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Community and Q&A

Insulating vaulted ceiling

Devin86 | Posted in GBA Pro Help on

I am building a tiny house / cabin and have 2×6 joists my current plan is to use 1/2″ osb on the deck covered with synthetic underpayment then a radiant barrier on top of that follower by 2×4’s strips to gain 1.5″ of air space then attach my metal roofing to that. On the underside I was going to do closed cell spray foam for a sealed roof the. T&G pine over the insulation are there any faults with this plan? I read in a article I should install drywall with a vapor retarding paint then the T&G over then any reason why? Also plan on installing a lunos e2 HRV.

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  1. user-2310254 | | #1


    Where are you located?

  2. Devin86 | | #2

    Northern WV on the border of zones 4A and 5A.

  3. GBA Editor
    Martin Holladay | | #3

    The maximum amount of closed-cell spray foam that can be installed between 2x6 joists is about 5 inches. (Because cured closed-cell spray foam is very hard to trim, installers won't fill the entire framing bay.) That means that you'll have a maximum R-value of R-30 -- which is OK, but less than the code minimum of R-49.

    Q. "I read in a article I should install drywall with a vapor retarding paint, then the T&G over that. Any reason why?"

    A. If you had chosen a different type of insulation -- for example, fiberglass batts -- you would need an interior air barrier. (Since tongue-and-groove boards do not create an air barrier, ceilings insulated with fiberglass need taped drywall as an air barrier.) The advice doesn't apply in your case, since closed-cell spray foam is an air barrier.

    That said, there is still the issue of fire safety. Most codes require spray foam to be covered with a thermal barrier (defined as gypsum drywall with a minimum thickness of 1/2 inch) for fire safety. Your local code authority may not consider tongue-and-groove boards to be an acceptable thermal barrier, so before you proceed with your plan, check with your local building department or code official.

  4. Devin86 | | #4

    Thanks Martin
    Another question should I use the synthetic underpayment or something like some 15lb felt to let the sheathing breath into the air space. Also I only have about 3" overhangs on the sides of the roof should I use some ice and water shield?

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