Vapor Barriers or Vapor Retarders
A vapor retarder is a specially treated paper, thin plastic sheeting, or low permeance paint that prevents condensation of water vapor inside wall or ceiling materials. This trapped moisture can cause damage to the wallboard and paint as well as structural deterioration.
The vapor retarder must have a vapor permeance (the rate that moisture can pass through materials) of not more than 1.0 perm (tested in accordance with ASTM Standard E96-80).
The term "vapor barrier", which is also commonly used, is somewhat misleading since it does not completely bar the transmission of water vapor. A vapor barrier is actually a vapor-resistant membrane, and is more properly called a "vapor retarder."
Interior moisture tries to move out of a building. Vapor retarders are important, because they keep this moisture in a warm area where it will not condense. For this reason, vapor retarders should be applied (in colder climates) behind the drywall of a wall or ceiling next to existing insulation and on the warm-in-winter side (between the insulation and the conditioned space) of insulated floor sections over crawl spaces.
: Because moisture may be trapped between the two vapor retarders and eventually ruin the installation, if additional insulation is added during your remodeling project, be sure that a second vapor retarder is not applied between layers of insulation. There is a chance that this second vapor retarder might be added inadvertantly during an insulation upgrade because some batt insulation comes with a vapor retarder integrally attached in the form of paper or aluminum-like sheating.