be located in a chimney as a false flue
which has a screened opening to the attic area.
A, Low
ventilator type;
M 139 217
turn, tends to keep attic temperature only slightly
above outdoor temperatures. This combination of
good ventilation and insulation is the answer to
reducing ice dam problems.
Another protective measure is provided by the
use of a flashing material. A 36-inch width of
45-pound roll roofing along the eave line will
provide such added protection (fig. 32,B).
Protection at unheated areas.--Walls and doors
to unheated areas such as attic spaces should be
treated to resist water vapor movement as well
as to minimize heat loss. This includes the use
of insulation and vapor barriers on all wall areas,
adjacent to the cold attic (fig. 33). Vapor barriers
should face the warm side of the room. In addition,
some means should be used to prevent heat and
vapor loss around the perimeter of the door. One
method is through some type of weather strip
(fig. 33). The door itself should be given several
finish coats of paint or varnish which will resist
the movement of water vapor. Table 1 lists a
number of coatings which provide some vapor
Figure 32.--Ice dams:
and ventilation can cause
ice dams and water damage; B, good
flashing minimize problems. M 134 787
Other Protective Measures
Snow and ice dams.--Water leakage into walls
and interiors of houses in the snow belt areas of
the country is sometimes caused by ice dams and
is often mistaken for condensation. Suchproblems
occur after heavy snowfalls, followed by temperatures somewhat below freezing when there is
sufficient heat loss from the living quarters to
melt the snow along the roof surface. The water
moves down the roof surface to the colder overhang of the roof where it freezes. This causes a
ledge of ice and backs up water, which can enter
the wall or drip down onto the ceiling finish (fig.
Ice dam problems can be minimized if not
entirely eliminated. By reducing attic temperatures by adequate insulation and ventilation, snow
melting at the roof surface is greatly reduced.
Good insulation, 6 inches or more in the northern
sections of the country, greatly reduces heat loss
from the house proper. Adequate ventilation, in
FPL 132
Figure 33.--lnsulating
attic space.
to unheated
M 139 238
If further resistance to heat loss is desired, a
covering of 1/2 inch or thicker rigid insulation,
such as insulation board or foamed plastic, can
be attached to the back of the door.
Protection at outlet boxes.--Outlet or switch
boxes or other openings in exposed (cold) walls
often are difficult to treat to prevent water vapor
escape. Initially, whether the vapor barrier is a
separate sheet or part of the insulation, as tight
a fit as possible should be made when trimming
the barrier around the box (fig. 34). This is less
difficult when the barrier is separate. As an
additional precaution, a bead of calking compound
should be applied around the box after the dry
wall or the plaster base has been installed (fig.
The same calking can be used around the
cold-air return ducts or other openings in exterior
walls. This type of sealing may appear unnecessary, but laboratory tests have shown that there
is enough loss through the perimeter of an outlet
box to form a large ball of frost on the back face
during extended cold periods. Melting of this
frost can affect the exterior paint films. In the
colder areas of the country and in rooms where
there is excess water vapor, such as the bath and
kitchen, this added protection is good insurance
from future problems. Some switch and junction
boxes are more difficult to seal than others
because of their makeup. A simple polyethylene
bag or other enclosure around such boxes will
provide some protection.
Figure 34.--Protection
in exposed walls.
outlet boxes
M139 224
Figure 35.--Results of
around outlet box.
vapor loss
M 139 215
Condensation problems caused by water-vapor
movement through unprotected outlet box areas
in exposed walls are often due to poor workmanship during application of the insulation. Figure
35 shows a section of exterior wall with the vapor
barrier loosely stapled to the face of the studs.
Because of poor application, a small space is
sometimes left at the top and bottom of the insulation in the stud space. Water vapor escaping
through the unprotected outlet box travels by
convection, on the warm side, to the top of the
wall, where it moves to the cold side and condenses on the inner face of the colder siding or
sheathing. Continued movement of vapor can
saturate these materials and in severe conditions
cause decay. Buckling of single panel siding,
such as hardboard or similar materials, can
result as moisture content of the material increases. Such problem can be minimized by
“enveloping” of the inner face of exposed walls with
a vapor barrier, as previously outlined (fig. 19).
Sealing the outlet box in some manner will also
aid in restricting water vapor movement into the
33 wall cavity.
