Thinnest R-10/13 in basement with XPS and batt?

tigerninetyNovember 5, 2012


Paying attention to Worthy's excellent and much appreciated posts on the use of EPS/XPS for basement interior insulation, I'm trying to figure out what the thinnest combination of insulation types would be to achieve the desired goals.

Reviewing various sources for the recommended R-value for my area (Silver Spring, MD, which apparently has HDD=4377), I should apparently be shooting for between R10 and R13. (The sources don't agree, but R-9 is the lowest and R-13 is the highest.)

Locally available Foamular is available at 3/4-in (R3.8), 1-in (R5), 1.5-in (R7.5), and 2-in (R10).

Ideally, we'd take the thinnest XPS that gets us the moisture benefits of using XPS against the walls and then supplement with batt that gets us to the R value that we should have.

So, that's my question: how thin can we go on the XPS knowing that we'd supplement batt to get to somewhere between R-9 and R-13?



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A 2001 study cited by Lstiburek at p.8 of the linked Building Science Corp. paper says as little as 3/4" of XPS would perform well in a climate equivalent to Minnesota, as long as the basement was also dehumidified during the summer.

Personally, I'd rather leave a little more margin. So in a similar climate to yours (4,066HDD) I've used 1" of XPS against the walls followed by sufficient fibreglass to reach the mandated RSI 2.11 in my jurisdiction for foundations enclosing heated space.

Still, in winter, the heads of the concrete screws would frost up. And, in summer, I needed to run a dehumidifier to keep rh levels below 50%. At our peak time utility rate of 11.8 cents a kwh, the costs of dehumidification aren't negligible.

Here is a link that might be useful: Building Science Corp.: Basement Insulation

    Bookmark   November 6, 2012 at 10:56AM
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