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Community and Q&A

One layer of exterior foam in climate zone 5

climbing_carpenter | Posted in Energy Efficiency and Durability on

I’m sourcing materials for a project in the Seacoast area of New Hampshire.

I’d planned on standard 2×6 construction with an engineered roof. My intention was 2 layers of 2″ foam on the walls (not too partial to type of foam) and 6″ of foam on the roof. Cavity insulation of some sort, cost dependent. I’d even be OK with 2×4 construction for a total all R value somewhere around R30. I’m building this house to sell.

The quandary: I’ve found an excellent price on 1.5 lb factory second 4″ EPS. I could use it beneath the slab, on the exterior foundation walls and continue it up the outside walls of the house and use two layers on the roof. How much do I have to worry about the joints opening if I were to only have one layer of foam on the walls? Could I spray foam and tape the joints and be OK? The cost savings of using 4″ EPS throughout is half. Price is $19 for a 4″ 4×8 sheet.

Could be possible to use a layer of 1″ or 1.5″ of ISO inside of the EPS, but I wouldn’t be psyched to deal with trim and wall cladding of any sort over nearly 6″ of foam.


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  1. climbing_carpenter | | #1

    * not too partial to type of foam because I'm using reclaimed/ factory seconds, obviously I wouldn't use exterior ISO below grade or beneath a slab

  2. GBA Editor
    Martin Holladay | | #2

    Q. "How much do I have to worry about the joints opening if I were to only have one layer of foam on the walls?"

    A. That's a tough question to answer. Historically, there are documented cases of rigid foam that has shrunk over time. For a discussion of the issue, see "Using Rigid Foam As a Water-Resistive Barrier." (Scroll down to the section under the heading, "Do rigid foam panels shrink?")

    Assuming that the rigid foam is not the wall's primary air barrier -- and that the sheathing seams have been taped (for example, with Zip System tape) to create an air barrier at the sheathing layer -- I think the downside to the possibility of foam shrinkage is small.

    Q. "Could I spray foam and tape the joints and be OK?"

    A. If I were you, that's what I would do -- and I wouldn't worry about it.

    That said, for any builders who can afford two layers of rigid foam with staggered seems, I would still say that the approach using two layers of rigid foam is somewhat better than the approach using one layer of rigid foam. (Probably not better enough to ignore a good deal on used 4-inch foam, though.)

  3. Expert Member
    Dana Dorsett | | #3

    With 6" / R25-ish foam on the roof you would have to limit the cavity fill in the rafters to R38 or less. To hit the IRC 2015 code minimum performance on a U-factor basis with R25 foam on the roof could happen with as little as R19 between the rafters.

    The 4" of continuous 1.5lb EPS (R17-ish) would meet code min on a 2x4 wall on a U-factor basis even without cavity fill.

  4. climbing_carpenter | | #4

    Thank you! Seems like the best path for me to follow with this project. I'm attempting to be resourceful and deliver a product that far exceeds code minimum at a similar price point. So far, it seems possible.

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