In recent years there have been remarkable changes in the methods and materials used in house construction. The average house has grown tighter and provides more livability in less space. Ceilings are lower. Basements are often omitted and thus utilities such as furnaces and laundry equipment are installed on the first floor. Exterior design also has changed. Many homes no longer have any overhang at eaves and gable ends to protect the walls.
Today people make greater use of water and of appliances which discharge moisture into living areas. Daily activities such as bathing and cooking add gallons of water vapor to the air. The use of sheet materials for sheathing, plaster base, and interior wall finish has reduced air exchange through walls. Insulation is walls, roofs, and under floors has helped to seal houses more tightly. Weather-stripped doors and windows and the modern window frame, while increasing comfort within the home, also trap air and moisture inside the house.
In homes built without vapor barriers, good vapor protection can be achieved by applying two coats of a low-permeability paint system to inside walls and ceilings. However, such measures do not provide complete protection. Paints of this type are intended to supplement the action of vapor barriers in new homes.
Vapor barriers in ceilings are considered necessary only where winter temperatures commonly fall below -20degree F or where roof slopes are very low. In these cases, vapor barriers should be installed in ceilings under attics and flat roofs in the same manner as in walls. Sheet metals, metal foils, asphalt-laminated papers, and foil laminates are highly vapor resistant and perform well as vapor barriers.
Except in very dry climates, the soil in crawl spaces should be covered with a layer of vapor-resistant durable material. Asphalt-saturated felt roll roofing or polyethylene film may be used. The ground surface should be leveled, and the cover material turned at walls and piers and lapped at least two inches. This material need not be sealed.
If a concrete slab is to be placed on the ground, a vapor barrier should be installed directly under the slab. Otherwise moisture from the ground will move up through the slab and into the basement. The barrier must be strong enough to resist puncturing when the slab is poured. It must also be a type that will not deteriorate with age.
Unit-masonry basement walls should be thoroughly damp-proofed by applying mortar and asphalt coating to the outside surfaces.
Good ventilation controls condensation in buildings and homes. In the winter, fresh, dry air from outside replaces moisture-laden air within the house. In this way, high vapor pressures which produce condensation are reduced.