HL 258 August 1998
R E S I D E N T I A L
H O U S I N G
Water. It causes doors to stick, salt shakers to clog, draperies to sag, and odors to linger. Unfortunately, excess moisture in warm humid climates creates ideal conditions in which fungi (molds) thrive. Home construction in humid climates must control moisture flow into the structure to minimize future problems with these destructive organisms. An extra
$500 spent on moisture management at the time of construction will save more than $5,000 in energy costs over the life of the air conditioning equipment.
When the environment’s temperature, humidity and nutrient sources combine properly, spores germinate and mold becomes a problem. Wood, paper, cloth and other home construction materials are damaged when molds use them for a source of food.
Unfortunately, the warmth and humidity levels that humans prefer overlap with the temperature and humidity levels that encourage mold growth. Figure
1 shows the ranges of temperature and humidity in which the human comfort zone and mildew growth overlap.
Consider this relationship:
Nutrients +
Warmth +
Humidity +
Spores =
Mold Growth
Mildew Comfort Zone
Human Comfort Zone
Mildew Alert Area
RH - %
100
80
Molds need nutrients to grow. Molds feed on organic materials used in home construction. Other organic materials in the home such as residue of soap on bathroom tiles, the slightest smear from sticky fingers, or body oils on interior furnishings support mold growth.
60
40
Warmth (heat) that provides comfort for people encourages mold growth. As temperature (a measure of heat) rises, molds grow faster.
Humidity supplies the moisture for mold growth.
Relative humidity (RH) is a measure of moisture in the atmosphere at the air’s temperature. Remove moisture by lowering the RH and mold growth slows or stops.
Spores, the tiny “seeds” of molds, are everywhere.
They easily enter homes through open doors and windows or are carried in on our clothes. Spores cannot be eliminated from our environment.
50 60 70 80 90
Temperature
°
F
Figure 1.
Temperature and relative humidity range for mold growth and human comfort
Mold is the indication of a serious problem. The mold growth relationship shows how the environment includes all the necessary ingredients. When the relationship is out of balance with too much of one or more ingredient, molds grow! A building can be built and managed to prevent moisture-related problems that encourage mold growth. In a home, built-in humidity management is crucial.
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Source of Moisture in Warm,
Humid-climate Buildings
Excess moisture is a problem in South Carolina homes and other buildings. This excess moisture comes from both indoor and outdoor sources.
Moisture is produced inside homes in many ways: bathing, cleaning with water, laundering clothes, cooking, perspiration, and growing plants. Even breathing contributes moisture to indoor air. A person exhales as much as 1 /
3
of a pint of water per hour into the environment. Most air-conditioning systems have been designed to control moisture developed from indoor sources. They work well in drier regions where outside air does not add to the moisture removal demands of the air conditioning unit.
Moisture enters buildings from the outside. Moisture comes into a building by infiltrating (mass movement) through doors, windows, cracks, and other openings to the outside world. About 95% of the moisture entering a building enters through air infiltration. Only 5% of the moisture enters by diffusion through building materials. A high concen- tration of water vapor on the outside of a building forces it through walls into drier inside air.
The concentration of water vapor (the gaseous form of water) in air depends upon the air’s temperature.
High-temperature air holds more water vapor than a cooler-temperature air. Water moves as a vapor
(diffuses) from spaces that have a high concentra-
tion to spaces with lower concentrations. This is why warm, humid outdoor weather is related to excess moisture inside South Carolina homes.
Cold-climate Moisture Problems
South Carolina’s home moisture problems should not be confused with moisture problems in cold-wintertemperature climates. Indoor moisture problems in cold climates occur during winter months if tight building construction to slow heat loss reduces infiltration and ventilation. Moisture produced inside diffuses through wall surfaces and condenses on cold surfaces such as insulation, studs, and siding.
During warmer weather in these climates, this condensed moisture encourages mold and fungi to grow and damage the wall structure. The same condensation will cause exterior paint to peel. Peeling paint is often the first sign of a serious moisture problem in cold climates.
Builders in cold climates put vapor retarders on the
inside of exterior walls to reduce diffusion of moisture.
This minimizes the amount of interior moisture available for condensation on cooler surfaces within the wall. In cold climates, family activity inside the house is the primary source of moisture. Thus control of moisture within the house is an important management factor for reducing mold and structural problems.
