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Basement Window Wells and Exterior Rigid Insulation

SustainablyBuilt | Posted in Energy Efficiency and Durability on

I’m working with a builder who is having difficulty detailing a window well with exterior rigid insulation. All the great details on this website and on swear by exterior rigid insulation for basement foundation walls, etc. But for the life of me, I can’t find any details on how to deal with a window well space. The foam is exposed to the outside, so what’s the best way to protect it and cover it for appearance? How do you trim this out around the window? Does anyone have a link to good details for this? I’ve been scouring the net for quite a while and have not come up with anything.

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  1. Expert Member
    Peter Yost | | #1

    Hi Adam - You are does not have any window well details...yet. In the meantime, it makes sense to use the same cover material around the window well that you are using for the other exposed portions of your exterior insulation: cementitious board or a fiber-reinforced parging. You just need to use that same material to return the jamb extension covering the thickness of the exterior rigid insulation.

    And I would not say that the votes are all in at regarding interior versus exterior foundation insulation. There are clear advantages and issues with each approach. Stay tuned on this one; we are contemplating a pro/con article for the Insider on this very topic.


    We've been using the Carlisle CCW861 mira dry peal and stick for our ICF foundations and it works well for direct application to concrete as well. at $150 per 200 sf roll. It gets it's joints sealed with the LM 800XL mastic $160 per five gallon bucket. Then we parge over the above grade areas with spider lathe and hard coat stucco. We've done it with 4X8 Hardi stucco board as well to save money but the joints show and some find that sloppy looking.

    The cheapest way to do it is to waterproof the masonry with a paint-on fibered foundation sealer, Lowes and Home Depot sell that stuff in five gallon buckets, not low VOC by any means though. Do not apply it with a rectangular "Foundation Brush" but instead use an in-expensive plastic broom. Then you can install your window on the masonry and tape it to the tar with flashing tape and go over all that with the foam, using the foam as a protective over-layer for the cheap waterproofing and glue the Hardi stucco board to the foam. Bostik makes a good Low VOC panel adhesive sold through Stock that is great for this. Make sure you flash the top of the foundation over and down onto the foam and stucco board because if ice gets in there it will push it all apart. Then all you have to do is bump your well up against the finished stucco and you're done. For us the well is generally site built out of PT 6X6's with egress steps built into the design.

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