Research has shown that air barriers can result in substantial energy 
savings, in addition to reducing moisture damage and improving interior air quality.
Necessary design criteria for air barrier tie-ins at window perimeter details include:
- Installation must "work" in sequence
- Attachments must accommodate shimming,
adjustment, anchorage and building movement
- Connections must maintain continuity of air/water/vapor
permeability
- Thermal "short circuits must be avoided
- Flashings must drain effectively
Coordination between trades is key.
There are two classes of barriers, non-permeable and vapor-permeable, both of which can be fluid-applied or hung as sheets. All air barriers should exhibit a low air permeance, not to exceed 0.004 cfm/sqft at 1.57 psf. Some sheet products are self-adhesive.
NON-PERMEABLE AIR and VAPOR BARRIERS
Non-permeable air barrier membranes not only act as an air and water barrier, but also as a vapor barrier. For this reason, in cold climates they must be located inboard of wall insulation, to avoid entrapment of internal condensation. Non-permeable membranes have a very low vapor permeance, usually less than 0.1 perms.
Non-permeable air barrier membranes should "tie into" window frame perimeters inboard of the thermal barrier, and frame perimeter insulation should not be placed on the warm side of these membranes.
VAPOR-PERMEABLE AIR BARRIERS
As the name implies, vapor-permeable air barriers are not vapor barriers, and hence, can be used at any location within the wall cavity. By allowing vapor diffusion, they can exhibit a minimum of 3 perms, while maintaining air tightness. They are often used in "mixed-humid" Climate Zones 3 and 4; those with about the same number of heating and cooling degree-days. They provide a mechanism to avoid moisture entrapment as the Dew Point Temperature "migrates" through the wall section with the seasons. A separate vapor barrier is necessary.
SEALING and WINDOW FRAME INTERFACE
At window openings, the barrier is framed with strips of flexible sheet 40 mils or more in thickness, called a perimeter closure strip, detail membrane, or transition sheet. Often, corner and folds are gusseted to prevent pinholes and tears.
Air barrier membranes can present challenges for sealant compatibility and adhesion. Some architects and consultants require sealant-free interface detailing, using clamping strips, continuous interior trim clips, reglets, interlocks or other methodology.
Air barrier manufacturers' websites show generic interface details. Most sites show non-permeable membranes "framing" the window opening, completely sealed prior to window installation. No "flaps" or extensions hang free in the opening for window installers to manipulate. Seals are accomplished through perimeter sealant joints or compression-sealed trim.
Vapor-permeable membranes are usually shown clamped or taped to the window frame at the center seal or exterior seal, depending on system type.