Warm, Humid-climate Moisture Problems
The largest source of moisture for mold growth in warm, humid climates is the outside air. Both ventilation (intentionally bringing in outside air) and infiltration (unintentionally bringing in outside air) can cause moisture and mold problems in these climates. Moisture brought in with outside air condenses on the cool, air conditioned, indoor surfaces creating ideal environments for mold growth.
Inappropriate use of ventilation can combine with infiltration to create mold problems during spring and fall. On spring days when heating is off, inside surfaces are often cooler than outside temperatures.
On fall days when air conditioning is off, moisture is not being removed from the air. The excess water vapor in the infiltrating air condenses on cool interior surfaces and supports mold growth.
Summertime moisture problems take several forms.
Infiltration brings moisture-laden air into the house though open doors and windows. Unless this high humidity air has water vapor removed from it by the air conditioner’s cooling coils, the water will condense on cooled interior surfaces to support mold growth and damage to the furnishings. Designing the air conditioner’s dehumidification load becomes an important part of moisture control.

When interior temperature controls are set below the outside air’s dewpoint temperature, outside air diffuses through the exterior wall. Moisture in this air condenses in the cooler wall space where molds and decay can grow.
Use of the air conditioner’s fan when the compressor is not operating creates moisture problems. Water that has collected on the evaporator drain pan is picked up by the moving air and pushed back into the house.
Unless the air pressure and HVAC in a house is balanced, running kitchen and bath exhaust fans for moisture removal may create a negative air pressure within homes. This negative air pressure increases infiltration of outside air into the home. This can bring more moisture in than is removed by the ventilating fan.
Construction for Moisture Control
A well built, tight house may have from one-fourth to one-half air exchange every hour. Poorly constructed buildings may have more than ten times this air exchange rate. A builder may not be able to control the ventilation practices of home buyers, but the builder can, during construction, eliminate many sites where moisture enters.
Vapor retarders can be used to reduce diffusion of moisture through walls, ceilings and floors. Location of these barriers, however, is important. Vinyl wall coverings or other moisture-inhibiting finishes used on the interior of exterior walls can trap moisture in the stud space.
Diffusion of moisture through building materials is very small compared to the amount of moisture that enters by air infiltration through the cracks and crevices of a building. For example, approximately one cup of water can move through a 10" x 10" section of painted wall per year by diffusion, but as much as 320 cups (20 gallons) can move through a
1" hole by infiltration.
The major infiltration locations are under wall soleplates that are not well sealed; around electrical outlets and windows; through duct systems; through vent fans and fireplace dampers; and around exterior doors. The specifics for controlling infiltration in well built housing are known. Tight-fitting components, weather stripping, and caulking control both heat loss and entrance of humid air into homes.
In climates that have both cold winter and hot humid summer conditions, misuse of vapor retarders and infiltration barriers is very common. A vapor retarder placed on the inside wall stops moisture moving into the living space. Moisture then condenses within the cooled wall during hot humid summers.
A vapor retarder placed on the outside of the wall keeps interior moisture from exiting to the outside air. Moisture then condenses within the wall space during cold winters. Thus in these climates, an
infiltration barrier on the outside of exterior walls is recommended instead of a vapor retarder within the wall.
Water can enter a structure through the floor slab, vents, and wall openings for plumbing and electrical wiring. Properly installed flashing closes holes, cracks, and crevices in the housing envelope where driving rain and moisture-laden air can enter the structure. Flashing plus caulking is needed at any openings. They help assure leakproof drainage of water away from the opening.
Many of these potential moisture entrances can be dealt with by a builder. The lesson is clear. Tight construction is a must in warm, humid climates. . . not only to keep out heat, but also to keep out moisture.
The Role of Air Conditioning
An air conditioner not only reduces air temperature
(cools), it also lowers moisture content of the air
(dehumidifies). Most air conditioning equipment is designed to meet the mass market needs of the entire
United States. It works well in parts of the country where the latent heat (moisture) infiltration is less than the sensible heat (temperature) infiltration. In many areas of South Carolina, however, the air conditioning load (amount) is needed more for dehumidification than it is needed for cooling the air.
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Air Conditioning Design
When air passes over the evaporator coil of an air conditioner unit, the moisture content of the air stream is reduced. The coil temperature is below the air’s dewpoint temperature. Thus water vapor condenses on the coil, flows onto the drain pan and then out through the condensate drain pipe.
