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Community and Q&A

Do I need drywall between rafters and T&G wood plank ceiling?

ecfelts | Posted in Energy Efficiency and Durability on

I have read a number of articles about unvented cathedral ceiling design. I’m a diy guy rennovating a 1978 brick ranch in Nashville, TN. Im not sure what climate zone I’m in. By the map, I think I’m in 4, but code requirements seem to be based on 3. I think I’ve read just enough to get unsure of myself. I will hire out all of this work, but I want to make sure the right things are being done to optimize my insulation and not cause more problems. I know there have been several posts on this subject, but I haven’t found anything that is related clear on my exact setup. Thanks for any help anyone can offer.

My existing situation:
…cathedral ceiling
Asphalt shingles roof
Plywood sheathing
2×8 rafters
R11 fiberglass insulation with vent cavity left between sheathing and insulation (no baffles)
Painted Drywall ceiling
Non functioning ridge vent

What I am considering:
..cathedral ceiling
Metal standing seam roof
2″ of rigid foam
Existing plywood sheathing
Open cell foam cavity insulation (7″ to 7.5″)
Toungue and groove plank ceiling

Do I need drywall between the t&g planks and the rafters? Is there another alternative? Does everything else look ok?


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  1. GBA Editor
    Martin Holladay | | #1

    E.C. Felts,
    First, I suggest that you read this article: How to Build an Insulated Cathedral Ceiling.

    Nashville, Tennessee is in Climate Zone 4A.

    Q. "Do I need drywall between the tongue-and-groove planks and the rafters?"

    A. Probably. If you install spray foam between the rafters, you need to have a thermal barrier -- that's code-talk for 1/2-inch drywall -- on the interior side of the spray foam for fire protection. If you want to confirm this requirement, talk to your local code authority.

    If you install fiberglass batts or mineral wool batts between the rafters, you should probably have the 1/2-inch drywall as an air barrier.

    Q. "Is there another alternative?"

    A. There are always alternatives. If you specify rigid foam above the roof sheathing with a minimum R-value of R-15 (for example, 2.5 or 3 inches of polyiso), you could substitute fiberglass batts or mineral wool batts for the spray foam. The resulting roof assembly would be a little more robust (because the roof sheathing would be warmer, and therefore dryer, during the winter) and the entire assembly would probably be cheaper.

  2. ecfelts | | #2

    So, to clarify...the stack up that I have suggested would be fine as far as moisture goes and a thermal barrier may be required as a firewall? One of my insulation contractors suggested a spray on thermal barrier in lieu of drywall. Are you familiar with that type of product? I suppose the codes official is the one I need to satisfy. I would like to think the contractor would have only suggested the spray on thermal if it meets code requirements. I will start my research on thermal barriers.
    Thanks for your response.

  3. user-2310254 | | #3

    You should talk to your code enforcer before moving ahead with the spray on barrier. If if they give you the green light, I would want to confirm that there would be no indoor air quality issues. If you install something that has a lot of VOCs, it would be a PITA to address down the road.

  4. GBA Editor
    Martin Holladay | | #4

    E.C. Felts,
    Sorry -- my earlier recommendation was wrong. I decided to re-read the GBA article on thermal barriers and ignition barriers, and realized my mistake.

    According to 2009 IRC R316.5.2, an interior thermal barrier is not required when the spray foam is installed in a roof assembly and is protected on the interior side with a layer of tongue-and-groove boards or plywood measuring at least 15/32 inch thick. So you won't need an additional thermal barrier; your tongue-and-groove boards are enough.

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