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Energy-Smart Details

Weatherizing a Boxed-Bay Window

A detail for ensuring continuity of the four control layers on a structurally complicated building component

The geometry of a simple house will always favor building-science best practices and the four control layers we talk about. The problem is, most people prefer more architecturally complex houses. The challenge is to maintain simplicity in terms of the control layers while developing a more aesthetically evolved building exterior.

The detail illustrated here is a good example of solving for this issue. It is the head assembly of a boxed-bay window. The stepped head/crown is topped with a curved copper metal roof, which is applied to the insulated exterior wall.

To keep water out of the assembly, the exterior weather-resistive barrier (WRB)—in this case Zip-R sheathing—runs down the framed wall, across the top of the boxed bay, and along its exterior face. The WRB, which we term our “Alamo,” maintains continuity along the exterior face of the wall and around the boxed bay, with the exception of the window rough opening.

The first level of water management is addressed at the top of the copper roof. Flashing connecting the roof to the WRB is installed underneath the exterior siding. At the bottom of the copper roof a metal “drip” is installed at the outer fascia. Typically, on bay windows the roof will come down to the window head, so the fascia is installed directly above the window and in roughly the same plane. For better water management, we bump the roof fascia out in front of the window head by 4 in., which allows any water coming off the bay roof to run off and onto the ground, not toward the window head. This assembly provides a high level of protection for the window below.

For the air control layer, the connection between the flanged window and the exterior air barrier (Zip-R sheathing) has been simplified. It…

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