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Community and Q&A

Roof assembly

ABakke | Posted in GBA Pro Help on

I’m insulating the roof of my log cabin in northern MN, zone 7. Trying to get to r49 topped with standing seam metal. Question is what underlayment do I put on the first layer of decking under the insulation and what do I put on the sheathing on top just under the metal? Also, vent the metal or not? Thank you, Andy

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  1. Expert Member
    Michael Maines | | #1

    User-68xx, first, could you tell us your name? The software isn't showing the name you registered with.

    Do you have a specific type of insulation in mind? There are many options available, and the choice of insulation affects the other details.

  2. ABakke | | #2

    Andy Bakke is my name. Considering 2 four inch layers of foil faced polyiso top one with nailbase if venting is not needed or perhaps Raycor 7.25 inch r52 polyurethane roofing panels.

  3. Expert Member
    Michael Maines | | #3

    Andy, I would be leery of Raycore's marketing claims. Nearly every other polyurethane foam on the market is less than R-6/in once the blowing agents off-gas, not the R-7+ per inch they are when brand new. I don't think Raycore includes the R-value reduction of the framing they include, either. I don't think their system is bad; I just think they exaggerate their performance claims.

    Is this an existing or new home? The 2015 IRC building code calls for R-49 insulation in a zone 7 roof; based on your question I assume you planning a new home and want all of the insulation above the roof deck.

    To get R-49 with polyiso, you should use at least 9" and preferably 10" because it loses R-value as it ages and it performs worse than advertised in cold conditions. If using Raycore panels I would also want at least 9" of foam, regardless of their claims.

    In any case, when over-insulating a roof, I like to make the structural sheathing layer the airtight layer, since insulated panels can be hard to keep airtight at the seams. If you have plywood or ZIP sheathing, you can just tape the seams. If you have board sheathing or commodity OSB, a self-adhered membrane like Siga Majvest SA would be a good choice.

    Standing seam metal installers generally prefer to install over solid decking, but some will allow installation over vented roofs with a strapping and purlin system. An advantage of installing on sheathing is a generally lower risk of condensation occurring below the roofing, due to night sky radiation cooling the metal below the dewpoint temperature.

    You don't need to vent a roof like this. You should include a roofing underlayment rated for high temperatures below the metal.

  4. Expert Member
    Dana Dorsett | | #4

    Raycore doesn't do LTTR age derating of the foam (they don't have to- they're selling panels, not insulation, a loophole in the FTC requirements). They are also using HFC245fa blowing agent for their foam (not the much more benign HFO1234ze, which they claim to have experimented with and rejected.) In my view they are on the dim brown edge, not even the pale green middle of "green" building materials. YMMV.

    Robert Brusman of RayCore weighed in about a year ago in an attempt to defend the product, starting in response #4:

    I wasn't convinced of the factuality of the statements in response # 7:

    "But, please note all things “green” have been considered, and simply put, in RAY-CORE's case HFO1234ze won’t work."

    (Is that "we tried many ways and it simply can't", or "we can't be bothered to learn how to MAKE it work"? )

    But even if that part is true, it doesn't make a brown leaf green.

  5. ABakke | | #5

    Thanks gentlemen, this is an existing home, 50 year old cedar cabin built over a crawl space. I’m lifting it up and placing it on a proper foundation to make it a 4season home. Hoping the roof can stay under a foot thick.

  6. Expert Member
    Dana Dorsett | | #6

    The roof doesn't need to be anywhere near a foot thick. The problem with R49 is that it presumes thermally bridging rafters/joists passing through the insulation layer, undercutting it's performance. With ANY amount of continuous insulation it's better to calculate the U-factor to hit the or better performance point as R49 between framing.

    To meet IRC 2015 code performance for roofs on a U-factor a basis the entire stackup needs to be no more than U0.026:

    U0.026 is equivalent to a whole-assembly R of (R=1/U) R38.5, and that includes the R-value of the roof deck itself (typically ~R1), the roofing & underlayment (~R0.5) the interior + exterior side air films (~R1), so really he insulation layer doesn't usually have to be more than R36. If there is going to be a nailer deck above the foam layer through-screwed to structutral roof deck, and a gypsum ceiling under the rafters (empty cavites) the insulation layer doesn't have to be more than ~R35.

    That could be 5" of Dow Thermax, or 6" of 2lb fiber faced roofing polyiso, and that is with ALL of the insulation being on the exterior of the structural roof deck.

    With 2x4 rafters (or deeper) it will cleanly duck under U0.026 with R15 rock wool tight to the underside of the roof deck, and a continuous R30 above, which could be 4" of Thermax or 5" of 2lb fiber faced roofing polysio. That would have huge dew point margin at the roof deck with 67% of the total R on the exterior side, more than sufficient to account for any age or temperature derating of the exterior foam.

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