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Rigid foam exterior, spray foam interior rot problems?

ryem | Posted in Energy Efficiency and Durability on

My friend is building a small house in Vermont. He has framed 2×4 walls with exterior plywood sheathing and 1″ of blue board outside of that. The facade will be stone with an air space behind of 1″. He is planning on using spray foam to fill in the stud bays. I am more familiar with building materials than he is, and this plan struck me as questionable. Will there be rot problems with the sheathing in between the two foam layers? Should he use open or closed cell foam or a different material all together? He needs the R value of closed cell, but open cell has the ability to dry.

He is also planning on using spray foam in between 2×10 rafters for his unvented cathedral ceilings. Any thought?

Another question I have is about the stone. There is going to be one exterior wall that spans into the interior. This creates a thermal bridging issue. Is there a way to isolate the exterior stone from the interior stone in the same wall without compromising structurally? The interior stone is load bearing. Any comments are much appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    In Vermont, the minimum R-value for rigid foam installed on the exterior side of a 2x4 wall is R-7.5, so one inch of foam isn't thick enough. More information here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    If the wall has exterior rigid foam, then the only type of spray foam to use is open-cell spray foam.

    For more information on insulating a cathedral ceiling with spray foam, see How to Build an Insulated Cathedral Ceiling.

  2. Expert Member
    Dana Dorsett | | #2

    There are more options than just open cell on the interior- could get away with an inch of closed cell on the interior, and the rest fiber or half-pound foam or he could go with 3" of thet 2.0 or 2.2lb density sorta-closed sorta-open cell stuff from Icynene (the MD-R-200 or MD-R-210 not the MD-C-200). With typical half-pound open cell foam it would be advisable to use a "smart" vapor retarder such as Intello Plus or Certanteed MemBrain on the interior side, since half pound foam is way too vapor open to protect the plywood without help.

    On the underside of the roof deck he would need a couple inches of a lower permeance 2lb close closed cell foam to get the vapor redardency down to ~0.5 perms, and half-pound for the rest. As with the walls, half-pound foam is too vapor open for this application, but in conjunction with a couple inches of closed cell it would be fine, no interior side vapor retarder (other than latex ceiling paint) needed.

    The interior/exterior stone issue isn't clear enough to comment on without a better picture of what he's shooting for.

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