June, 2005
Inspection News and Views from the American Society of Home Inspectors

Moisture Control in Homes


Part 2 of 3

Vapor barriors

Many materials used as interior coverings for exposed walls, such as plastic drywall, wood paneling and plywood, permit water vapor to slowly pass through them. When the relative humidity 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, where it will condense if it comes into contact with surfaces colder than its dew point temperature. Vapor barriers are used to resist this movement of water vapor or moisture in various areas of the house.

B924_4.gifAll construction materials have some resistance to moisture flow, but only those materials highly resistant to vapor flow should be used as vapor barriers. The permeability of the surface to such vapor movements 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 (1.0 or less) is a barrier, while one with a high perm value (greater than 1.0) is a breather. Membranes which best serve this purpose include polyethylene film (four to six mil.), asphalt-coated or laminated papers and kraft-backed aluminum foil. Oil base or aluminum paints and /or vinyl wallpaper are often used in existing homes which did not have vapor barriers installed during their construction.

B924_5.gifApply vapor barriers on the warm side of the wall. In home construction this is usually between the framing and the interior sheathing or wall finish. For such uses it is a good practice to select materials with perm values of 0.25 or less. This vapor barrier can be a part of the insulation or a separate film. The membrane must present a solid surface with no holes in it, and where joints or layers are made, they must be formed over a framing member for backing. Openings for electrical outlet boxes should be sealed to prevent moisture flow.

Vapor barriers under concrete slabs resist the movement of moisture through the concrete and into the living areas. Such vapor barriers should normally have a maximum perm value 0.50. Heavy asphalt-laminated films, roll roofing and heavy films such as polyethylene are commonly used as vapor barriers under slabs. Figure 5 illustrates a standard construction procedure to install both gravel and polyethylene plastic sheet vapor barrier under the concrete. The function of the gravel is to slow capillary water movement toward the concrete. The polyethylene impedes vapor movement above the gravel.

Vapor barriers in crawl spaces prevent ground moisture from moving up and B924_6.gifcondensing on wood members or entering the home. A perm valued of 1.0 or less is considered satisfactory for such use. Asphalt-laminated paper and polyethylene (four to six mil.) are commonly used. The vapor barrier should be used to cover about 2/3 to 3/4 of the crawl space area (see illustration). Some ground area needs to be exposed, particularly if the house has hardwood floors. Some moisture is needed to prevent excessive drying of oak flooring and trim around doors and windows. If the floor begins to open, or the head joint in trim begins to open, expose more ground by rolling back the vapor barrier. When the floors in a house are covered with carpet or vinyl products, all the crawl space can be covered with a vapor barrier.

As a final step to the installation, one or two inches of sand may be placed on top of the vapor barrier. This step is optional, however it assists with the maintenance and inspection of the house. The sand weighs down the vapor barrier, preventing the condensation of moisture on the undersurface and absorbs the small water droplets that condense on the top surface in cold weather.

Installing a vapor barrier on crawl space surfaces will only assist in the control of excess moisture vapor and should be used in combination with an effective ventilation system.


Attics and crawl spaces are the predominant areas requiring ventilation. In both B924_7.gifplaces it is necessary to have good distribution of air movement over the entire area.
Attic ventilation is essential. Without it, moisture that moves through the ceiling will be trapped in the attic because most roofing materials prevent moisture from escaping. Basically, the idea of cold-side venting is to relieve the vapor pressure in the attic by providing a vent to the outside air, which usually
has a lower vapor-pressure.

Ventilate the attic with inlet vents distributed along the eave and with the outlet vents near the ridge. You'll get the best results when the ventilation is uniformly distributed along the roof and is equally divided between the high and low. Warm air in the attic rises and escapes through the ridge vents: cooler outside air enters at the eaves (see illustration). In this way, ventilation is continuous and does not depend on the wind.

B924_8.gifFor proper ventilation, attics require one square foot of unobstructed ventilation area for each 150 square feet of attic area. Five vent types are common: eave (soffit), gable, turbine, roof or continuous ridge (see illustration).

Crawl spaces should be vented to the outdoors to permit water vapor to escape. If the vents are located near each corner, the vents will permit good air movement through the crawl space. A standard metal foundation vent is eight inches by 16 inches and is usually located in the top eight inches of the foundation. It has a metal grid of one-inch squares, may have screen wire to elude mice, etc. and may have an operating metal shutter (see illustration). One standard B924_9.gifsuggestion for vent sizing is one square inch of unobstructed ventilating area for each square foot of crawl space area. Thus, each standard eight inch by 16 inch vent has about 60 to 75 square feet of unobstructed area and is adequate to ventilate about 75 feet of crawl space area. The function of the foundation ventilator is to dissipate the moisture vapor in the crawl space, therefore the ventilator should remain open year round except during the coldest few days.

Insulation, storm windows and insulation windows

Insulation is important in controlling moisture problems because it increases the B924_10.giftemperature of the inside surfaces of walls, ceilings and floors, preventing condensation on those surfaces. In cases where mildew or dampness is appearing on the ceilings at its edges near the outside walls, there is a possibility that the ceiling insulation is not properly installed. Insulation must extend over the top plate of the wall and be fitted tightly to the top plate. Cold air can blow under insulation and chill the ceiling where vapor will subsequently condense. Similarly, wall insulation can settle, allowing cold spots to occur at the top of walls. In both cases, insulation must be repositioned or fitted in (see illustration).

In the average home, moisture condensation appears first on the glass in windows and doors, because these are usually the coolest surfaces in the house. This condensation can be reduced or eliminated by installing storm window units. The air space separating the storm unit from the regular window becomes an insulator. This space allows the temperature of the storm window unit to approach the temperature of the cold outside air, while the temperature within the house or at least stay above a temperature that will cause condensation to take place on the inner unit.

If you are building a new home or want to replace your window unit, double or insulating glass within the sash, coupled with weatherstripping, is another effective way of reducing or eliminating condensation.

Occasionally, after a storm unit has been installed, the regular or existing window will continue to have condensation. This means the storm unit does not have a tight fit and is permitting an excessive amount of cold air to reach the regular unit. Caulking around the storm unit usually corrects this problem. After a storm unit has been installed, if the storm unit begins to have condensation, it is an indication that the regular window does not have a tight fit and should be taped around the sash to reduce air leakage.

Reprinted with permission from the University of Georgia, College of Family and Consumer Services.

View this article in its entirety here: http://www.fcs.uga.edu/pubs/current/B924.html