It has long been recognised that the control of air flow is a
crucial and intrinsic part of heat and moisture control in
modern building enclosures [Wilson 1963, Garden 1965]. That this
statement is true for all climates has been a more recently
developed awareness [Lstiburek 1994]. A large fraction of a
modern, well-insulated building's space conditioning energy load
is due to uncontrolled air leakage. Wintertime condensation of
water vapor in exfiltrating air (or summertime condensation of
infiltrating air) within assemblies is one of the two major
sources of moisture in the above-grade enclosure (driving rain
being the other). Air flow through the enclosure can also carry,
exhaust gases, odours, and sounds through enclosures as well as
mold spores and off gassing generated within the enclosure.
Uncontrolled air leakage through the enclosure is therefore
often a major cause of performance (e.g. comfort, health,
energy, durability, etc.) problems. Water vapor diffusion, while
amenable to simple analysis, is