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Another “rigid-foam attached to the underside of rafters” plan

Richard Carpenter | Posted in General Questions on

This is new construction with an all hip roof with a very short ridge located in the northern part of zone 3. I’ve ended up with a large cathedral ceiling area which opens to additional living space tucked under the roof resulting in an unvented attic. Is my insulation plan workable? Might I have future water issues? Will my stack-up breathe and dry sufficiently? What issues will the trades find with the plan? is there a better plan?

Top down cathedral ceiling roof stack: metal roof, 30# felt, OSB, 6” rafters, 5” open-cell spray foam in rafter bays, 2” polyiso (fire rated) rigid foam board taped and attached to the rafter bottoms, drywall and paint. Rigid foam above the OSB is not an option. Would 3” of closed cell foam be an improvement?

Non-cathedral attic rafter bays and rim joist area to be air sealed with eight inches of open cell foam.

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  1. GBA Editor
    Martin Holladay | | #1

    For this type of unvented roof assembly, the code requires a spray foam layer with a minimum R-value of R-5 in your climate zone (Zone 3). That can be achieved with 1 inch of closed-cell spray foam or about 1.5 or 2 inches of open-cell spray foam. I recommend that you install the minimum amount of spray foam against the underside of your roof sheathing.

    Once the spray foam is installed, it's a good idea to choose a vapor-permeable insulation for the rest of the needed insulation. (That way, the insulated assembly can dry to the interior if necessary.) If you are aiming for R-38 as the total R-value of the assembly, you'll need about R-33 of vapor-permeable insulation -- which means about 9 inches of insulation. If you have 2x6 rafters, you need to fur down the rafters with additional framing -- to add about 4 1/2 inches or 5 1/2 inches to your rafter cavities, so that you have a total cavity depth of about 10 or 11 inches. Then you'll be all set.

  2. D Dorsett | | #2

    If the metal roofing is tight to the #30 felt it's a moisture trap. If the roofing is mounted on purlins it's fine, from a moisture point of view, but with only 5" foam in 6" (full depth rafters?) there's a thermal bypass channel that can rob performance. Using 3" of closed cell between rafters under-performs 5" of open cell due to the higher thermal bridging, and leaves an even bigger thermal bypass channel. (But at 3" closed cell foam adds some structural rigidity.)

    At only ~R30 it's not up to IRC code min (R38) for zone 3 either.

    Whether the finished ceiling is at inside of the rafters or not, the space below insulated rafters with the 8" of open cell foam is still conditioned space. Without a smart vapor retarder or a class-II vapor retarder under that 8" open cell there is some risk to the roof deck, a risk that is higher on north facing or shaded roof areas, or everywhere if it's a "cool roof" finish with an SRI north of 25. The vapor permeance of most 8" open cell foam isn't low enough to mitigate that risk.

    A better plan would be to put the 2" polyiso above the roof deck (or better yet, 3-4", to be more like IRC code min), between the #30 felt and the metal roofing, a plan which you claim is not an option.

  3. GBA Editor
    Martin Holladay | | #3

    The technique you describe, the "cut-and-cobble" technique, is not recommended for unvented cathedral ceilings. There have been reports of failures. Here is a link to an article on the topic: Cut-and-Cobble Insulation.

    Here is a link to an article that discusses all of your options for insulating this type of roof assembly: How to Build an Insulated Cathedral Ceiling.

    In general, it's not a good idea to create an insulation sandwich that has rigid foam on both sides and fluffy insulation in the middle -- because this type of assembly can't dry out if it ever gets damp.

  4. Sean Sellers | | #4

    For a cathedral assembly with 2x6 rafters, and assuming careful attention paid to air tightness, would it be sound to fasten 3/4" polyiso to the underside of the roof sheating (ripped into 16" widths), fill the cavity with fiberglass, and then fasten another 3/4" polyiso sheet below the rafters, finished with drywall and then T&G cypress?

    Or would it be better to simply fasten both sheets of polyiso to the underside of the roof sheathing, with the fiberglass in the rafter bay? That seems like it would certainly allow for easier installation of the drywall (which would serve as the air barrier) and T&G.

    I am also frankly unsure whether I should build a vented or unvented assembly. The geometry of the roof is simple. Only one side of the ceiling will be joined to the roof rafters; the other side of the ceiling is exposed to a vented unconditioned attic (soffit and gable vents).

    This is for a retrofit in a relatively small area (250sf). I am in climate zone 2.

    Thank you. Great site.

  5. Sean Sellers | | #5

    Thanks, Martin. I appreciate the feedback, and you nailed my nagging concern -- dampness inside the "sandwich."

  6. Expert Member
    Dana Dorsett | | #6

    Definitely NO to foil facers, either at the under side of the roof deck or on the the interior side of the rafters on an unvented assembly.

    In zone 2 you'd probably get away with cut'n'cobbled 1-1.5" unfaced EPS at the roof deck, with 1-1.5" EPS behind the gypsum board though. At 1.5" Type-II EPS (1.5lbs per cubic foot nominal density) is still in the 1-1.5 perms range, which would have adequate drying rates while being more vapor-tight than latex paint.

    A better solution would be a flash of 3/4" of closed cell polyurethane foam on the under side of the roof deck. A 200 board foot DIY kit runs about $300- call it $350 by the time you add in a Tyvek suit and some extra spray tips. With a bit of practice you should be able to get at least a half-inch of coverage on a 250' room. Then cut 1-2 sheets of 3/4-1" polyiso into 1.5" wide strips that you can tack to the rafter edges, and install compressed R25 fiberglass batts into the 5.5-6" of space. Better yet, install 2" polyiso rafter edge strips and compressed R25 batts. You might still be a bit shy of R30 at center cavity, but with 2" of polyiso on the rafter edges you'd have a really great thermal break on the rafters to make up for it. Use a 4" putty knife with one of the sides sharpened as the cutting tool. With a metal straight edge you can get pretty clean cuts for the edge strips if you keep it well honed.

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