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Insulating an unvented roof with foam boards on the interior

brpbrown21 | Posted in Energy Efficiency and Durability on

Hello,

I am hoping for advice on my historic home. The house is a 150 year old italianate home in climate zone 6a, with a low-hipped roof and no true top “ridge.” The roof assembly is unvented, and accessible from the unfinished 3rd floor space, which is a large, open 40×40 ft. room, The rafters are full dimension 2×8 boards.
I want to use this large space, and need to insulate the rafters. I have read multiple posts here, and understand that foam board insulation on the exterior of the sheathing would be best, followed by closed cell spray foam on the interior as second best. However, the roof does not need to be replaced anytime soon, and I am looking for options to avoid interior spray foam for health/environmental concerns. Are there any other options? Would installing full size foam board insulation on the surface of the rafters (not cut and cobble) work, followed by tongue and groove interior board siding? How thick of foam boards? If so, would I leave the spaces between the rafters empty?

Also, the rafters come down onto a 3 foot hip wall of 3 courses brick (which is the exterior shell of the house). Would the foam board work over this as well, leaving a space between bricks and foam board that communicates with the wide overhanging eaves?

Thanks in advance for any advice.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    BRP Brown,
    For an unvented roof assembly, you're stuck. If you are working from the interior, your only choice is to install closed-cell spray foam.

    You don't have to provide all of the R-value with spray foam, however. You can use the flash-and-batt approach. In your climate zone (Zone 6), building codes require that the spray foam layer have a minimum R-value of R-25 for a roof assembly. That means that you'll need about 4 inches of closed-cell spray foam. The rest of the insulation (another R-24) can come from fiberglass, mineral wool, or cellulose. For more information on the flash-and-batt approach, see Flash-and-Batt Insulation.

    For information on insulating a multi-wythe structural brick wall, see this article: Insulating Old Brick Buildings.

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