Vapor barriers are materials that restrict the movement of water vapor through the home's envelope. Commonly made of materials like plastic sheets, treated papers, and metallic foils, they often come attached to insulation materials. In homes without them, warm inside air, which contains water vapor, can move into the walls, ceilings and floors and condense causing serious moisture damage and significant loss of insulating value.
In a home with no vapor barrier and no insulation, moisture laden air moves freely into the walls where it cools and moisture condenses. If it's cold enough outside, this moisture freezes.
If the home has insulation but no vapor barrier, moisture condenses in the insulation. In homes having insulation and a vapor barrier, warm, moist room air stays in the room, blocked from entering the walls.
Placement of vapor barriers is important. In most parts of the country, particularly in colder climates, the vapor barrier goes on the warm side, the lived-in side, of the home.
In walls, that means insulation already faced with a vapor retarder should be installed with the vapor barrier facing toward the inside of the home. Unfaced insulation must be covered with a vapor barrier material like plastic. In ceilings, vapor barriers go down toward the living space, and under floors, they face up.
The only exception to this placement is in particularly hot, humid areas of the country, primarily located in this strip between the southern tip of Texas and the Florida-Georgia border on the Atlantic Ocean. In these areas, vapor barriers should be either omitted or applied to the outside surface of walls. Homes in these areas should be evaluated by a professional to determine correct vapor barrier placement.
Vapor barriers can take several forms. They can be purchased separately as sheets of polyethylene for installation with loose fills. Often, with batts, blankets and rigid boards, they come as treated paper or aluminum foil already bonded to the insulation. Some wall papers such as vinyls make fairly good vapor barriers. Aluminum and latex vapor barrier paints work well, as do oil paints when applied in two or more coats to plaster. Wall papers and paints are especially useful when the walls are finished and you don't want to damage them installing insulation with a vapor barrier.