Air Barrier Membranes and Aluminum Window Frames

by Jonathan McGaha | 31 March 2017 12:00 am

Energy savings, reduced moisture damage and improved air quality

Air and/or vapor retardant membranes are increasingly required by specifiers across the U.S. Research has shown that these barriers can result in substantial energy savings, in addition to reducing moisture damage and improving interior air quality. They have been used effectively in Canada, the United Kingdom and Germany for many years.

Membranes or coatings used to block airflow generally are called barriers, while membranes used to block water vapor transfer generally are called retardants. It can be more difficult to stop the flow of water vapor, due to the driving force of diffusion. While most U.S. window manufacturers and glazing contractors do not design, provide or install these membranes, they are expected to take interfaces into account. Detailing, sequence of work and inspection may all be affected by air barrier product selection and installation.

 

Barrier Types

There are two classes of membranes, 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. As a system, they can be tested like windows to ASTM E283. Some sheet products are self-adhesive.

Non-permeable air barrier membranes not only act as an air and water retardant layer, but also as a vapor retardant layer. For this reason, in cold climates, they must be located inboard of wall insulation to avoid entrapment of internal condensation. For a given set of interior and exterior conditions, the wall’s dew point temperature must occur somewhere through the thickness of the insulation. 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.

As the name implies, vapor-permeable air barriers are not vapor retardant, and hence, can be used at any location within the wall cavity. So they allow vapor diffusion, they should exhibit a minimum of 3 perms, while maintaining airtightness. They often are 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.

If the membrane is detailed at the exterior of wall cavity insulation, or on the exterior of an insulated stud wall’s sheathing, it likely is intended as a vapor-permeable air barrier. Since the wall cavity itself does not need to be insulated, the wall section can be thinner in profile; however, a vapor retardant still is necessary inboard of the insulated stud wall.

 

Sealing and Window Frame Interface

At window openings, the membrane 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. Remember, the thicker the membrane, the easier it is to self-seal fastener penetrations. Strips can be set atop or underneath the membrane, but the latter usually requires a line of adhesive tape or a liquid lap seal.

Most air barrier membranes, whether liquid or sheet, are based on asphaltic chemistry. This presents challenges for sealant compatibility and adhesion, especially for silicone sealants. Certain silicones and primers can decompose membranes. Polyurethanes and polysulfides can be better choices. EPDM membranes, while exhibiting useful extension characteristics, may contain plasticizers that inhibit adhesion of certain sealants. Metal flashings can lack the flexibility necessary to accommodate movements. For these reasons, some architects and consultants simply require sealant-free interface detailing, using clamping strips, continuous interior trim clips, reglets, interlocks or other methodology.

Translucent silicone sheets or boots similar to those used as unitized curtainwall splices can be an ideal interface material because of easy application and high extension, but as noted, sealant compatibility and adhesion must be verified up front. Waterproofing subcontractors may not be as familiar with the use of formed silicone transitions as they are with other air- and water-membrane system components.

Air barrier manufacturers’ websites show generic interface details, which are useful in developing wall sections and plan details. Most show non-permeable membranes framing the window opening, completely sealed prior to window installation. No flaps or extensions hang free in the opening; instead, seals are accomplished through perimeter sealant joints or compression-sealed trim. Vapor-permeable membranes usually are shown clamped or taped to the window frame at the center seal or exterior seal, depending on system type.

These details involve the work of several subcontractors, and the work is specified in several different specification sections. It is best to leave this detailed design to the architect. Remember that coordination is the key to a successful project. All parties’ respective scope of work needs to be clearly defined in the bid documents, and the general contractor or construction manager must buy the project accordingly.

Steve Fronek, PE, leads Wausau, Wis.-based Wausau Window and Wall Systems’ new product development, marketing, field service, technical support and general research. To learn more, visit www.wausauwindow.com[1].

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