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Is the use of rigid foam on both sides of OSB an issue for moisture avoidance?

jzender | Posted in Green Building Techniques on

I am about to start construction on a new residential house in Seattle (3-Marine).

Would the following wall construction (from outside to inside) create any issues?

3/4 furring strips for rainscreen
2 inch R-Max Thermasheat 3 – aluminum foiled
2×6 wood wall construction with following insulation between studs
– 2 inch R-Max Termasheat 3 – aluminum foiled
– 3.5 inch batt insulation unfaced

After reading various articles and posts, I am concerned that the OSB sandwiched between the two rigid foam layer might have moisture issues as it can’t dry neither to inside nor outside.

I am also investigating the option to put both foam layers on the outside of the OSB or not use OSB at all (with structural bracing), but wanted to know if the above configuration would be OK.

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  1. GBA Editor
    Martin Holladay | | #1

    You guessed correctly: it is not a good idea to sandwich OSB between two layers of vapor-impermeable foam.

    The best way to proceed would be to put all of the rigid foam on the exterior side of the OSB.

  2. Expert Member
    Dana Dorsett | | #2

    It's the foil facers more than the foam which are the problem. If you replaced the exterior 2" R-MAX with 2" (or even 3") of unfaced Type-II EPS, the EPS would be roughly as vapor permeable as the OSB itself.

    Putting high-R foam between thermally bridging studs is a waste good foam. If in stead of the 2" R-Max in the cavities you put 3" R-Max on the exterior and only fiber in the cavites (even crummy low density R19 batts, though R23 rock wool would be a better choice) you'd have a higher performance assembly for less foam, less labor, and less money.

  3. jwkaren | | #3

    Using mineral wool batts (such as Roxul) has additional benefits besides better R-value: 1) increased fire resistance and 2) is much easier to install properly than fiberglass batts.

  4. user-1109130 | | #4

    I just completed a Passive House in Portland using a similar wall: 2x8's, plywood sheathing as the air-barrier, and 3" of exterior foam. I chose to fill the stud cavity with blown in fiberglass, but cellulose is a great option for a 2x6 wall.
    Overall the build went well and it's a very high performing wall. Next time I would consider cellulose for the cavity fill and roxul as the outsulation.

  5. user-1109130 | | #5

    I hope you're going to install a Zehnder, Mr. Zender.

  6. jzender | | #6

    Thanks for all the quick responses. Very helpful.
    I do like the 3" exterior foam and Roxul mineral wool cavity insulation approach.

    I definitely did look at a Zehnder HRV, but most likely will go with an UltimateAir or Venmar.

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