High-Performance Window Header – GreenBuildingAdvisor – GWC Mag

This detail was used on a new-construction project that was pursuing Passive House certification and net zero–energy performance. The double-stud wall system has 2×6 exterior load-bearing framing 24 in. o.c. and 2×4 non-load-bearing framing 24 in. o.c. to the interior. A 3-in. cavity between makes for a 12-in. wall assembly.

The aluminum-framed triple-glazed windows are flangeless, which means they could have been installed anywhere within the 12-in. wall. In most high-performance homes, windows are installed in the center third of the wall. This is our preferred location because if the window is pushed too far toward the exterior of the wall—a colder location—the window cavity has the potential to become a microclimate that could lead to condensation problems; and if a window is pushed too far inward, it becomes difficult to manage water at the exterior sill.

That said, almost all our clients want a deeper interior stool, which is among the reasons we installed this window closer to the exterior. The main reason was that it was an exceptionally large window, which we handled with a triple-LVL header on the exterior wall. The interior drywall extends into the window cavity creating a 10-in. window stool.

Here, the center of the window frame is out about 9 in., which is the beginning edge of the outer third of the wall. If we brought the window closer to the interior wall, we would need to add blocking and would lose the full value of the double-stud wall’s thermal break. Also, the window-fastening system relies on metal clips rotated into the aluminum frame and screwed to the wood frame, as shown. By installing the window closer to the exterior framing, we can easily screw the metal clip into the 2×6 wall.

This location, which puts the window roughly 1 in. inboard of…