Moisture Control in Houses:
"The Effect of Interior Moisture on Exterior Finishes"
Introduction..........(Return to Table of Contents)
Moisture movement and control is a major concern to building designers, builders, and
homeowners. Where moisture problems exist, they can range from simply being a nuisance to
actually threatening the life of the building. Moisture in vapor form is usually not a
problem, but it becomes a problem when it condenses as free water on cold surfaces. The
following discussion addresses the mechanisms of moisture movement and condensation, the
effects of interior moisture on exterior finishes, and methods of controlling this
movement to prevent problems in houses.
Condensation problems in houses were of little consequence prior to the introduction of
thermal insulation and attempts to more closely control the indoor environment. When
builders began using thermal insulation during the 1930's, condensation problems developed
because insulation resulted in lowering the temperature of wall sheathing below the dew
point temperature of indoor air. The use of vapor retarders near the warm face of walls
was proposed to prevent moisture from migrating to the exterior. Initially, these vapor
retarders provided adequate control, but the energy crisis of the 1970's resulted in much
more insulation and more airtight houses. This increased condensation potential two ways;
(1) The added insulation resulted in colder sheathing surfaces and consequently a greater
potential for dew point temperature to occur. (2) Airtight houses meant higher humidity
because less indoor moisture was lost through air exchange; therefore, more moisture found
its way into walls by diffusion through the vapor retarder or leakage where the vapor
retarder was not continuous.
Condensation can cause several problems for exterior finishes. Water soaking into the back
of siding causes it to swell and consequently stress the film-forming finishes. In some
cases condensation occurs at the interface of the siding and the finish film, and causes
blisters. Water in siding may also leach extractives from certain extractive-rich woods
and cause staining. Wet siding also promotes mildew growth on the surface.
A certain amount of condensation occurs in most houses. In many cases, it does not create
a problem. Some materials are able to absorb and store this moisture for short time
periods and return it to the air when conditions change. Condensation may occur in walls
for a period of several days during times of extreme cold, but if it evaporates at the end
of that time, there may not be any damaging effect. Whether or not there is damage depends
on the quantity of condensation, length of time over which it occurs, and ability of the
building materials to absorb and store the water.
There are four basic ways of controlling moisture in walls:
1) ventilation to keep indoor humidity at a reasonable level,
2) elimination of air leakage into wall cavities,
3) effective use of vapor retarders, and
4) ventilation of crawl spaces to prevent soil moisture from entering the building.
The theory of moisture movement and control is not an exact science because data for the
many variables involved are difficult to establish. Research to quantify these variables
is ongoing. However, much is known about good practice. While moisture control is best
accomplished in new construction, the same principles can be applied to solving existing
problems.
Moisture Control Methods- Cold Weather Conditions..........(Return to Table of Contents)
Indoor Humidity
The most critical item in preventing moisture damage is to keep indoor relative humidity
at reasonable levels during the heating season. While exceptionally dry conditions may
cause respiratory problems and shrinking of wood furniture or trim, humidities of 30 to 40
percent appear to prevent these problems. When indoor humidity exceeds 40 percent during
cold weather, moisture problems begin to appear. It is difficult even with proper vapor
retarders to construct a house that will not have condensation problems when indoor
humidity exceeds 40 percent. When a house is retrofitted with insulation without the
benefit of vapor retarders and air leakage control, an even lower humidity may be
required. Persistent condensation on double-glazed windows is a good indicator that
relative humidity is too high and may cause damage to the exterior finish.
Some interior moisture control is possible by using exhaust vents in kitchens and
bathrooms. These may be manually controlled by a conscientious homeowner or automatically
controlled by humidistats that turn on the fan when relative humidity exceeds a
predetermined level. A more positive measure is to connect a small duct from the outdoors
to the return side of a forced-air heating system, so that fresh air is drawn into the
house whenever the system is operating. A damper placed in this duct will allow the
homeowner to control incoming air. While ventilation results in energy requirements to
heat the incoming air, it is still much more energy efficient than purposely building with
uncontrolled natural air leakage. With natural leakage, the greatest air exchange occurs
during the coldest weather when the least air exchange is needed for humidity control
because incoming air is very dry. By controlling the ventilation to the actual amount
needed the benefits of energy efficient construction are preserved.
