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Exterior foundation insulation and basement windows

New build, CZ 6A (Ottawa ON)

I would like to insulate the above-grade portion of my basement walls to at least a foot below grade, in addition to the code required interior insulation (R20 with no thermal bridging), most likely a layer of EPS foam board with a framed 2x4 wall and cellulose or fiberglass.

Assuming an 8" thick foundation wall, how do you suggest approaching the insulation details around basement windows? Can the exterior insulation wrap inside the rough opening and connect with the interior insulation? If so, how is the window installed onto the insulation?

My concern is, leaving the foundation wall exposed around the outside of the window will effectively bypass the exterior foundation insulation for quite a ways around the window opening, as well as perhaps offer a path for water to get behind the exterior insulation.

Asked by Lance Peters
Posted Jun 19, 2017 9:02 AM ET
Edited Jun 19, 2017 9:59 AM ET


1 Answer

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If you plan ahead, and your rough openings are large enough, I don't see any reason why you couldn't wrap rigid foam around the rough openings for your basement windows.

Most builders don't do that, however. For a foundation with exterior rigid foam, most builders simply install a pressure-treated window buck in the rough opening, with the exterior edges of the buck flush with the exterior face of the rigid foam.

Regardless of your approach, you have to plan ahead to make sure that your rough openings are big enough.

Answered by Martin Holladay
Posted Jun 19, 2017 9:40 AM ET

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