Understanding It's Importance
Moisture control is a major concern associated with thermal insulation and the "health" of a building. The warm air inside your house contains water vapor. If this vapor passes into the insulation and condenses, it can cause significant loss of insulating value. The R Value of a mass type insulation is reduced by over 35% with only 1 1/2% moisture content. (ie. from R-13 to R-8.3). The moisture content in insulation materials for most homes typically exceeds 1 1/2%.
Besides reducing the effectiveness of in the wall insulation, water vapor can cause mold growth, peeling paint, and eventual rotting of structural wood.
A vapor barrier or vapor retarder is a material that reduces the rate at which water vapor can move through a material. The older term "vapor barrier" is still used , however, this is incorrect since it implies that the material stops all of the moisture transfer. Since everything allows some water vapor to diffuse through it to some degree, the term "vapor diffusion retarder" is more accurate. The main reason to retard the transmission of water vapor is to prevent it from condensing to liquid water within the structure.
Vapor retarders should be used in most parts of the country. In colder climates, place the vapor retarder on the warm side--the lived-in side--of the space to be insulated. This location prevents the moisture in the warm indoor air from reaching the insulation.
If you live in an area where the climate is predominantly hot and humid the process is in reverse. As the humid outdoor air enters the walls to find cooler wall cavities it condenses into liquid water. Placing the vapor retarder on the outside will reduce water vapor movement from the outside from getting into the building envelope. Any vapor that does get into the walls or roof assembly can evaporate to the inside and therefore dry out before the moisture can lead to mold, mildew and rot problems.
Attics: Even when not required to prevent condensation problems, attic vapor retarders may be worthwhile; their presence may help maintain more comfortable humidity levels.
Check with a local builder to determine the correct placement or need for a vapor retarder. More detailed guidance on regional differences in moisture control recommendations can be found in the Moisture Control Handbook published by US Department of Energy . To guard against moisture problems, use Barrier Coat on interior walls and ceilings and provide adequate ventilation for the house. If you have a crawl space you should spray the underside with Hy-Tech #85 Barrier Coat.