The amount of moisture removed from air during the conditioning process is expressed as the sensible heat ratio (SHR). Air conditioners with typical
SHR ratings of 0.70 have been designed to handle average year-round loads that are 70% temperature reduction and 30% dehumidification.
If infiltration is not limited via tight construction, the typical 0.70 SHR air conditioner will not remove enough moisture to prevent mold growth and other damage. This makes proper building design and construction critical in order to control moisture infiltration.
Air conditioning ducts with interior fiberglass lining become a problem when they trap dust. Dust is a nutrient source on which molds grow. Moisture for mold growth comes from the cooled, high-relativehumidity, conditioned air. When the ducts warm, mold grows on the damp dust.
Air conditioning ducts used in warm humid climates need to be made of metal, tightly sealed, and well insulated. Insulation must be on the outside of the duct. Fiberglass ductboard or any AC ducts with interior fiberglass lining are not recommended.
Tightly sealed joints prevent leakage of cooled air into the insulating cover. If ducts are located in unconditioned space (i.e., attic, crawl space) the Rvalue of the insulating cover should be equivalent to the recommended attic or underfloor R-value.
Air Conditioning Load Due to Infiltration
Moisture coming into a house adds to the air conditioning load. Figure 2 shows the latent and sensible heat loads for Beaufort, SC. In this figure, the base line is a comfortable, mildew-safe temperature of 75
° F with a 50% relative humidity. The sensible curve shows the effect of infiltrating air’s temperature. The
latent line illustrates how the moisture content of outside air affects the air conditioning load.
Figures 3, 4 and 5 show that temperature and humidity conditions vary across the state. The period during which air conditioning is needed to reduce the latent heat load shortens as you move inland from the South Carolina coastline and go into the mountains. On a summer day in the South Carolina midlands and coastal plains, one third of the total energy
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Figure 2. þ
Average daily air conditioning loads due to infiltration at Beaufort, SC

Figure 3.
þ
Average daily air conditioning loads due to infiltration at
Florence, SC
Figure 4. þ
Average daily air conditioning loads due to infiltration at
Columbia, SC
Figure 5.
þ
Average daily air conditioning loads due to infiltration at
Walhalla, SC
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6 cost for air conditioning goes to cooling the air.
Two-thirds of the air conditioning costs are needed to remove moisture. In the South Carolina foothills and mountains, 60% of the air conditioning costs are needed to remove moisture in the summer.
Incorrectly Sized Air Conditioners
In energy-efficient homes, the heating and cooling load is reduced enough to make sizing air conditioners very difficult. Removal of moisture from the air stream takes time. Oversized air conditioning units will cool the air quickly but will not run long enough to remove excess moisture from the air stream.
Unless both infiltration and ventilation are carefully controlled, houses with oversized air-conditioning equipment will feel cool and clammy. These conditions are in the mold growth comfort zone. It is important that the SHR rating of the equipment match both the latent and sensible cooling load.
Twelve Golden Rules For Builders in Warm
Humid Climates
To guard against moisture problems in warm humid climates, the architect’s and builder’s techniques must be reviewed at every step in planning and construction. To avoid moisture problems, follow these Twelve Golden Rules for building in warm humid climates.
1. A vapor retarder (minimum 6 mil thick) under a concrete slab floor stops ground moisture and gases before they enter the house.
Make sure there are no holes or tears in the vapor retarder. Overlap the sheets by
12" or more and seal with tape along all seams before concrete is poured. Any tears or holes must be tightly sealed.
2. Eliminate cracks and crevices in the exterior walls by using tight building construction practices.
Use a sealant where the soleplate (wall sill) meets the floor. Caulk around plumbing and wiring holes through exterior walls. Flash, caulk, and weather-strip around windows and doors. The housing envelope should prevent outside air from entering into the stud space as well as into the house interior.
3. An air infiltration barrier should be used on the outside of exterior walls in warm, humid climates.
About 95% of the moisture entering homes comes by infiltration; 5% by diffusion. South Carolina homes should have air infiltration barriers installed on the outside of exterior walls. Vinyl wall coverings or other vapor-retarding finishes used on the interior of exterior walls trap moisture. Trapped moisture encourages mold growth and structural decay.
Research continues on use and placement of vapor barriers.
