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How-To

Part 1: Prepare a Rough Opening for a Nail-Fin Window Over Exterior Rigid-Foam Insulation

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

By Jake Bruton

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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Videos in the Series

  • Part 1: Prepare a Rough Opening for a Nail-Fin Window Over Exterior Rigid-Foam Insulation

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

  • Part 2: Installing a Nail-Fin Window in a Wall With Exterior Rigid-Foam Insulation

    Use a manufacturer-approved type of sealant to bond the window to the preliminary flashing before setting, squaring, and fastening the window to the framing.

  • Part 3: Flash a Nail-Fin Window in a Wall With Exterior Rigid-Foam Insulation

    Mask off the window to keep things clean before using liquid flashing to seal the window flanges to the wall.

  • Part 4: Air-Seal and Insulate a Nail-Fin Window in a Wall With Exterior Rigid-Foam Insulation

    Spray foam and high-quality sealant bridge all of the gaps between the framing, shims, and window for a durable and energy-efficient installation.

  • Have a different type of wall? Check out the full video series

    See additional window-flashing details for walls with Zip System sheathing, Housewrap, Peel-and-stick Membranes, and Fluid-applied WRBs

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About This Video Series

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… More about this Video Series

View Comments

  1. 876543 | Feb 11, 2020 04:26pm | #1

    So the exterior foam is insulating the zip sheathing, keeping it above the dew point and preventing condensation on the backside. Assuming that the 3.5"-wide window buck isn't really providing any r-value, is there any risk of vapor condensing on the framing or sheathing around the window?

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