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Community and Q&A

Window details

user-1105327 | Posted in General Questions on

We’re taught to air seal the interior side of a window with foam or tape. We’re also taught to provide some sort of back dam or positive slope to the sill.

What is the point of sloping the sill to the exterior if the window is foamed/taped/caulked to the framing? We are also taught to not caulk/tape the exterior nailing flange to the housewrap for the same purpose, i.e., give water a path to escape.

What gives?

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  1. Expert Member

    Erik, Here in British Columbia our code acknowledges that by making us either mount the window outboard of the rain screen strapping so it can drain behind the flange at he sloped sill, or drill holes in the lower flange for drainage.
    The trade off is that in both cases you now have a path for cold air to get right to the window's inside surface at the sill.

  2. GBA Editor
    Martin Holladay | | #2

    I suggest that you read this Fine Homebuilding article: Windows in Double-Stud Walls.

    In that article, Jesse Thompson advises:

    "Treat the window flange as an installation aid only, not as a water or air seal. Expect that water will be able to enter behind the flange during extreme conditions, and always provide a pathway out. ...

    "Wrap rough openings properly, with tape or flashing that is lapped top over bottom, and install a sloped sill pan below the window. It’s now part of most window manufacturers’ instructions to place a clapboard or sloped shim at the rough sill, but we still rarely see this recommendation followed until we ask for it. The critical and often-over-looked air-sealing area is at the interior of the sill. If
    the window is not shimmed at its sill—the usual situation we see—there needs to be a bead of caulk or a gasket seal at the inside face of the window to prevent water from blowing up under the open exterior window flange, right under the window frame, and into the building. ...

    "Establish a long-lasting air-and-water seal at the inside face of the window unit, and connect that seal to the rough opening. The best material for creating this seal is a flexible and long-lasting construction tape such as Dow Weathermate or 3M 8067 Flashing Tape, or a European tape such as Siga Rissan or Pro Clima Tescon. ..."

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