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OK to use commercial roofing boards for wall insulation?

C Talwalkar | Posted in Energy Efficiency and Durability on

I am insulating an early 1950s Cape Cod style ventilated attic in Lexington, KY (IECC Zone 4) and have reached the following tentative conclusions:

1. I can’t replace the roof, so I can’t insulate above the deck.

2. There is too much thermal bridging through the rafters, too much thermal mass in the concrete board finished walls, and not enough room in the 2×6-framed rafter bays to effectively insulate between the rafters.

3. The best way to make the attic living space habitable is to take out the existing finished ceiling and knee wall, install 4″ of polyiso below the rafters and inside the wall studs, and refinish the living space with drywall.

My questions:

1. Does this sound like a reasonable approach, or is there a better/easier way? Is this enough insulation?

2. I can get 2″ Firestone ISO 95+ boards for a much better price than XPS foam here ($22 vs ~$30 for 4×8) but the ISO 95+ boards are faced with a fiberglass reinforced craft paper on both sides. Manufacturer says boards are intended for commercial roofing, and offers no guidance on sealing seams. Has anyone used commercial roofing insulation for interior walls and ceilings? How did you seal the seams? Is one kind of foam or tape better than another?

A search on the GBA site showed one thread from 2011 questioning whether the ISO 95+ boards were impermeable. The technical data sheet on the manufacturer’s web site says ASTM E96 “Moisture Vapor Transmission” is <1 perm. Any reason not to use this product as wall insulation? If this question has already been answered, please point me to that thread. Thanks for you consideration!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    C Talwalkar,
    Your plan will work. As to whether the R-value is sufficient: you'll get between R-24 and R-26 from the 4 inches of polyiso. That should be a significant improvement -- even though your house won't be superinsulated.

    I'm not sure which tape is best; here is a link to a blog with a discussion of tapes: Air-Sealing Tapes and Gaskets. In addition to the tapes mentioned in that blog, you might want to consider using one of the European tapes sold by Small Planet Workshop or Four Seven Five.

  2. C Talwalkar | | #2

    Thanks, Martin. I read your referenced thread and am now thinking of using a thicker weathersealing tape instead of normal housewrap tape. I'm also contemplating using gaskets under the 1" firring to seal the seams on the outer layer of foam.

    Presumably, if I do both the screws that attach the firring to the framing will puncture the tape at the seams. In this situation, is taping the vertical seams a waste of time and $?

  3. GBA Editor
    Martin Holladay | | #3

    C Talwalkar,
    I can't quite visualize your plan to "use gaskets under the 1-inch furring to seal the seams on the outer layer of foam." Are you planning to install 1x3 or 1x4 furring strips to cover the foam seams?

    My advice: Just tape the seams and don't worry about the screw holes. If the screw holes have screws in them, the holes won't be significant air leaks.

  4. C Talwalkar | | #4

    Thanks for your advice, Martin!

    About the gasket -- I assumed it would be easier to attach the drywall to 3" wide furring on the surface of the foam than to 1.5" wide framing under 4 inches of foam. The vertical seams of each foam layer should land on a stud or rafter. To airseal the foam, I’d tack sill seal to the rafters and studs where the first layer of foam’s seams landed, and tack sill seal to the furring that landed over the second layer of foam’s seams. Horizontal seams could only be taped.

    Would it be wiser to skip the furring and try to screw the drywall directly to the framing? It would certainly save the time and cost of attaching the furring. If I’m reading the linked document below correctly, it would also be important to eliminate air gaps between the drywall (the warm-side “air barrier”) and the wall insulation (to minimize heat loss via convective loops):
    http://www.aecb.net/PDFs/Impact_of_thermal_bypass.pdf

    Directly attaching the drywall would also, then, save the cost and work of filling in between the furring. Seems like a good idea, except for the part about using 6" fasteners to hit 1.5" framing through 1/2" drywall + 4" foam. Also, seems like the gaskets could go under the drywall as easily as they could go under the furring.

    WWMD (What Would Martin Do)?

    Thanks again!

  5. GBA Editor
    Martin Holladay | | #5

    C Talwalkar,
    WWMD? I'd probably install the furring strips, although I've done it both ways. It's perfectly possible to install drywall with long screws through a layer of interior rigid foam. Not all drywallers like to do it that way, but I've built several homes using that method, and I always hang my own drywall.

    The furring strips make it much easier to install the drywall. Whichever way you choose, be sure that your electrical boxes are at the right depth, and strive for an airtight installation. I think it would be much easier to tape the seams of your foam than to try to seal the foam seams with gaskets and furring strips.

    If you are installing drywall on furring strips over rigid foam, there is no need for your air barrier to be in direct contact with the insulation layer, as long as you are paying attention to airtightness.

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