Other openings.--The same principles used in
sealing outlet boxes should be applied to all openings in an outside wall or ceiling. Openings may
include exhaust fans in the kitchen or the bathroom, hot air registers, cold air return registers,
and plumbing. Openings are also required in
ceilings for light fixtures, ventilation fans, and
plumbing vents. Regardless of the type of opening,
the vapor barrier should be trimmed to fit as
tightly as possible.
problems can be eliminated by
specifying proper
construction details during
planning of the house. Correct placement of vapor
barriers, adequate insulation, the use of attic
ventilation, and other good practices can be
incorporated at this time. These recommendations
have been outlined and illustrated in the preceding
sections. However, when one or more of these
details have not been included in an existing house
and condensation problems occur, they are often
more difficult to solve. Nevertheless, there are
methods which can be used to minimize such
condensation problems after the house has been
surfaces.--Visible surface condensation
on the interior glass surfaces of windows can be
minimized by the use of storm windows or by
replacing single glass with insulated glass. However, when this does not prevent condensation on
the surface, the relative humidity in the room
must be reduced. Drapes or curtains across the
windows hinder rather than help. Not only do they
increase surface condensation because of colder
glass surfaces, but they also prevent the air
movement that would warm the glass surface and
aid in dispersing some of the moisture.
Attic areas.--Condensation or frost on protruding nails, on the surfaces of roof boards, or
other members in attic areas normally indicates
the escape of excessive amounts of water vapor
from the heated rooms below. If a vapor barrier
FPL 132
is not already present, place one between joists
. under the insulation. Make sure the vapor barrier
fits tightly around ceiling lights and exhaust fans,
calking if necessary. In addition, increase both
inlet and outlet ventilators to conform to the
minimum recommendations in figures 25, 26, and
27. Decreasing the amount of water vapor produced in the living areas is also helpful.
Crawl spaces. - -Surface condensation in unheated crawl spaces is usually caused by excessive moisture from the soil or from warm humid
air entering from outside the house. Toeliminate
this problem, place a vapor barrier over the soil
as shown in figure 14; if necessary, use the
proper amount of ventilation as recommended in
table 2.
Concrete slabs.--Concrete slabs without radiant heat are sometimes subjected to surface
condensation in late spring when warm humid air
enters the house. Because the temperature of some
areas of the concrete slab or its covering is
below the dewpoint, surface condensation can
occur. Keeping the windows closed during the day,
using a dehumidifier, and raising the inside
temperature aid in minimizing this problem. When
the concrete slab reaches normal room temperatures, this inconvenience is eliminated.
Reducing Relative Humidity
Reducing high relative humidities within the
house to permissible levels is often necessary
to minimize condensation problems. Discontinuing
the use of room-size humidifiers or reducing
the output of automatic humidifiers until conditions
are improved is helpful. The use of exhaust fans
and dehumidifiers can also be of value in eliminating high relative humidities within the house.
When possible, decreasing the activities which
produce excessive moisture, as discussed in a
section, is sometimes necessary. This
is especially important for homes with electric
condensation.--Concealed condensation is, in essence, a surface or similar condensation that takes place within a component such
as a wall cavity when a condensing surface is
below the dewpoint. In cold weather, condensation
often forms as frost. Such conditions can cause
staining of siding and peeling of the paint and
often decay in severe and sustained conditions.
These problems are usually not detected until
34 spring after the heating season. has ended. Installing a nonpermeable metal o r plastic siding is not
a solution, but only hides the symptoms. The
remedies and solutions to the problems should
be taken care of before repainting is attempted.
Several methods might be used to correct this
(1) Reduce or control the relative humidity
within the house as previously discussed.
(2) Improve the vapor resistance of the wall
and ceiling by adding a vapor barrier between
the ceiling joists. Add a vapor-resistant paint
coating to the interior of walls (table 1).
(3) Improve attic ventilation (figs. 25, 26, and
(4) When repainting the outside of the house,
use permeable paints which allow some vapor
movement through them (table 1).
Ice dams.--Several methods can be used to
minimize this problem caused by melting snow.
By reducing the attic temperatures in the winter
so that they are only slightly above outdoor
temperatures, most ice dams can be eliminated.
This can be accomplished in the following manner:
(1) Add insulation to the ceiling area in the
attic to reduce heat loss from living areas below.
This added insulation and ventilation will also be
helpful by reducing summer temperatures in the
living areas below.
(2) Provide additional inlet ventilation in the
soffit area of the cornice as well as better outlet
ventilation near the ridge.
(3) When reroofing, use a flashing strip of 36inch-wide roll roofing paper of 45-pound weight
along the eave line before reshingling. While this
does not prevent ice dams, it is a worthwhile
(4) Under severe conditions, or when only some
portions of a roof produce ice dams (such as at
valleys), the use of electric-thermal wire laid in
a zig-zag pattern and in gutters may prove effective. The wire is connected and heated during
periods of snowfall and at other times as needed
to maintain channels for drainage.
