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Roof rafters and spray foam

Mill_house | Posted in General Questions on

Climate zone 7 Canadian prairies; unvented unconditioned attic 
Cape cod style house with 2×4 fir roof rafters.  House was built in 1940s.  the roof is in good shape despite the rafters not being to today’s depth standards.  I am going to leave them as is.
I understand that when insulating from the inside one of the options for insulating the sloped portion of the roof is to use closed cell spray foam. (Spray foam the underside of the roof sheathing.)  Because I am not building out the rafters, I will only get about 3 inches of foam.  I understand this is below the recommended r value.  But if I proceed with it, is there anything I should be concerned about in terms of condensation, etc?

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  1. GBA Editor
    Martin Holladay | | #1

    Three inches of closed-cell spray foam will yield an R-value of R-19 or so -- perhaps a bit less -- and the insulation's effectiveness will be diminished by the thermal bridging through the 2x4s.

    You should be aiming for R-49, so your approach is far from ideal.

    That said, there are no moisture concerns with your approach.

  2. Mill_house | | #2

    I understand that it's difficult to seal the transition zones - where the spray foam ends just above the collar ties and transitions to the flat ceiling where I'll be insulating "old school" style (drywall, poly, mineral wool batts). Sealing this area "sounds" easy. But I don't think it will be as I don't have a flat surface (in between the rafters) to connect the poly to.
    So what I'm thinking of doing instead is to staple cardboard to the attic side of the collar ties and have the spray foam sprayed on the slope portion of roof (3"), across the flat ceiling (2") and back down the other side of the room on the slopes. Both slope sides will seal in that the spray foam will start and end on the top plate on the kneewall.
    I'll leave the cardboard in place and then just lay mineral wool batts on top.
    How does this sound?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "How does this sound?"

    A. I would use OSB, not cardboard, for the spray foam substrate.

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