An alternative to direct ventilation is the use of an air-to-air heat exchanger. This type
of device removes a large portion of the heat from air being exhausted and transfers it to
incoming air from outdoors. However, the initial cost of this equipment has to be compared
to savings in operation. Dehumidifiers have also been used in extreme cases, but they
generally are not effective in drying the air below about 50 percent relative humidity,
and they are major consumers of energy.
Protection of Walls
Control of moisture in walls has traditionally been by the use of vapor retarders near the
warm face of the wall. More recently it has been recognized that air leakage into the wall
cavity may be more critical than diffusion of water vapor through the materials of the
wall. Moisture-laden air can enter the wall through joints at the base of the wall, around
windows and doors, or through electrical outlets. Field examinations as well as simulation
tests have shown excessive buildup of moisture behind electrical outlets. For good
moisture control, a complete air barrier is required. Vapor retarder materials generally
serve as air barriers if they are continuous. It is important that the vapor retarder fit
tightly around outlet boxes and other openings such as windows and doors. Airtight outlet
boxes should be used, and the openings around electrical cables sealed with electricians
putty. An alternative is to use an airtight insert behind the cover plate and use plugs in
the outlets not in use. All joints should be made as airtight as possible.
A good vapor retarder is required even where air leakage into the wall is stopped. Tests
have shown that where the vapor retarder has gaps in a building with 40 percent relative
humidity, condensation can stain the siding during very cold weather even though there is
no air leakage into the wall.
The asphalted paper backing on blanket insulation is technically a vapor retarder, but
flanges must overlap each other over the edge of studs for it to be effective. Research
has shown that stapling the flanges between studs without overlapping does not result in a
good vapor retarder. It is also difficult to provide coverage of window and door framing
with this type of vapor retarder. Better coverage is generally possible with wide sheets
of polyethylene covering an entire wall with accurate cutouts for windows and outlets.
This assures good coverage of all areas including framing. While 2-mil polyethylene
provides adequate vapor resistance, 4-mil or 6-mil polyethylene is preferred because of
its increased resistance to tearing during installation. An alternative is the use of
foil-backed gypsum board, but it also must be continuous with tight joints at openings.
Vapor retarders are recommended near the inside face of all walls in all geographic areas
of the United States except the gulf coast, Florida, and Hawaii. Application to these hot,
humid climate zones is discussed in a later section.
Some recommendations in the past have advocated high permeability for materials on the
outside of the wall in order to let any moisture getting into the wall escape to the
outside. This is good practice for thin, non-insulating materials because moisture will
condense on these cold materials if it cannot pass through them. However, the use of
insulating foam sheathings present a different condition. Although many of these materials
are vapor retarders, they are also good insulators, so the inside face generally remains
above dew point temperature most of the time. For this reason, they have proven both in
research studies and in practice to perform quite well.
Some sources have suggested ventilating wall cavities to let moisture escape, especially
where low permeability sheathing is used. However, studies in instrumented buildings have
shown that ventilating actually increases the potential for condensation problems. Vents
provided only at the top tend to draw more humid indoor air into the wall cavity, and thus
provide more moisture to condense. Vents provided at both top and bottom allow cold air to
pass through the wall, which may cool the sheathing surface below dew point temperature.
While retrofit vents have sometimes alleviated paint peeling by letting moisture escape
when the weather warms up, they also increase condensation potential. Even without vents,
moisture is not trapped in the wall since it will eventually escape around and through the
top plate.
Ice Dams
In cold climates, the effects of ice dams are sometimes mistaken for condensation
problems. Snow can melt over the heated portion of the attic and run down to the roof
overhang where it re-freezes. This ice can build up to form a trough to catch water right
over the wall. The water then backs under the shingles and runs down through the wall or
ceiling, causing stains on the inside face of the wall or ceiling, or paint peeling from
siding. Good ceiling insulation and attic ventilation at the eaves keep the roof at
temperatures near outdoor air temperature and thus prevent melting until warmer weather.