American Society of Heating, Refrigerating, and
Air-conditioning Engineers (ASHRAE)
Handbook of Fundamentals, Heating,
Refrigerating, Ventilating, and Air
Conditioning. 544 pp.
Anderson, L. O.
1970. Wood-Frame House Construction. U.S.
Dept. Agr., Agr. Handb. No. 73.
Building Research Advisory Board
1952. Condensation Control in Buildings. Nat.
Res. Counc. 118 pp.
Lewis, Wayne C.
Thermal Insulation from Wood for
Buildings: Effects of Moisture and Its
Control. U.S.D.A. Forest Serv. Res.
Pap. FPL 86. 42 pp. Forest Products
Lab., Madison, Wis.
(c) Rigid.--Includes insulating board or other
materials in sheet or block form. Often
used as sheathing material, as perimeter
insulation, etc.
(d) Reflective.--A polished surface such as
aluminum foil which has high reflectivity
and low emissivity.
k (see Thermal conductivity)
Perm.--A measure of vapor movement through
a material, i.e. grains per square foot per hour
per inch of mercury difference in vapor pressure at standard test conditions.
Permeance.--Rate of water vapor transmission
through a material, measured in perms. Thus,
the lower the permeance, the better the vapor
R.--Resistance of a substance or assemblytoheat
Radiation (see Energy transfer)
Relative humidity.--The amount of water vapor
expressed as a percentage of the maximum
quantity that could be present in the atmosphere
at a given temperature. (The actual amount
of water vapor that can be held in space
increases with the temperature.)
Roll roofing--Roofing material composed of fiber
saturated with asphalt and supplied in 36-inchwide rolls which cover 100 square feet including
a lap seam. It can be obtained in weights of
45 to 90 pounds per roll.
Sheathing paper.--A paper for use between wood
board sheathing and the exterior covering to
reduce air infiltration. Materials such as 15pound asphalt felt or red rosin paper with a
perm value of 5 or greater is commonly used.
Condensation.--Free water, frost, or ice extracted from the air and deposited on any cold
surface. It may occur on the surface of a window
glass, for example (surface condensation), or
on a cold inner face of wall sheathing (concealed
condensation), or within a material. Excessive
condensation especially in the walls, can cause
problems which often result in excessive maintenance.
Conduction, convection (see Energy transfer)
Crawl space.--A shallow space below the living
quarters of a house. It is generally not excavated
and may be constructed with a foundation wall
or with piers and a skirt board enclosure.
Dew point.--The temperature at which the water
vapor in space becomes saturated and can hold
no more moisture. Water vapor cooled below
the dew point appears in the atmosphere as
fog and on the surface as dew or frost.
Energy transfer
Conduction.--Transfer of heat or energy within
and through a material (or gas) as a result
of temperature gradient.
of heat energy by the
circulatory motion that occurs in a fluid or
gas at a non-uniform temperature owing to
the variation of its density and the action of
Radiation.--Transfer of heat energy in the form
of waves or particles which have no effect
on the medium through which they pass.
Humidifier.--A device designed to discharge
water vapor into a confined space for the purpose of increasing or maintaining the relative
humidity. It may be attached to the central
heating plant or consist of small room-size
Ice dam.--Ice forming at the eave line from
melting snow pocketing. Water which can enter
walls and cornice.
low-density material
Insulation.--Normally a
used to reduce heat loss. It is made of wood
fiber, cotton fiber, mineral or glass wool or
fiber, vermiculite, expanded plastics, and
others, It is made in several forms including:
(a) Flexible.--in blanket or batt form.
(b) Fill.--A loose form which can be poured
or blown.
FPL 132
Thermal conductivity (k).--The amount of heat
expressed in British thermal units (B.T.U.)
that will pass through 1 square foot of uniform
material, 1 inch thick, in 1 hour when the
temperature difference between surfaces of the
material is 1° F. The lower this value the better
the material is for insulating purposes.
U.--Over all heat transmission coefficient; the
amount of heat expressed in British thermal
units transmitted in 1 hour through 1 square
foot of a building section for each degree
temperature difference between air on the
inside and air on the outside of the building
Vapor barrier.--A film, duplex paper, aluminum
foil, paint coating or other materials which
restrict the movement of water vapor from an
area of high vapor pressure to one of lower
pressure. Material with a perm value of 1.0 or
less is normally considered a vaporbarrier.
Vapor permeability.--The property of a material
that allows the passage of water vapor.
Vapor, water.--Water vapor i s an invisible gas
present in varying amounts in the atmosphere.
There is a maximum amount that can be held at
a given temperature.
Ventilation.--The replacement, by outside air,
of the a i r within the building.
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