Building Envelope Durability

The durability of the building envelope of wood-frame structures is directly affected by four inter-related physical processes of moisture, air, vapour and heat moving through the various assemblies. These processes act on the building envelope in different ways and with different intensities. The assemblies and materials that comprise the building envelope are selected and arranged to effectively control these four processes:

Water penetration control prevents water from entering the interior space or building envelope assemblies and causing premature deterioration of materials. This is the most important function because water potentially causes the most rapid and destructive deterioration of the building envelope.

Air leakage control through the materials, interfaces and joints of the building envelope provides occupant comfort, minimizes heating and cooling energy use; and limits the potential for condensation. Uncontrolled air leakage allows moisture to move through the building envelope, which can also result in severe deterioration of the building envelope assemblies.

Vapour diffusion control through the materials that comprise the building envelope limits the potential for condensation. Water vapour diffusion through the building envelope is a very slow and uniform physical process. As a result, the accumulation of condensation-related moisture occurs slowly and is less likely to lead to damage.

Heat flow control through the materials that comprise the building envelope provides occupant comfort, minimizes heating and cooling energy use; and limits the potential for condensation.

Source : Canada Mortgage and Housing Corporation (CMHC)