John Leeke's Historic HomeWorks™
207 773-2306 26 Higgins St. Portland, ME 04103
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John Leeke's Historic HomeWorks™ |
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207 773-2306 26 Higgins St. Portland, ME 04103 |
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(Return to the List of Articles)
"Moisture Control in
Houses: The Effect of Interior Moisture on Exterior
Finishes"
Joint Coatings/Forest Products Committee
Chairman: Gerald Sherwood, National Forest Products Association,
Washington, DC Edward Lewandowski,.Rohm & Haas Company,
Spring House, PA Kenneth Peterson, Georgia-Pacific Corporation,
Atlanta, GA Harry Warren, Masonite Corporation, Chicago, IL
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.
Acknowledgment..........(Return to
Table of Contents)
The Moisture Control Task Group wishes to thank the contributing
joint committee members from the Federation of Societies for
Coatings Technology and National Forest Products Association for
their valuable time and cooperation.
Webpage creation by Dan Mitchell, working in
the Historic Home Works Assistantship Program. Dan is a
preservation enthusiast on Cobbosseeconte Lake in Maine.
(Return to the List of Articles)
|
John Leeke's Historic HomeWorks™ |
|
207 773-2306 26 Higgins St. Portland, ME 04103 |
|
| [Home][Library][Restoration Reports][Seminars][Forum][Internships][Office][Workshop][Front Porch][Search] info.© 1994-2007 JohnC.Leeke |