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Community and Q&A

Insulating top plate

erikdavitt | Posted in Energy Efficiency and Durability on

I am curious about the best way to insulate my top plate?

currently the soffits are open and if I insulate the exterior wall from the outside I will be able to hide the insulation inside the soffit and it will insulate the top plate (see attached diagram) 

The house is currently in climate zone 5a.  The roof has r31 foam board with fibrous insulation between the rafters. The walls are flash/batt with 3 inches os closed cell spray foam.

initially I was thinking putting closed cell spray foam but then I would sandwich the blocking between the rafters with two vapor impermeable substances. Could open cell be used in this case? Are there any other methods out there?

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Erik.

    The exterior insulation seems like it could work, but I don't think there is a double vapor barrier concern in this situation. If you insulate the top plate from the interior with closed cell foam, the blocking will be exposed to the soffit area with nothing to stop it from drying outward. And this is such a small area that I can't imagine it stopping the sheathing from drying towards the interior or soffit area as well. Maybe I'm missing something about your assembly.

    1. erikdavitt | | #2

      Thanks for the reply. The blocking on the exterior side in the soffit along with the top plate would be covered with insulation. because closed cell was already used on the interior of the blocking using closed cell on the exterior (inside the soffit) would sandwich the blocking with a double vapor barrier. Because of this I am wondering if open cell foam would be better to use so the blocking can dry to the exterior. If open cell is not a good idea in that area I could always remove the closed cell on the interior so the blocking can dry to the inside. The spray foam contractors accidentally spray foamed that area and it’s not completely covered. I just think by insulating in the exterior soffit I can add r value to the top plate and it will be hidden when the finish work is done. Otherwise the top plate is impossible to insulate from the interior side.

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