Condensation occurs on the inside face of a wall when water vapor from warm air within the house (resulting from cooking, washing, bathing, and similar sources) forms fine droplets on the windows or wall if such surfaces are colder than the dew point of the air in the room.
Water vapor may pass through materials, such as wallpaper. plaster, and wood, and condense within a wall. An accumulation of moisture of this kind is often very troublesome. and can be prevented by the installation of a vapor barrier placed at or near the inner or warm surface of the wall.
Vapor barriers may consist of smooth-surfaced asphalt roll roofing, asphalt-saturated and coated sheathing paper, some grades of asphalt duplex papers, and papers backed with metal foil. The effectiveness of the barrier is increased by lapping and sealing the joints. Metal foil should not be placed in contact with either plaster or masonry. There are some types of paints which can be used to provide a vapor barrier when applied to the plaster. Such paints should be applied in two or three coats and include lead-in-oil paints, an outside type of varnish, and paints containing flake-type aluminum and .spar varnish.
Masonry walls may be insulated by providing an air space through the use of furring strips nailed to the wall or, if desired, the application of a layer of insulating material between the strips before surface coating is applied. If the exterior surface of a masonry wall has become badly deteriorated, a weatherproof coating, such as stucco, shingles, wallboard, or siding, may be applied over the furring strips. A new finish of plaster or wallboard applied to furring strips on the inside face of a masonry wall will make the surface warmer and prevent condensation on it. Wherever furring is installed, window and door frames and trim will have to be built out to conform to the new surface.