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What is the best way to build and insulate a bay window in new construction?

Dan Easton | Posted in GBA Pro Help on

I have two bay windows to build and I want to keep the insulation levels consistent. I using 1 1/2 exterior rigid foam with lap siding on the exterior and 2×6 framing

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Dan,
    A bay window needs structural support; if you understand the basics of framing, this shouldn't be a problem. Beyond that, a bay window should be treated just like any other ell on a building.

    Pay attention to air sealing and to the continuity of your thermal barrier. If this is a projecting bay, the underside of the projection can be level or sloped. In either case, you want to design the bump-out so there is enough room for insulation -- below the window, at the sides of the window, and above the window.

    It's fine to include insulation between the framing members if you want, just as you might include insulation between studs. However, the performance of this insulation will be degraded by thermal bridging. That's why multiple layers of continuous rigid foam on the exterior of the sheathing perform so much better than insulation between framing members.

    Here is a GBA detail for retrofit insulation at a bay window:
    https://www.greenbuildingadvisor.com/cad/detail/exterior-insulation-retrofit-existing-bay-window

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