Air and vapor barriers

If building a new home or about to make a large remodel to your home's envelope (exterior walls, ceiling, floor, foundations...) pay attention to the air and vapor barrier issue. They are crucial in wood and steel frame homes...

The wind, the stack pressure (the chimney effect or buoyancy) and the pressure of the HVAC/ventilation equipment are common causes of air leakage and air movement through building cavities (and also of moisture). Air barriers prevent air movement through cavities and air leakage, while vapor barriers are part of moisture control...

The whole Home's Envelope should be covered by a continuos blanket of insulation and an air barrier Wood and steel frame homes in cold and harsh climates should have a continuous air barrier all over their envelope. That’s the best way to prevent air leaks and to get higher energy savings.

Image at right (DOE): the home's envelope is represented by the orange-red line.

Typical air barriers

Sheet goods such as plywood or drywall are the most typical air barriers in home building. Housewraps, plaster, heavy asphalt impregnated (building paper) and poured concrete are also common air barriers in wood frame walls.

Rigid foam boards can also be used as air barriers, besides being an excellent insulation material...

Many air barriers are available in wide sheets, and should be protected from sunlight. To be effective, the seams and the joints between the air barrier sheets must also be sealed with an appropriate caulking or weatherstripping.New wall building strategies and air barriers
Some new wall building technologies eliminate the need of air barriers. That's the case of he ICF technology, which includes built-in elements like insulation and sealing layers and reinforcement bars...

Sheet goods & air barriers

Housewraps and rigid foam sheets are better air barriers than sheet goods (drywall, sheathing, decking..).

To get a reasonable barrier solution with sheet goods, make sure they are properly installed and sealed. The installation should ensure a continuous layer, without open holes or non-sealed seams.

Air and Vapor barriers

Air and vapor barriers or retarders are basically different materials. Air barriers are primarily designed to restrict the flow of air, while vapor barriers are specifically designed to restrict the flow of water vapor...

A vapor barrier does not have to be continuous or free of holes, and doesn't have to be sealed. An air barrier should be continuous, free of holes and sealed.

Vapor barriers are vital on exterior walls in locations experiencing temperatures below freezing. They will control water and vapor transmission, preventing condensation and moisture.

Some materials can be both air barriers and vapor barriers. That's, namely, the case of some housewraps. House-Wrap

Housewraps

Housewraps are excellent air barriers. They are typically made from fibrous spun polyolefin plastic, matted into sheets, and are applied during new construction (wrapped around the home's shell - exterior walls, ceiling, floor, foundations...).

To get a better strength, some housewraps incorporate specially woven or bonded materials, while others incorporate water-repelling materials.

In hot-humid climate zones, use an housewrap with a water vapor diffusion retarder - often a polyethylene plastic sheet, a foam insulation board or some specific sheathing material... In rainy and wet climates, install felt paper over the housewrap, or use heavy asphalt impregnated building paper instead...