DESIGN/AIR
BARRIERS
Air Barrier Location within the Envelope
From the perspective of controlling heat transfer alone, the location of the air barrier within the
envelope is not important. However, from the perspectives of constructability, durability and
envelope condensation, the location is very important.
From the perspective of durability, it is preferable to have the air barrier within the exterior cladding
and outward of the structural frame. Having the air barrier within the cladding protects the air
barrier materials from the detrimental affects of weather, i.e., sunlight, rainwater and extreme
temperature fluctuations. The preferred approach to realizing this design is the use of a pressure-
equalized rain screen cladding, as discussed in the section on Rain Penetration Control. In this
approach a well vented cavity behind the facade controls pressure-driven rain penetration and a
well protected air barrier controls air leakage.
Keeping the structural frame of the building within the air barrier makes the air barrier system
design more straightforward in terms of maintaining continuity at penetrations associated with
structural elements.
In cold climates, positioning the air barrier on the interior side of the insulation protects the air
barrier from outdoor temperature fluctuations. Furthermore, the envelope elements to which it is
attached are similarly protected, minimizing the thermally induced movement of these elements and
the resultant physical stresses on the air barrier components. In this situation the air barrier can
also serve as the vapor retarder since it is on the warm side of the insulation. In warm climates, it
will generally be more advantageous to locate the air barrier outside the insulation from the
perspective of airborne moisture transport. If the air barrier is located interior of the thermal
insulation, special care is required to avoid infiltrating water vapor from condensing on the air
barrier.
As discussed in the next section on Vapor Retarders, if the air barrier is not also serving as a vapor
retarder, the relative position of these two elements must be given careful consideration. Whether
or not this is the case, the position of the vapor retarder should be based on an analysis of the
temperature and water vapor profiles through the building envelope, using the techniques
presented in the ASHRAE Handbook of Fundamentals. If the two systems are separate, i.e., the air
barrier is on the low vapor pressure side of the envelope, then the water vapor permeability of the
air barrier must be well above the permeability of the vapor barrier. Recommendations on the
permeability ratio of the air barrier to the vapor retarder range from 5 to 20, however, each system
needs to be analyzed individually for its particular climate.
PAGE 3.1-5
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