Definition - What does Vapor Barrier mean?
A vapor barrier is a layer of material that is used to prevent the transfer of water or water vapor from the environment into a constructed structure. A vapor barrier prevents water vapor movement to protect structures from the water damage that can occur when moisture is allowed to migrate into them.
Corrosionpedia explains Vapor Barrier
A vapor barrier is material that prevents water vapor transmission, as compared to a vapor retardant material that has varying degrees of permeability that allow some vapor to pass back and forth through it. The amount of permeability is measured in units called "perms." Although they are technically two separate terms, the term vapor barrier is also used to mean vapor retardant.
Common materials for vapor barriers and retardants are divided into classes:
Class I (less than 0.1 perms): Rubber, glass, metal foil or sheet, polyethylene sheet
Class II (0.1 to 1 perm): Plywood, polystyrene, asphalt coated paper
Class III (1 to 10 perms): Lumber, cellulose, brick, concrete
Different environments will require different types of vapor barriers. Recommendations for the correct vapor barrier for a given application are typically given by the vapor barrier manufacturer.
Air barriers are commonly confused with vapor barriers. This lack of understanding stems from the fact that air can carry water vapor. If the purpose of the barrier is to prevent the transport of water vapor via the air, then it is a vapor barrier. If the intention of the barrier is to prevent the movement of the air itself, then it is an air barrier.