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Mixing EPS and XPS–zone 5

user-6259794 | Posted in Energy Efficiency and Durability on

I am currently building a house in zone 5. On the walls, I am doing 2×6 with cellulose and 3″ of rigid insulation on the exterior. I couldn’t decide between EPS(cost effective) and XPS, so I purchase enough for 1.5″ layer of each. Since EPS is a little more vapor permeable does it matter if it’s the inside layer or the top layer? I do plan to have a house wrap on the outside of the foam.


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  1. user-6259794 | | #1

    Also, I am using Prosoco joint and seam filler on the OSB layer so do I need to tape any of the rigid foam insulation seams?

  2. GBA Editor
    Martin Holladay | | #2

    I can't think of any reason why it matters whether the XPS goes on before the EPS or vice versa.

    Depending on the density of the EPS, it's possible that the XPS is denser than the EPS -- and if that's true, then putting the XPS on the exterior may make it easier to install furring strips.

    Even if you seal the seams in the OSB, you still might want to tape the seams of at least one of the rigid foam layers. It's a belt-and-suspenders thing -- one more way to reduce air leakage to the lowest possible value.

  3. user-6259794 | | #3

    I was thinking the same thing with the XPS on the outside...easier to install. I kept thinking I could go without taping the seams, but it just kept bugging me that I probably should just to make sure.
    Thank you for your help!

  4. Expert Member
    Dana Dorsett | | #4

    The compressive strength of XPS is about the same as EPS of the same density. Most XPS sheathing is Type-II (1.3lbs -1.5lbs per cubic foot density), and rated at 15 psi, just like Type-II EPS. XPS that is 1.55lbs density- (eg Foamular 250) or higher is usually rated 25 psi, and 1.8-2lb XPS is typically rated 40psi (but more expensive than the ~1.5lb-ish density goods.)

    Type-I EPS (1lb per cubic foot) is usually rated 10 psi, in which case putting the XPS on the exterior makes sense. If you're not sure what you have, the labeled R of 1.5" Type-I EPS is usually R5.75- R5.9, whereas Type-II or denser would be labeled R6.3. With 1.5" XPS it's always R7.5 (in N. America, anyway), independent of density, but you can cut out a similar sized chunk of both to compare weight.

    Tape the seams of BOTH layers, and lap the seams of the layers so that any potential shrinkage over time has the thermal break of the other layer over the gap.

    Over the life cycle of a house the thermal performance of the XPS will drop to that of EPS of similar density- any thermal advantage is temporary.

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