Exterior Air Space

Exterior Air Spaces are typically installed behind the cladding.  These spaces are key components in managing rainwater.  The space acts as a drainage plane for any bulk water that makes it past the cladding.  Rain that makes it past the cladding and enters the assembly, typically will drain down the back of the cladding.  In some cases, where the rain bridges the gap, it drains down the water resistive barrier in the assembly, preventing it from getting to the interior.

The size of the air space is often determined by the type of cladding that is installed on the exterior wall assembly.  Depending on how much air is allowed to circulate in the air space, an air space can be categorized as vented or ventilated.  Vented air spaces typically have openings such as weep holes only at the bottom of the assembly, whereas ventilated air spaces have both openings at the bottom of the cladding and at the top of the wall.  Vented air spaces are most common in low rise housing.  These have limited air circulation and serve primarily as a drainage plane.  Ventilated air spaces, on the other hand, are designed to allow much more air to circulate in the space, allowing the space to not only act as a drainage plane, but also as a space that allows moisture to be dissipated.  Ventilated air spaces often use furring or strapping to create deeper, circulating air spaces.  See the section on cladding for additional discussion of how air spaces are used for different cladding types.

References

Air Space. (2013). In Glossary of housing terms (Rev. ed. of: A glossary of house-building and site-development terms, 1982 ed.). Ottawa: Canada Mortgage and Housing Corporation

National Research Council Canada. (2015). National Building Code of Canada 2015 s. 9.27.2.2.(1)