Another good preventive in areas where ice dams are prevalent, is the use of a wide roll
roofing under the shingles parallel to the eave and extending over the wall. Then, if ice
dams do occur, water cannot get through the roof.
Crawl Spaces
Moisture from crawl spaces may eventually enter the living space and raise the humidity,
or it may move directly into wall cavities. The major source of moisture is the soil. An
effective solution is the installation of a soil cover using a vapor-retardant material,
which is tear and puncture resistant. The material is simply laid on the soil with all
joints lapped and held in place at the edges by gravel, bricks or other weights. Roll
roofing has been used for many years, but more recently, 6-mil polyethylene has become
more popular.
The effectiveness of soil covers is recognized by major codes and standards, which allow
reduction of ventilation to one-tenth that required where no soil cover is used. The usual
ventilation requirement without a soil cover is one sq. Ft. per 150 sq. ft. of soil area,
with vents distributed for cross ventilation to all areas. This normally means a minimum
of four openings with one near each corner. Where a soil cover is used, the ventilation
can be reduced to one sq. ft. per 1500 sq. ft. of soil area, but good cross ventilation is
still required. Drainage away from the building is always critical, since water standing
on top of the soil cover negates any advantage. In problem areas, a sump drain is a good
precaution.
Slab On-Grade Foundations
Where houses are built on concrete slabs, a moisture barrier should be placed under the
slab to prevent soil moisture from rising into the house. The material should be tear and
puncture resistant such as a heavy polyethylene. Good drainage away from the house is
especially important.
Hot, Humid Climate Considerations..........(Return to Table of Contents)
In some cases, condensation problems resulting from air-conditioning have been observed in
the hot, humid area along the South Atlantic and Gulf Coast. In these areas,
air-conditioning is operated for long periods of time, and nighttime temperatures remain
high. A large vapor pressure drive exists from the outside in. While condensation from
these conditions would normally be on the inside face of the wall, damp conditions could
be created in the wall, which could affect exterior finishes.
Surveys conducted in the 1960's showed condensation in walls was not generally a problem
where indoor temperatures were maintained at 75 degrees F or higher. Two major studies
were conducted in the early 1980's in instrumented buildings exposed to the hot, humid
coastal climate. These studies showed that where an inside vapor retarder was used, there
was daily cycling of moisture with condensation on the vapor retarder forming during the
day and being dissipated at night. While there was no damage to any materials in this case
the potential was there. Where no inside vapor retarder was used, no condensation
developed.
Based on such studies and observation in hot, humid areas, it is recommended that if an
inside vapor retarder is used on walls, some vapor resistance should also be provided on
the outside. One way to provide an outside vapor retarder is by using a closed-cell foam
sheathing. The use of a polyethylene vapor retarder on the inside of walls and a
closed-cell foam sheathing on the outside has proven to provide effective moisture control
in both cold and hot climates.
Trouble-Shooting Existing Problems
The moisture control measures that have been described will generally assure a house that
is free from condensation problems, but problems in existing houses may be more difficult
to solve. All exterior finish problems are not attributed to condensation, so first the
cause must be established. Other causes may be incompatible coats of paint, painting under
the wrong weather conditions, condition of the substrate, or simply a poor quality of
paint. If peeling is due to condensation, it will probably be worse on the north side and
outside of high moisture areas such as bathrooms and kitchens, and will not occur on
unheated areas such as garages. If condensation is the problem, the most effective
treatment is to reduce indoor humidity. Also, stop air leakage through electrical outlets
and cracks where possible. If there is no vapor retarder, application of a vapor retarder
paint, particularly on bathroom and kitchen walls, may also be helpful. Venting the walls
is not recommended. As discussed previously, studies have shown that the introduction of
cold outside air can actually result in more condensation.
New Houses
One of the most common problems is excessive humidity in new houses. The primary reason is
that concrete contains a great deal of water, which is given back to the air over a period
of several months. This is particularly critical when the house is enclosed just as the
heating season begins, so there is little natural ventilation to carry off this water. The
only solution is to provide extra ventilation either by running exhaust fans for longer
periods or periodically opening windows. The homeowner should be made aware of these
conditions and reassured that the problem will be alleviated after the first heating
season.
