Air barriers and air leakage

If building a new home or about to make a large remodeling to your home's envelope - exterior walls, ceiling, floor, foundations - pay attention to the air barrier issue. Air barriers will prevent air movement through building cavities, and air leakage and even moisture...

Home Envelope Homes – namely wood frames homes in cold and harsh climates - should have a continuous air barrier all over their envelope. That’s the best way to prevent air leaking and to get higher energy savings.

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

Why your home needs an air barrier

Wind, other environmental pressures, stack pressure (the chimney effect or buoyancy) the and HVAC/ventilation pressure cause air movement through building cavities, and air leakage or even moisture.

Air barriers will prevent it, and may also help to control water vapor, though only partially (air barriers and vapor barriers shouldn’t be confused).

Air barriers vs. Vapor barriers

Air barriers and vapor barriers are different materials. Air barriers are primarily designed to restrict the flow of air into a home assembly, while vapor barriers are 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.

Though different, in practice some air barriers act also as vapor barriers, and vice-versa. That's why common housewraps are used both as vapor barriers and air barriers. House-Wrap

Housewraps

House wraps are applied during construction: they are wrapped around the home's shell - exterior walls, ceiling, floor, foundations.

House wraps are typically made from fibrous spun polyolefin plastic, matted into sheets. Housewraps may have materials woven or bonded to them to improve their strength. They also vary in their water-repelling capacities.

Their type and use depends on construction techniques and climate. In hot-humid climate zones, use an housewrap with a water vapor diffusion retarder. This last type of material – typically placed under the exterior finish, around the home’s perimeter - consists in polyethylene plastic sheet, foam insulation board insulation, or specific sheathings.

Wet climates, house wraps and asphalt impregnated

In rainy and wet climates, to avoid water problems and to get a water-repellent surface, instead of housewraps you may use heavy asphalt impregnated (building paper). Or install felt paper over the housewrap…

Sheet goods & air barriers

Instead of housewraps and specific air barriers, many constructors use sheet goods - drywall, sheathing and decking - as air barriers. In this case, besides structural purposes sheet goods also act as air barriers.

To get a reasonable or even good air barrier solution with sheet goods, male sure they are properly mounted and sealed. The installation should ensure a continuous barrier, without open holes or non-sealed seams.