Building Science

Diagram of warm and moist inside air condensing on the exterior gypsum board when outside is cool and dry.

Without a vapor retarder, moisture can penetrate wall and condense on cold surface

Diagram of moisture barrier blocking warm and moist air from the interior of house from getting through the wall to the cool and dry outside.

With a vapor retarder, moisture can’t reach cold surface to condense


To allow excess water vapor to escape, provisions should be made for proper ventilation when installing insulation. In attics, the most common approach is a static system with some combination of simple gable, eave or ridge vents. The basic rule of thumb is to provide one square foot of vent area for every 150 square feet of net floor area. When a vapor retarder is
present, that figure doubles to 300 square feet—but note that an attic vapor retarder is not required if there is sufficient ventilation (unless the home is located in a very cold region or has higher than normal interior humidity).

Air Tightness

Only when the building envelope is air tight can warm air be retained within the building and cold air kept outside. Preventing leaks gives greater thermal comfort, increased energy efficiency, and protection for the building materials against damage, helping the building maintain its appearance and extending its life. But the benefits are not purely financial, as high levels of air tightness also ensure healthy indoor air quality for the occupants.

CertainTeed MemBrainTM Continuous Air Barrier & Smart Vapor Retarder can be installed to perform as a building envelope air barrier to create an air-tight home.


For compliance information on CertainTeed insulation, see the specification sheet for each individual product. Specification sheets can be found at www.certainteed/insulation

Achieve Continuity

Air constantly moves from high to low pressure, finding every penetration into and out of a home, which is why proper air sealing is crucial to create a continuous seal at all joints and penetrations.