4. Moisture enters a home through leaks in the air-conditioning duct work.
All joints in air conditioning ducts must be carefully sealed during installation.
Inspect and maintain duct seals.
5. Air conditioning and heating ducts should be metal with exterior insulation.
Many metals contain zinc which inhibits mold growth. Ducts with interior fiberglass lining trap dust. Dust is the nutrient source on which molds grow. Moisture comes from the cooled, high-relativehumidity air. When the air conditioning cycle is off, ducts warm and mold grows on the damp dust.
6. Install return air ducts in each bedroom area or undercut the doors 1 1 /
2
to 2 inches.
Unless bedroom air can enter the return air stream, a negative pressure will form in other parts of the house and pull humid outside air indoors.
7. Install exhaust fans in each bathroom to remove moisture from the tub and shower area.
Air travels both ways through open bath-

room windows. Air coming in may be more humid than bathroom air. Vent the fan to the outside, not into the roof or crawl space.
8. Install a timer on each bathroom exhaust fan. Bathroom exhaust fans create a slight negative pressure drawing in outside air through cracks in outer walls. Continuous operation of exhaust fans can increase the latent air conditioning load. Using the exhaust fan for 15 or 20 minutes will remove most of the excess moisture in air after bathing.
9. Provide air circulation into and through closets.
Louvered doors, open shelving, and air conditioning supply vents will speed drying of damp clothing.
10. Cure poured concrete and dry the structure before painting, putting on wallpaper, and installing floor coverings.
Wet building materials create future problems. Moisture trapped by vinyl wall coverings or vapor retarders enables molds to grow, reduces insulating value, and enhances decay in stud spaces. This destroys the strength of the structure.
11. Calculate both sensible and latent cooling loads to determine the load SHR
(sensible heat ratio) when sizing air conditioning equipment.
Incorrectly sized air conditioning equipment will not provide proper dehumidification. Consider factors such as shading,
R-value of insulation and type of lighting fixtures in the sensible heat and cooling load calculation. Match the SHR of the equipment with both the latent and sensible components of the load. Do not oversize the sensible load.
12. Use innovative air conditioning for a low sensible heat load and a high latent air conditioning load.
Heat pipes, hot gas bypass, or desiccant wheels may be installed to help the sensible-latent heat load balance by increasing dehumidification.
GLOSSARY
Dehumidify: remove moisture from the air stream.
Diffusion: movement of liquids, gases, or solids through materials by molecular motion.
Molecules travel from a region of high concentration to regions of low concentration.
Infiltration: mass movement of exterior air into a structure through openings such as doors and cracks.
Infiltration barrier: a material used to minimize mass movement. Sometimes called a house wrap.
Latent load: the part of an air conditioning load needed to remove moisture from the air stream.
Relative humidity (RH): the ratio of the amount of water vapor actually present in air to the greatest amount possible at the same temperature and pressure.
Sensible load: the part of an air conditioning load needed to remove sensible heat (lower air temperature).
Vapor retarder: a material used to slow movement by diffusion.
Ventilation: the exchange of inside air for fresh outside air.
For other publications and videos on building moisture management, contact your county office of the
Clemson University Cooperative Extension Service.
Selected publications are available through our Web site at http://www.clemson.edu/psapublishing/pages/ fyd/fydhmnt.htm.
Adapted from Moisture, the Enemy: Building to Keep
It Out of Florida Homes by Gary Cook, Energy Extension Specialist for Building and Indoor Air Quality,
M.E. Rinker, Sr., School of Building Construction; and Virginia Peart, Extension Housing Specialist
Emeritus, IFAS, University of Florida.
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Revised for use in South Carolina by Linda L. Redmann, Ph.D., Extension Residential Housing Specialist, Department of Family and Youth Development; and Dale E. Linvill, Ph.D., Professor of Agricultural Meteorology, Agricultural
Weather Office, Department of Agricultural and Biological Engineering, Clemson University.
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Community Sustainability Center
The Clemson University Cooperative Extension Service offers its programs to people of all ages, regardless of race, color, sex, religion, national origin, or disability and is an equal opportunity employer.
Clemson University Cooperating with U.S. Department of Agriculture and South Carolina Counties. Issued in Furtherance of
Cooperative Extension Work in Agriculture and Home Economics, Acts of May 8 and June 30, 1914 6993