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Community and Q&A

Insulation plan for a cathedral ceiling

MTTim | Posted in General Questions on

After studying the subject to some extent and reading articles and responses from Martin Holladay I would like a response to my planned method of insulating my cathedral ceiling on log home. I have 2×6 T&G ceiling over my log purloins.  I think the following layup might work:  first a 2″ layer of Rmax polyiso on top of T&G for vapor barrier and continuous insulation to provide a thermal break on rafters, then my 2″x6″ rafter with the cavity filled with 3-layers of polyiso (2″ plus 2″ plus 1.5″), then a 5/8″ layer of OSB, titanium underlayment and standing seam metal roofing to complete roof. I live in south central Montana in climate zone 6.  seems like this layup would provide ~46.5 R I suppose an alternative would be to attach my 2×6 rafters direct to my T&G, fill cavity with fiberglass batts  (R19), then 5/8″ OSB, then 4″ of polyiso (two 2″ layers w/ offset seams for R27.4), then another layer of OSB beforemy titanium and metal roofing.  Question is on this layup is 4″ of polyiso enough in zone 6 for hot roof setup? Thanks in Advance for your responses. Tim

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  1. GBA Editor
    Martin Holladay | | #1

    You don't want to cut the rigid foam into narrow rectangles, and then insert the cut-up foam between rafters. (If you do, you'll get thermal bridging through the rafters, and an increased chance of air leakage.)

    All of the rigid foam should be installed as a continuous layer of rigid foam above your roof sheathing. If there is more than one layer of rigid foam, you want to install the second layer with staggered seams. Once you have installed all of the rigid foam, you need to install a second layer of roof sheathing (either plywood or OSB) on the exterior side of the rigid foam. The sheathing is fastened with long screws that extend through the rigid foam to the log purlins.

    For more information, see this article: "How to Install Rigid Foam On Top of Roof Sheathing."

  2. Expert Member
    Akos | | #2


    Generally cut and cobble is a waste to time. The first layup you propose is pretty decent performance, but it is still unvented assembly without proper condensation control. Your cut and cobble can (and will) leak air, with no way to dry, you are making something that could result in a similar situation as the SIP roof rot problems.

    The 2nd assembly is much better, you also get an extra R3 of insulation value with way less foam. Check this for the proper ratio for your climate:

  3. MTTim | | #3

    First let me thank you Martin and Akos for your replies and apologies for not responding sooner. However I have learned a lot from your comments and the links you provided to articles and other info. That said here is my hopefully good planned layup for my roof - first the 2x6 T&G ceiling over my log purlins, then 2x6 rafters on top of the T&G with R21 Unfaced fiberglass batt insulation. Then a layer of 7/16 OSB followed by two layers of 2" polyiso rigid insulation (4x8 shts) with offset and taped joints, then a final layer of 5/8 inch OSB that would be screwed thru the polyiso into the rafters with ~6 inch screws.
    what do you think of this layup?
    Also, I plan on using the foil backed sheets of RMax thermasheath-3 polyiso probably bought at Home Depot and am wondering IF the foil on the first layer of polyiso over the 7/16 osb layer will provide the vapor barrier i need at that point or if I nee something more? Based on the article Akos sent I should have my 50% insulation ratio met with permeable versus impermeable at my 7/16 layer for my region of 6A here i Montana.
    couple last questions are: 1) should i glue my sheets of polyiso or is just screwing thru my last layer of osb adequate, and 2) as long as the only screw penetrations thru mi polyiso are thru into my rafters i have not created any cold spots for condenstion?
    Thanks in advance for more of your good comments.
    PS - I hope to be doing my roof come next Monday and would really appreciate your response asap. Thanks again

  4. MTTim | | #4

    Anyone have any ideas on this? Am I on the right track?

    1. Expert Member
      Akos | | #5


      Your stack up sounds good. I would tape the seams of your first layer of OSB as the primary air barrier. Make sure this air barrier somehow connects to the air barrier of your walls. Also check that your 2x6 T&G doesn't poke through the air barrier if you are extending it into your eaves. The grooves on all the T&G combined for a whole roof add up to a pretty big hole which you don't want.

      Foil faced foam is easier to tape and can also be your primary vapor barrier. There is no problem with multiple layers of foil faced foam.

  5. Expert Member
    Dana Dorsett | | #6

    The air leakiness of the t & g ceiling is still a problem, even though you have sufficient exterior R for dew point control. R21 fiberglass is fairly air retardent but highly vapor permeable, and the size of the air leak of all that t & g seam is huge.

    Even though it wouldn't be the primary air barrier it really needs an air barrier between the t & g and the batts. Even a membrane type smart vapor retarder such as Intello Plus or Certainteed MemBrain detailed as an air barrier would do something.

  6. MTTim | | #7

    Thanks for the reply Akos and Dana:
    Ok I think I am getting it -
    1. so taping the seams on the osb itself (even where it may intersect my ridge rafter and/or wall line vapor barrier is okay (what kind of tape for taping to wood?) OR taping the seams on the foil backing on the first layer of polyiso above the 7/16 intermediate layer of osb is okay for the primary vapor/condensation barrier. (NOTE - the seams on the top side of the first layer of polyiso correct? seems very hard to tape the bottom side of that first layer of polyiso when installing.)

    2. also using the intello plus or certainteed membrain on top of T&G would reduce air and vapor flow substantially and not set up any condensation or vapor sandwich problems between that and the primary foil vapor barrier at the 7/16 osb interface - correct?

  7. MTTim | | #8

    Concerned about no. 2 condensation problems. Any thoughts?

  8. Expert Member
    Akos | | #9

    Around here, typical roofs with exterior rigid/interior fluffy have drywall, so this is never an issue. I've never really thought about it much.

    My read of the BSI paper is that the sheathing under the rigid stays above dew point (or close enough that it doesn't accumulate a lot of moisture) with the right ratio. This to me would mean that you don't need a warm side air barrier. I'm just a random dude on the internet so not sure how much trust to put into that.

    If you want to be on the safe side, a warm side air barrier directly above the T&G would not hurt. I would go for a permeable sheet goof of some kind. With a roof, you would want something you can walk on, so that would be a thin layer of OSB/CDX or permeable roofing underlayment.

    For your 1. question.

    If you tape the OSB under the foam, you don't need to tape the seams of the rigid. It doesn't hurt though. The faced foam will work as a vapour barrier either way.

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