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Community and Q&A

Tongue-and-Groove Ceiling and Exterior Rigid Foam Roof Insulation

johnMD | Posted in General Questions on

Hi.. the situation referred to is in a single story rancher zone 4a northeastern Maryland. I currently have non vented roof made of 4×6 exposed beams 4’ o.c. through out entire house except for to small “vaulted” sections on 1/2 of the width of each end of the ranch where beams are not readily accessible. The sheathing/insulation currently on my roof is 2” homasote panels that are original so I believe from 1972. A lot of the panels are sagging and water damaged. The panels are r-5 with vinyl barrier attached to underside of panels which is the finished surface you see inside the house. I have been researching and studying this topic of vaulted /cathedral non vented roof assemblies and the right way to re roof my structure to no relief .i don’t want to stick with homasote panels after seeing how bad they have failed due to mutiple reasons. I paid a local structural engineer to give me an awnser and he wants to demo out homasote (agreed is a must)seat cut and drop in 2×12’s 24”o.c. Then 5/8 cdx sheathing with 1-2 “ ccSF sprayed under the roof sheathing then r-38 fiberglass batts in order to reach current code of r-49 with Kraft faced seal to structure .. then smart vapor retarder before drywall . Some areas inside the house arent readily accessible to execute the insulation to underside of decking. I understand his approach but 1,700 sq ft of this style installation plus demo of multiple areas of fine wood work and bringing in multiple installers for multiple bids..loss of exposed beams and just the astronomical cost to do the extensive job( I’m on a budget and don’t think I can afford this method) I’m searching still for another awnser. One recommendation from a contractor the other day was to use t&g then insulate from outside with rigid foam panel insulation . I contacted my local code office they stated 2×6 t&g will suffice or 3/4 cdx using h clips for the 4’ span between rafters .Also they stated no r value met Is necessary just” get in” what I can since it’s existing structure and built before r -values were regulated in Maryland. So my question is that does anyone think the ladder idea of t&g and the rigid foam outside will be ok opposed to what the engineer wants? And for the sandwich would t&g or 3/4 cdx directly to beams- grace water and ice (or some other smart vapor barrier) – then 4 “ of polyiso then 5/8 cdx then roofing g materials . At edges of the sandwich use spray foam to seal for air barrier. Then inside the house between rafters at top plate cut 2” polyiso to fit and spray foam to seal edges. Any input or suggestions is needed direly.

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  1. Expert Member

    I think the exterior T&G solution with a self-adhered membrane would probably be the most cost effective, and maintain the look that you have now. With the OSB/CDX solution you would have to cover the sheathing from the inside with something, it wouldn't be pretty, unless you bought some very expensive plywood.

    You could effectively keep a vented roof if you added a 2x4 on flat between the 4" of polyiso and the 23/32, keeping a ventilated airspace.

    I wouldn't bother with a cut and cobble if you're already having to spray foam. Just have the guy shoot it in there and be done with it in a matter of hours instead of days. It'll do a better job sealing as it is.

    Seems like an ok plan to me.

  2. Expert Member


    I agree with Kyle. The only tricky bit may be at the exterior walls and how the overhangs are dealt with. You will need some way to either terminate the T&G or air-seal the bottom and the grooves where they pass over the walls.

  3. Expert Member

    I'd like to amend my response, John.

    If you're going to go with all of the insulation on the exterior, I'd consider upping the thickness while you have the opportunity to do so. 4" of polyiso in the cold, with 1.5" wood from the T&G only gives you about R24. You can do a little bit of games with the radiant barrier and the air space, but not enough to matter. If you can afford it, it might be money well spent.

    At this point the thickness doesn't really matter all that much. You're going to need 'the long screws' as it is, and you'll need to redo the fascia anyways.

  4. Expert Member
    Akos | | #4

    As the other have said, the T&G plus rigid is the way to go. Since the new roof deck is directly supported by the foam, you can save a bit and go for 7/16 OSB for 1/2" CDX for the new deck.

    The issue that Malcom brings up is important, the gap between the boards adds up very quickly to a pretty large hole to the outside. I like to drill through the T&G groove from the top above the wall plates and inject the gap with SPF to seal it.

    Make sure the peel and stick is butyl or acrylic based. Modified bitumen ones can react with sap in the wood (issue with newish pine T&G) and ooze. Not pleasant to clean up.

    Picture frame the rigid around the perimeter with 2x on edge cut to height to protect the edges. You don't want to have any space where critters can get in as they love nesting in rigid foam.

  5. johnMD | | #5

    Thank you guys.. I’m taking all this to my contractor.. stay tuned I may have some more questions . Thank you so much for the input.

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