Gas and vapor barriers are used on the exterior of buildings to allow air flow, but limit the passage of moisture. They usually have a permeance value or perm rating of 1.0 or less. Permeance is the physical property that describes the ease with which water molecules diffuse through a material. It is measured in perms according to specifications such as ASTM E96-95 and ASTM E96-00, standards from the American Society for Testing and Materials (ASTM) that establish test methods for water vapor transmission. Other standards for vapor barriers and permeance are defined in the National Building Code of Canada (NBCC); the International Residential Code (IRC); and the ASHRAE Handbook, a publication of the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE).
Gas and vapor barriers are an important part of a moisture control strategy for a building’s walls, ceilings, floors, and foundation. Typically, they are available as membranes or coatings. Membranes are relatively thin, but differ in terms of thickness and materials of construction. Thicker sheet materials consist of rigid foam insulation, reinforced plastic, aluminum or stainless steel. Often, these vapor barriers are fastened mechanically and sealed at the joints. Foam board insulation is placed between the exterior finish and the studs of exterior walls. By installing a vapor diffusion retarder on the warmest side of the wall cavity, problems such as wood rot and mold can be avoided. The most common types of materials in foam board are polystyrene (PS), polyurethane (PU), and polyisocyanurate.
Thicker membranes are available as rolls or integrated into building materials. Examples include aluminum-faced and paper-faced fiberglass roll insulation. These blankets and batts carry specifications for R-values, but also serve as vapor barriers. Vapor diffusion retarders (VDR) also include foil-backed wallboard and polyethylene (PE). In very cold climates, polyethylene sheets are used as vapor barriers in above-grade walls and ceilings. These cold climates have 8000 or more heating degree days, a unit which measures how often dry-bulb temperatures fail to exceed a base temperature. Thinner vapor barriers may include asphalt-coated paper backing on insulation, plywood with exterior glue, aluminum foil, and other types of plastic sheeting or foil sheeting.