Installing Flanged Windows – Part 5: Air-Sealing

This video walks through the inside finishing steps for installing a new window, once the outside work is done. The builder shims the window in place, then fills three sides of the gap with low-expansion spray foam, a foam that doesn't push the frame out of shape as it cures. He leaves a small gap between the shims and the back of the window's flange so foam can fill that space, which helps prevent condensation from forming inside the wall in cold weather. The shims themselves keep the window seated straight and give it something to push against when it opens and closes, so it keeps working smoothly over time.

The bottom of the window gets different treatment. Instead of foam, he uses two layers of a foam cord called backer rod, then seals over it with caulk shaped like an hourglass around the rod, so it bonds to the window and the wall but not to itself in the middle, letting it flex without cracking. No foam goes in the sill because foam can trap water if the window ever leaks, while backer rod lets any water drain out instead of getting stuck.

This kind of careful air-sealing around a window is what a blower-door test checks for afterward, a whole-house depressurization test that measures how much outside air is leaking in. Getting these details right at each window is part of what keeps a home's heating and cooling costs down and avoids moisture problems in the wall over time.