Installing a Nail-Fin Window Over Rigid-Foam Insulation
As we add thicker insulation and more water-sensitive materials to the walls of our houses, it becomes increasingly important to detail our exterior walls to keep water out. Window flashing is one of the parts of a house where this matters most. Getting flashing details right for the common nail-fin-style window means making leakproof transitions between the water-resistive barrier (WRB) and the flanges that fasten the window to the wall.
In this Fine Homebuilding Shop Class video series, professional builder Jake Bruton demonstrates the crucial steps for getting the flashing details right when installing a flanged window in a wall covered in a layer of rigid-foam insulation. There are many ways to detail an assembly like this; Jake has chosen to build extended wooden bucks that bring the rough opening out to the plane of the rigid foam, providing a solid surface for fastening the window. He will leave the insulation off of the wall until after the window is installed, allowing him to flash around the buck and to the Zip System sheathing covering the wall. He chose to bridge the gaps with Zip System’s Liquid Flash because of the complexity that the buck adds to the window opening.
In the first episode of the video series, Jake prepares the opening in the wall by applying sealant around the rough opening, across the buck, and onto the entire surface of the studs inside the opening before troweling it to create a flat, continuous moisture barrier. Next, he adds a bead of the same sealant around the sides and top of the opening before inserting, plumbing, squaring up, and fastening the window in place. The third episode outlines the methods for masking the window frame with painter’s tape and then covering the top and side window flanges with the liquid flashing to create the primary flashing details. Finally, he applies foam and quality caulk in the gaps between the window and the interior side of the rough opening to wrap up this airtight, energy-efficient installation.