Rigid foam thicker than existing siding
We live in Vermont and we have recently excavated the cinder block basement of our 1968 ranch down to the footer for moisture mitigation and insulation. The plan is to clean the walls, let them dry, paint with liquid rubber, line the wall with 4 inch rigid foam (Each 2in board is R10 so total of R20), add crushed stone to the bottom of the trench and then backfill.
The question is this: Since the walls of our house are filled with blown-in cellulose, we didn’t do a layer of rigid foam under the siding when we re-did the siding several years back. The 4in foam coming out of the ground up to the bottom of the siding will be significantly thicker than the siding (by at least 2in or more). What do I do now?
I was thinking of stopping the outer foam board at ground level and then continuing from ground level to the bottom of the siding with just the back board, but there will still be a little bit of a lip.
Could I just stop both foam boards at ground level and cap them with metal trim? Completely stumped! Help!
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User-7553592 (do you want to share a name?), the usual approach in a situation like yours is to bend a heavy-gauge metal flashing that goes from the wall to a kickout beyond the foam. In cold climates the most important part of the foundation to insulate is the portion above grade, because that's the part that is exposed to the coldest temperatures.
It sounds like you are probably planning to use XPS insulation, advertised as R-5/in. Be aware that the blowing agents used are some of the most potent greenhouse gasses of any product, so if you care at all about the environment, another foam such as polyiso or EPS is much more benign. (Even better would be a low-carbon material like rigid wood fiber insulation.) Also be aware that XPS does not remain R-5/in; it ages to R-4.2 to 4.4 per inch after a few years.
Sorry name is Jeff! We plan on doing 2 inch x 4ft x 8ft pieces of rigid foam up against the exterior of the basement so the thickness of the foam would be 4 inches total. Thickness of the siding is 1.75 inches. It would look weird to have 3 inches of foam sticking out from under the siding right? Even covered in flashing?
Hi Jeff, it's not my favorite look, so as a rule I design homes and renovations to avoid that detail. But energy efficiency matters more than aesthetics, and some people like the look of a foundation that is fatter than the wall above. Check out the Renaissance buildings in Florence, Italy for assurance that it's not inherently bad looking; it's just not common here in the US.