High Relative Humidity..........(Return to Table of Contents)
Regardless of the symptoms, many problems can be traced to excessively high indoor
humidity. If a humidity gauge registers more than 40 percent relative humidity indoors
during the heating season, something should be done to reduce humidity. If no humidity
gauge is available, excessive condensation on double-glazed windows is a good indicator of
high humidity. If this has been occurring very long, the bottoms of window sash will be
stained from water running over them.
The first step toward a solution is to look for large sources of moisture. A crawl space
with no soil cover, water standing under the house, or a damp basement can substantially
increase humidity. The problem can often be traced to poor drainage around the house,
which can be corrected by regrading and channeling discharge from downspouts away from the
house. Other sources may include numerous house plants, an unvented clothes dryer,
firewood stored indoors, or a humidifier.
If there are no unusual moisture sources, the high humidity may simply be a result of
living habits that are difficult to change. The only solution in this case may be to
provide more ventilation by more use of existing exhaust fans, or the addition of an
exhaust fan in a kitchen, bathroom or laundry. However, in exceptionally tight houses
small inlets may have to be added at several locations throughout the house to provide the
necessary air exchange. Where a forced air heating system exists, a more positive means of
providing ventilation is to connect a fresh air duct to the return side of the system, as
described previously.
Crawl Spaces
Moisture in crawl spaces was discussed previously under moisture control measures. Most of
these measures apply equally to trouble-shooting situations. The most common problems are
lack of good drainage away from the house and lack of a soil cover. In particularly
troublesome cases a sump may be installed to collect excessive ground water and drain or
pump it away. In addition to correcting these situations, check for adequate ventilation
with distribution to all parts of the crawl space.
Ice Dams
Ice dams are usually visible when they occur. There is little that can be done at that
point except keep snow removed from the roof to prevent melting. The main corrective
measure is to provide attic ventilation with good distribution of air movement at the
eaves. A high level of ceiling insulation is also helpful to prevent snow on the roof from
melting. When re-roofing, lay a wide strip of roll roofing along eaves before applying
shingles to prevent water from ice dams from getting through the roof.
Summary of Good Practice Recommendations..........(Return to Table of Contents)
The recommendations discussed for condensation control in walls to prevent exterior finish
problems are summarized in this section:
· Keep indoor relative humidity at 40 percent or lower during cold weather. This may
require using exhaust fans operated either manually or by humidistat. In very tight
houses, small inlets at several locations throughout the house may be required. A damper
controlled fresh air duct to the return side of a forced air heating system is a more
positive method.
· Install vapor retarders on the warm side of wall in all areas of the United States
except in the very hot humid climate zones of the Atlantic and Gulf Coast and in Hawaii.
For continuous coverage and good tear resistance, a 4-mil or 6-mil polyethylene is
recommended.
· Stop air leakage from living space into walls, particularly at electrical outlets and
other punctures in the vapor retarder.
· Provide a puncture-resistant soil cover over all the area of crawl spaces and be sure
water is drained away from the house.
· Provide cross ventilation to all areas of the crawl space. Area of vents must be at
least 1/150 of the crawl space area without a soil cover, or 1/1500 of the crawl space
area with a soil cover.
· Use a puncture-resistant vapor retarder, such as 6-mil polyethylene, under concrete
slabs to block moisture from the soil.
· In the hot, humid climate of the South Atlantic and Gulf Coast regions, provide some
vapor resistance on the outside of walls especially if a vapor retarder is used on the
inside.
Current Test Methods..........(Return to Table of Contents)
The only standard test methods currently available apply to materials rather than
components or systems. While material properties are important to performance of wall
systems, the way materials are combined and the air leakage involved is also critical to
performance. Moisture movement tests are being conducted for wall systems, but no standard
procedure has been developed for them. Current standard test methods for materials
include:
· ASTM D 2366 -- Blister Resistance of Exterior House Paints, on Wood Substrates,
Accelerated Test.
Comment: This is an adhesion test for paint. Water vapor under pressure moves through the
wood substrate and pushes the paint from the surface.
