An attic vapor barrier is installed to ensure that warm air passing through the walls and ceiling of the attic space does not get cold enough to condense and become droplets of water that can become trapped in the building envelope. The attic of a building is an important area for insulation and vapor barriers because warm air rises through a building and comes into contact with cold air close to the exterior of the loft space. Ratings are awarded for different types of vapor barriers depending on their effectiveness at retarding the passage of water.
The term "vapor barrier" is seen as misleading because it tends to make people believe that the material stops the passage of all moisture through it. A more commonly used term is a "vapor retarding material," which refers to the fact that all materials allow some moisture to pass through them with the correct type of vapor barrier needing to be chosen for the environment. All vapor barriers are given a number rating that shows the amount of moisture passing through them per square foot of material compared to the amount of mercury passing through one inch of the material. Materials rated 1.0 or less are considered to be usable as an attic vapor barrier.
An effective attic vapor barrier is installed on the warm side of a wall regardless of whether this is in front of or behind insulation installed in the attic. In cold climate areas, installation of the vapor barrier should be on the interior side of the wall or ceiling and on the exterior of attic walls and ceilings in warm climates. Vapor barriers that are installed on the wrong side of the building envelope can trap large amounts of moisture within the building and cause damage to the structure. A barrier installed on both sides of the wall can trap excessive amounts of moisture and damage a building at a faster rate than incorrectly-installed vapor retarding materials.
Some types of vapor barriers are manufactured on one side of a piece of insulation used to keep warmed or cooled air inside of a structure. Insulation installed in an attic often has coated Kraft paper attached to it that acts as an attic vapor barrier that overlaps onto the next piece of insulation to form an effective barrier. Foil and plastic sheathing vapor barriers can be attached to insulation during the manufacturing process or can be installed separately in an attic space.
Vapor barriers are not only installed in attic spaces, but can also be placed on walls, crawl spaces, and basements. Some roofing materials, such as asphalt shingles, can act as vapor barriers and cause problems with rusted steel and rotting wooden beams within the structure of the building. Effective ventilation is required when an attic vapor barrier is installed in a loft space to ensure that moist air does not remain trapped and should be vented to the exterior of the building.