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We are building a house in Hanover, N.H.

user-6724719 | Posted in GBA Pro Help on

Based on discussions from GBA, we are adding 2 1/2″ XPS to the exterior of the sheathing. What is not clear is what (if any) treatment occurs near the ground level — i.e., on the bottom edge of the 4×8′ panel of XPS.

If left exposed, is it susceptible to rodents, bee nests, etc? The siding will be Hardiboard.

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  1. GBA Editor
    Martin Holladay | | #1

    The bottom of your insulated wall needs metal flashing and insect screening. GBA has produced a video that shows how these materials are installed. Here is the link:
    Video: How to Install Rigid Foam Insulation Outside a House.

    -- Martin Holladay

  2. chiefsilverback | | #2

    I can't expand on Martin's link, but howdy nearly neighbour, we're in Peterborough!

  3. Expert Member
    Dana Dorsett | | #3

    If the 2.5" of XPS is to push past the IRC's R11.25 minimum for dew point control on a 2x6 framed house in zone 6, it's not really enough margin. As XPS loses it's HFC blowing agents over the decades it's performance drops to about R4.2/inch when measured at 75F mean temp through the foam.

    Although it's performance increases with falling temp, counting on more than R4.5/inch wouldn't be prudent. At 2.5" that would leave the bare minimum R11.25 specified by the IRC.

    A greener choice would be 3" of Type-II EPS, which tests at R12.6 @ 75F mean temp, and R13.5 @ 40F mean temp. It's blown with a much more innocuous blowing agent, pentane, most of which is released from the foam & recaptured during manufacturing.

    Alternatively 2.5" of polyisocyanurate would get you there. Even though polyiso's performance drops with temperature, at 2.5" it would still be good for at least ~R12 during the coldest weeks, and much higher during most of the heating season. Polyiso is also blown with low-impact pentane.

    The greenest foam though is reclaimed/reused foam of any type. Just be sure to derate re-used XPS to the fully depleted R4.2/inch.

    These folks in Barre VT deal in reclaimed and factory-seconds foam, and are probably close enough to your site to be worth the drive (they might even deliver... or not- it's about an hour each way):

  4. ethant | | #4

    Sorry to be the party pooper here, but I think it is becoming abundantly clear that XPS has no place in a "green" building (as I'm assuming are discussed at I don't have the calculations on hand, but I think the energy (carbon emissions) saved will never make up for the global warming potential (GWP) of the XPS.

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