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Unvented roof reshingling

danjezow | Posted in GBA Pro Help on

We have about a 19 year old unvented cathedral ceiling insulated with closed cell spray foam that needs new shingles. Lots of valleys and such…not simple geometry in the roof system. Actually our heating and cooling bills are very low so no problem there. Problem is we did not encapsulate the trusses and rafters in the roof system which leads to thermal bridging and constant melting when it snows. We have a 10/12 and 12/12 pitch so the huge ice damms rarely back up beyond the ice and water shield. Would like to insulate either on top or from underneath. Read your articles about adding rigid foam on top and that would prevent the sheating from drying either from the top or the bottom if i took that approach. If i did it from the underside would have to tear off lots of tongue and groove and drywall since that is directly attached to trusses and rafters. I am def in need of advice. How do i say…we are looking for a solution that is also least disruptive for my family. We are in climate zone 5. Thanks, Dan

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Replies

  1. maine_tyler | | #1

    user-THX 1138, I mean Dan,

    How deep does the cc spray foam fill the rafter bays? One issue that comes up with cc spray foam is that if there is a space left and its not filled with another insulation, your thermal bridging is especially bad since the sides of the rafters are exposed (more area to transfer heat).

    Check out this article: https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers
    In which it describes how you essentially already have an assembly (most likely) that can't dry to either side.

    Another thought (not sure if its good) is to find vapor-permeable insulation to add to the exterior, either rockwool or wood fiber. This might be considered 'experimental,' but it's been done (not by me).
    You may also be fine with relatively thin unfaced EPS since that has some vapor permeability, and you don't have any restrictions on minimum R-value above the deck due to your cc spray foam.
    In both of these cases, the issue becomes whether that vapor permeance gets you anywhere without venting above the deck coupled with a vapor permeable roof underlayment. So you could try to add some above deck venting but that might be a tad complicated with your roof geometry.

    Someone may say you're fine to just add as much exterior rigid as you want, but I can't personally make any recommendation due to lack of experience and knowledge on that matter.

    I'd start with looking at the depth of foam fill in your rafters.

  2. Expert Member
    Akos | | #2

    Closed cell foam is already vapor barrier, adding adding anothet layer of non permeable insulation is not a problem provided the deck is dry when the foam is installed.

    For exapmle is common to have a ice and water over the whole roof (true vapor barrier) with SPF on the bottom, no issues with it. Part of my roof at home is built this way.

    The exterior rigid is the way to go, even 1" will go a long ways to reduce thermal bridging. For example a 2x6 crafter is R6, so R5 of foam reduces the heat loss by 1/2. Usually the cost of more insulation is not much extra, if you are in snow country going up to even 2.5" might be worth it. Generally the more snow you have, the more insulation you need as snow is a good insulator.

  3. danjezow | | #3

    Wow thanks guys...great advice and great articleđź‘Ť. Yes Tyler you nailed it when you said the thermal bridging is especially bad cause the side of rafters are exposed. I believe i have about 2 1/2 to 3 inches of foam under the sheating. I would be tempted to tear off the tongue and groove and drywall. Encapsulate all the rafters and trusses....then drop the ceiling a couple inches. But when i suggestest tearing into the ceiling...it did not go over well. Plus that would be a lot of work. So yes you guys are right it really can not dry now through the top so i guess i could make that sandwich and add an inch or two of foam. Just have to hope it doesnt get wet on me. If i used standing seam...in part 3 of the article he said put 1 or 2x4s parellel to the ridge then your standing seam and it will be able to dry. Just because of the air gap? Because it would not be vented. And if i did that and used standing seam would you add more foam first or no on top of the deck? Oh and the idea of vapor permeable insulation on top is very interesting idea. And yes the roofs geometry would make it difficult to vent above the deck.

  4. 730d | | #4

    Another approach would be to create a ventilated roof by adding furing and than another layer of sheathing and shingles.

  5. maine_tyler | | #5

    Another good article to check out: https://www.buildingscience.com/documents/insights/bsi083-mea-culpa-roofs

    In my non-roofers opinion, a good underlayment and a well executed roofing job perhaps becomes more important with cc spray foam underneath than a roof with more drying potential. The addition of exterior foam, though, doesn't really make your situation worse, and perhaps makes it better, especially if the outermost layer of foam is taped. It becomes just another layer to shed water before getting to the sheathing.

    This vapor-open / venting business is a bit of a preferential thing, and I personally see value in venting and maintaining drying potentials, but you're kind of limited on options in both regards there, so embracing the mentality that you're fine just keeping the water out is probably the way to go. And its probably a sensible mentality in this case.

  6. Expert Member
    Akos | | #6

    If you go with standing seam, you can skip the top sheathing.

    You can put horizontal 2x3 on edge 25.5" OC and then use 2' wide 2.5" thick rigid insulation in between. Underlayment over this then the standing seam roof.

    This has slightly more thermal bridging than a full foam cover but it is cheaper and much quicker to install.

    The best way to do it is put down the first 2x3 (this has to be tapered to match your facia angle, if steep roof, you might need 2 pieces), foam against it, use the foam as guide for the next row of 2x3, repeat. Make sure to snap a chalk like to mark out your rafters beforehand as a guide where to screw.

    This was pretty much the assembly for one section of my roof.

    For budget permeable foam, your best bet is paper faced roofing poyiso. This is a standard item at most roofing places but you have to cut it to 2' wide strips.

    1. Jim12d | | #7

      Standing seam can be right on top of foam with no sheeting? That is cool. Do you have pictures?

      1. Expert Member
        Akos | | #8

        Here are a couple of pictures. The first is the peel and stick over the original T&G roof deck with some wood framing around the perimiter started. The foam installed between the 2x on edge. A layer of synthetic underlayment goes over the whole thing before the final metal roof is installed.

  7. creativedestruction | | #9

    Vented nailbase insulation panels on the top of the assembly would solve both the thermal bridging and the ice dams all at once. They're not a cheap product, but the speed of install can make up for some of that. You could install a peel and stick air barrier on the existing roof deck first, but with ccspf in place you should be reasonably airtight already. Know that it will change your eave and fascia profiles and the look of your roof considerably.

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