Critical Interface

Jim Snyder
May 9, 2015
COLUMN : From the Field | Methods & Techniques

The vertical separation between our living space and the elements is made up of three primary sectors: the common wall area, fenestration and the installation perimeter of the fenestration. All three share similar demands as part of the building envelope—to keep weather out and climate control in.

Even though a window (or door) is an independent component in itself, it’s still a window, in a wall, and thus part of the “wall system.”

In fact, the wall system is a conglomerate of components (fenestration, flashings, claddings, building membranes, etc.) that must work together to resist, or at least control, exterior elements. All three sectors are critical to this role. Yet by far, the biggest variable and most difficult to manage of the three primary sectors is the fenestration installation perimeter. To justify this statement, I’ll address all three.

Control Points

Given a membrane drainage wall system, the typical common wall area is made up of sheathed framing, covered by a building membrane of some sort, and capped by an exterior cladding. The make-up is fairly simple and uninterrupted. Throughout the continuous wall, all of these materials can be easily spliced or lapped, and it’s done numerous times around the entire wall structure of the building.

For instance, the installation of structural sheathing (typically 4-foot by 8-foot panels) leaves various horizontal and vertical joints in an exterior wall that can be easily managed in a variety of ways. All of these cases essentially result in a continuous membrane.

Exterior cladding can be fairly continuous as well. Joints are manageable when proper methods are used and the number of joints is largely dependent on the make-up of the cladding (stucco, lap siding, masonry, etc.). Additionally, minimal “allowable” leakage through the cladding is tolerable when used on a membrane drainage wall system (as described above), given that the membrane backs it up.

This combined make-up of common wall area is not complicated.

Fenestration is more complex than the common wall. However, since the products are factory assembled, the quality control is more manageable. Most are air, water and structurally tested and certified. They have proven resistance to air infiltration, water intrusion and structural failure based on the North American Fenestration Standard. But the NAFS certification is for the product only, tested in a lab, and excludes the installation perimeter.

The Critical Interface

That brings us to the third element: the installation perimeter of the fenestration, which is a very busy and potentially “hazardous” intersection.

AAMA 711-07 Voluntary Specification for Self Adhering Flashing Used for Installation of the Exterior Wall Fenestration Products terms this as the “critical interface.”

Here, all the continuity in the common wall terminates and the fenestration (a completely different profile) begins. The objective is to securely anchor the window to resist wind loads and, like the fenestration and common wall, resist air infiltration and water penetration.

In this case, completely sealing the installation perimeter (on the exterior plane) against air and water is a double-edged sword. Incidental water can find its way in to the rough opening through other avenues such as a failure in the membrane above the window, a roof leak that permits water to enter behind the membrane, or even window failure. This water, at all costs, cannot enter the wall cavity.

Ironically then, the installation perimeter must also allow and direct incidental water that approaches or enters the rough opening back to the exterior, or at least to the membrane drainage plane. Envision a watertight boat that has to rid rainwater collecting in the hull. It’s a seemingly impossible requirement.

Through a strategic installation of a drip cap, flashings, sill pan and weep exits, this can be accomplished. Industry documents such as AAMA 711 (and a variety of others) provide guidelines for contending with this critical interface of the installation perimeter.

Keeping the weather out and the climate control in is a team effort by every component of the wall system. The common wall area, fenestration, and installation perimeter of the fenestration are all important to install accurately. But, pay close attention to the critical interface. It’s your most hazardous intersection.

Jim Snyder is an AAMA-certified FenestrationMaster and InstallationMaster who shares his years of installation field experience as an industry writer, speaker, trainer and project/product consultant for dealers and manufacturers. A member of various industry organizations, Snyder also is involved in instructional document creation and revision. Contact him at