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Rigid foam board on interior ceiling

GBA Editor | Posted in Energy Efficiency and Durability on

This question concerns a gut rehab in the northeast. Its a 50’s ranch with 2×4 walls and 2×8 rafters. The previous owner installed painted cedar shingles on house wrap over the original T-111 siding. We’re gutting from the inside only, installing new windows but leaving the siding in-place. Ideally we’d strip the siding, add rigid foam and re-side, but we’re just not going there. For the walls we’re leaning toward a ‘flash+pack’ approach, with 1-1/2″ high density spray on the inside of the sheathing followed by 4-1/2″ cellulose (2×4 studs with 2×3 cross-stapping). You may comment on the walls, but our bigger question is the ceiling/roof.

The previous owner installed a 4:12 pitch roof over the original 2:12 gable roof – there’s about 30″ from ridge to ridge at the peak, which diminishes to a couple of inches over the eave walls. There are soffit & ridge vents into the space between the roofs. Half of the ceiling (~700sf) is original cathedral (2x8s). We’re considering either: 2x cross-strapping the ceiling and filling the cavity with cellulose, with high-R foam spray near the walls; or 2″ of poly-iso board on the inside of the ceiling, with cellulose in the rafter bays. What do you think of the rigid on the interior side? Not ideal, but its tough to imagine crawling between the roofs to put insulation on top of the old roof. Would 2″ styrofoam be any better considering drying potential? Either way, where there is a flat ceiling we’ll fill most of the shallow attic with cellulose.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Chris,
    I'm partial to rigid polyiso on the interior of cathedral ceilings -- it does a good job of addressing thermal bridging through the rafters.

  2. Riversong | | #2

    And I'm more than partial to leaving some drying potential, particularly when you don't know what's on top of the original roof. I would advise XPS over the non-forgiving foil-faced polyiso. Tape the seams and spray foam the edges for a good air barrier.

    As for the walls, if you're applying the 2x3 cross-hatching flatwise, then you'll have a 5" insulation cavity, not the 6" (1½" + 4½") you've specified.

  3. Chris | | #3

    I get that the XPS is more forgiving for the interior ceiling than polyiso. What do you think about 2" XPS for the interior of the exterior walls? My reluctance here is partly the penetrations required for outlets. 2" of XPS for R10, or 2-1/2" (2x3s on edge) celluolse for about the same R10.

  4. Riversong | | #4

    Chris,

    Either wall option would work and either one would require careful air sealing, particularly around electrical outlets.

    With the XPS option, you can tape the seams and foam the edges to use the foam board as the air barrier, but you'll have to extend the electrical boxes inward and thoroughly foam around and behind them.

    With the cross-hatching and cellulose option, you can install Lessco polypans behind new electrical boxes on the strapping and use the drywall as the air barrier by caulking it to the top and bottom plates, door and window openings and polypan flanges.

  5. Deleted | | #5

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