Flash the sheathing around the buck to the window opening with tape or liquid flashing before installing the exterior foam insulation.

To learn how to install nail-fin windows in 5 common wall types, check out this playlist: https://www.finehomebuilding.com/2019...

When installing a nail-fin window in a house with exterior rigid-foam insulation, the job is more complicated than with other claddings, but the extra steps are easy once you understand the process. As builder Jake Bruton points out in this video, if you aren’t using the rigid exterior foam as the water-resistive barrier (WRB) then the windows should go in before the foam, allowing the opening to be properly flashed behind where the foam will cover. The most common method, shown in this video, is to pad out the rough opening using framing lumber, often called bucks, that are cut to match the thickness of the exterior foam—in this case 2 in. Then the bucks are flashed as part of the rough opening, with a continuous seal extending from the sill and jack studs across and around the bucks, and tied into the WRB on the wall, which in this case is Zip System sheathing. For this installation, Jake demonstrates using liquid flashing, which is well-suited for weatherproofing a bucked opening. Just like paint on a canvas, the goal here is to cover the exposed wood completely with liquid flashing as well as a little bit of the wall sheathing to create a continuous barrier against the weather. Jake establishes layout lines around the outside of the window frame to ensure that he has sufficient coverage. Adding these lines as a guideline to make sure you properly protect your wall is a great idea, but don’t fret about adding a little too much liquid flashing—there’s no harm in coloring outside the lines. It’s important to recognize that liquid flashing, because of its nature, will act differently depending on the temperature and the humidity. Jake explains how to go about handling these quandaries as well as other problems you may run into when using liquid flashing.

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