Insulation must trap still air to be effective. Insulation must be protected from wind blowing through from the outside and from air escaping from the inside of the home.

Typically, the air barrier (sometimes referred to as a wind barrier or weather barrier) is under the exterior wall finish or cladding (wood or vinyl siding, brick veneer, stucco, etc.). Its primary roles are to shield the wall components from the weather (rain, wind, etc.) while also providing an escape route to the exterior for any water vapour that has penetrated the wall cavity. When rain leaks past the cladding or internal water vapour permeates through the air barrier, there should be a space or gap at the base of the wall where the moisture can escape. This space, called the drainage plane, allows moisture to drain down and away.

An air barrier blocks airflow through the building envelope. It reduces heat loss by preventing air from passing in and out through the envelope and protects the insulation and structure from moisture damage. The air barrier, when located on the exterior, may also act as part of the drainage plane. Standard building materials, such as exterior sheathing, building paper and house wrap, act as the air barrier.

The vapour barrier resists the diffusion of water vapour from the inside to the outside of the building envelope. It protects the insulation and structure from moisture damage that can be caused if water vapour moves into and condenses in the envelope assembly. In some applications, the vapour barrier also acts as the air barrier by reducing heat loss as it prevents air from passing in and out through the envelope (i.e. an air and vapour barrier).

A number of building materials resist vapour diffusion well enough to be used as a vapour barrier. These include polyethylene sheeting, smart retarders, oil-based and special vapour barrier paints, some insulation materials, exterior-grade plywood and oriented strand board (OSB).

New houses typically have an air barrier on the outside and a vapour barrier on the inside of the exterior walls. The ceiling typically has only a single air and vapour barrier, as an exterior air barrier is normally impractical due to air sealing constraints caused by the roofing and ceiling structures.

One material can work as both an air barrier and a vapour barrier, provided it meets the requirements of both types of barriers and is properly installed. Polyethylene sheets and foil-backed gypsum drywall can combine these functions. To avoid confusion of terms, when a material is doing both jobs it is called an air and vapour barrier.

Table 2-1 Recommended minimum insulation values

SAFETY WARNING:

Every home that has a combustion appliance should have carbon monoxide detectors. The local building code may require it. Typically, the requirement is to have one detector located near the furnace or appliance and one detector in each bedroom area.

Because the house envelope is made up of many components, it is impossible for any one material to surround the house completely and form the air barrier (see Figure 2-9). The air barrier is actually a continuous system made up of many parts that are sealed to each other with caulking, tape, gaskets and weatherstripping. Typical components of the air barrier system include the following:

• polyethylene, drywall or plaster - when used for large interior surfaces such as walls and ceilings
• windows, doors, hatches, vent dampers and any components that close an opening in the envelope
• structural parts of the building in some cases, such as the sill plate or rim joist