Once known as vapor barriers they are now called Moisture barriers/vapor retarders. Thier function is to stop or retard moisture in the form of vapor from migrating into cold areas of the structure and condensing on cold surfaces causing condensation. They are very important, misunderstood and controversial. Most ofter they are sheets of plastic (4 – 6 mil) material that are impermeable and highly resistant to the passage of moisture evaporation. They stop the movement of most of the moisture before it gets into the wall cavity, attic, ect.. and condenses, wetting the insulation and the wood.
As we build tighter homes, moisture barriers become more important. When houses were loose and drafty, moisture readily and harmlessly escapes along with the air and costly energy. The tighter the home becomes. the likely moisture is going to condensate inside walls, attics, and the crawlspace of a structure. Condensation will dramatically reduce the effectiveness of insulation. Fiberglass insulation that becomes wet can lose as much as 80% of it’s R-value.
Moisture barrier are difficult to install or retrofit to existing older homes. The colder the climate the more important it is to install a moisture barrier. Moisture barriers stop evaporation moisture not humidity driven condensation moisture. So having your home evaluated for all moisture intrusion problems will help protect it from future problems.