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Community and Q&A

Flash and Batt in Reverse – Modular

cp808 | Posted in Green Building Techniques on

Working on a modular residential project in CZ 6. In modular construction though, the batt is installed on the exterior facing edge of sheathing and closed-cell spray foam on the interior facing edge of the gypsum (serving as vapor barrier). Would this flash and batt insulation in reverse cause any problems with moisture transfer?

Also, if we could, does it make sense to add an inch of Poly iso or Roxul on the outside of the sheathing as a thermal break to the outside to increase R value and add a thermal break?

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  1. kevin_in_denver | | #1

    I really like this wall assembly for a lot of reasons, and it's apparently the standard for high performance prefab.

    One inch or more of foam on the exterior is really important to reduce thermal bridging. The problem is that nails don't work well if the foam is more than about an inch thick. Using screws really slows things down.

  2. Expert Member
    Dana Dorsett | | #2

    The vapor permeance of an inch of closed cell polyurethane is about 1 perm (give or take), which is a minimal class-II vapor retarder. As long at the exterior side has a higher permeance than that would not create a signficant moisture problem at the sheathing.

    Foil faced polyiso is a true vapor barrier, and you would need to put at least the IRC prescriptive level of R value for the framing depth on the exterior to keep the sheathing sufficiently warm to avoid moisture problems. (See: ) Also, in climate zone 6 for dew point control purposes climate the polyiso itself needs to be de-rated to about R5/inch (or slightly less) rather than it's labeled value in this application, since the average temp through the foam will be well below the optimum, and it's performance falls off a cliff when the mid-foam temp is well below 40F.

    Rigid Roxul board is very vapor permeable (about 30 perms) and would be far and away the better choice in this stackup.

  3. wjrobinson | | #3

    Roxul only as you have to give moisture and easy path out.

    Personally I think it's a bad idea.

    My easy rule of thumb is;

    Design and build wall to be either cold or warm.

    What you are doing with outside and inside foam is for one thing making the inner wall stay closer to moisture rot and mold temperatures. NOT GOOD. That is why Dana posted as he did.

    Time and time again I see people over do and cause worse problems. Over tighten bolts to be safe on a plane build and the AN hardware snaps. Insulate two sides of a wall and you have a lobster trap. Moisture will get in. Will it get out? NO. What temperature is the inside at now? Rot likes a mid temperature. Cold and no rot. Hot and the moisture may evaporate and no rot.

    Best idea would be to diagonal brace the exterior after ripping off the sheathing, then you could add as much of any breathable insulation in many manners that you wanted. You could also fill the wall with spray foam on the existing spray foam.

    Just don't trap the fiberglass batts between foam. Bad idea.

    An inch of any rigid insulation isn't in the end worth it anyway. Never would see a difference in the energy bills IMO. You are still losing energy via air leaks, windows basements roofs etc. May make a 5% difference and cause some rot eventually somewhere down the road that cost thousands to fix.

    So what are yaa gonna do?

  4. Expert Member
    Dana Dorsett | | #4

    I dunno AJ- I've seen mobile-modular homes built with 2x3 studs, in which case an inch of exterior insulation would cut the heat loss from walls by more than 25%. THAT would show up in the energy bills! :-)

    If it's 2x6 construction you would need R5 on the exterior just to meet IRC 2012 code-min, assuming the cavity insulation adds up to at least R20. You don't get "extra credit" if the cavity-fill is R23-R25 due to the spray foam, since the fact that it is thermally bridged by the framing means it doesn't deliver anywhere near R3-R5 more in whole-wall performance, but the exterior R5 does, since it thermally breaks the framing edges.

    With closed cell foam on the gypsum the foam is a sufficient air retarder to protect the sheathing from interior moisture drives, as long as the drying path to the exterior remains fairly vapor open.

  5. wjrobinson | | #5

    Dana, foam, fiberglass, foam is....... A rotting build.

  6. Richard Beyer | | #6

    I agree with AJ "foam, fiberglass, foam is....... A rotting build."

    Foam in general without a mechanical means to acclimate the home on a constant Rh could result in a rotting build.

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