· ASTM E 96-80 -- Standard Test Method for Water-Vapor Transmission of Materials.
Comment: In this method a material is placed over the top of a cup containing either water
or a desiccant (wet cup or dry cup). The cup is placed in a controlled humidity room and
the weight loss or gain is measured periodically to determine a rate of moisture transfer,
which is stated in perms.
· ASTM E 154-68 -- Standard Methods of Testing Materials for Use As Vapor Barriers Under
Concrete Slabs and As Ground Cover in Crawl Spaces.
Comment: The test measures water-vapor transmission after alternate wetting and drying, as
well as deterioration after long-time soaking. It also measures resistance to puncture,
resistance to plastic flow at elevated temperatures, and the effects on bending at low
temperatures.
· ASTM E 398-83 -- Standard Test Method for Water-Vapor Transmission Rate of Sheet
Materials Using a Rapid Technique for Dynamic Measurement.
Comment: The test measures the same properties as E 96, but involves the use of a complex
instrument that can only accommodate sheet materials. The test is used for control or
comparison testing.
Research Needs..........(Return to Table of Contents)
Research needs regarding the performance of finishes include quantification of overall
moisture generation and moisture movement through the entire structure. Development of
coatings or treatments that would stabilize wood may partially overcome the finish
problems created by moisture, but controlling moisture movement and the way moisture
condenses on surfaces, may provide a better solution to the total moisture problem. The
whole subject area is extremely complex since moisture occurs in three different states
depending on the temperature, and the mode of movement changes with each state. It is
further complicated by the difficulty in determining the relative amount of movement by
air and by permeating through materials. Research is needed to quantify the moisture and
temperature interactions in wall systems. This includes development of test methods and
mathematical models as well as both laboratory and field tests to verify the models.
Following are specific needs for improved building design techniques that would result in
better performance of finishes:
· Test methods for entire wall systems with variations in materials and environmental
conditions on both sides of the wall. Methods must include determination of quantity and
distribution of air flow.
· A data base for moisture related properties of materials. This should include permeance
of both liquid water and water vapor, as well as adsorption/desorption characteristics.
· Development and validation of mathematical models to predict moisture transfer through
the building envelope. Models must include transfer both by permeation and airflow.
· Determination of typical moisture release from occupant activities and other sources of
moisture, including crawl spaces and basements.
· Refinement and validation of mathematical models for indoor humidity and whole-house
ventilation. The models should include effects of weather, occupancy and other sources of
moisture, moisture storage, and various ventilation equipment controls.
· Field evaluation of wood-based sidings using a variety of installation techniques under
different climatic conditions.
References..........(Return to Table of Contents)
1. American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE),
"ASHRAE Handbook of Fundamentals," ASHRAE, 1990.
2. Building Thermal Envelope Coordinating Council, "Moisture Control in
Buildings," Workshop Proceedings, Washington, D.C., October 1984.
3. Burch, D.M., et al, "Transient Moisture and Heat Transfer in Multi-Layer
Non-Isothermal Walls--Comparison of Predicted and Measured Results," Presented at
ASHRAE/DOE/BTECC/ CIBSE Conference in Orlando, Florida, December 1989.
4. National Association of Home Builders (NAHB), "Controlling Moisture in
Homes," NAHB, 1987.
5. Sherwood, G.E., "Condensation Potential in High Thermal Performance Walls--Hot,
Humid, Summer Climate," Forest Products Laboratory Research Paper No. 455, 1985.
6. Sherwood, G.E., "Condensation Potential in High Thermal Performance Walls--Cold,
Winter Climate," Forest Products Laboratory Research Paper No. 433, 1983.
7. Sherwood, G.E. and Peters, C.C., "Moisture Conditions in Walls and Ceilings of an
Older Home During Winter," Forest Products Laboratory Research Paper No. 290, 1977.
8. TenWolde, A. and Mei, H.T., "Moisture Movement in Walls in a Warm, Humid
Climate," Presented at ASHRAE/DOE/BTECC Conference in Clearwater Beach, Florida,
December 1986.
9. Verrall, A.F., "Condensation in Air-Cooled Buildings," Forest Products
Journal 12:531-536, 1961.