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Hot roof in Massachusetts?

mFK35Pqseu | Posted in Energy Efficiency and Durability on

I have 2×10 rafters with a new roof. Can I use Demilec AgriBalance open-cell spray foam?

They tell me it has an R-value of 4.5. My main concern is using 3/4″ tongue-and-groove pine for the ceiling. Should I use a vapor barrier or not? Thinking of using CertainTeed MemBrain. Do not want mold issues.

Thank you for any input.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I use Demilec AgriBalance open-cell spray foam?"

    A. Yes, as long as you use enough to achieve minimum code requirements. Since that product has an R-value of R-4.5 per inch, so you would need to install at least 8.5 inches of foam to achieve R-38.

    Q. "My main concern is using 3/4" tongue-and-groove pine for the ceiling. Should I use a vapor barrier or not? Thinking of using CertainTeed MemBrain."

    A. You don't need a vapor barrier. However, the building code in cold climates usually requires a vapor retarder. If you install 8.5 inches of spray foam, that qualifies as a vapor retarder and an air barrier. Be careful, though: if you switch to a different type of insulation, especially fiberglass batts, you definitely need an air barrier. Tongue-and-groove boards are not an air barrier.

  2. mFK35Pqseu | | #2

    do you have any input on the certainteed membrain. thanks for the info.

  3. GBA Editor
    Martin Holladay | | #3

    MemBrain is a vapor retarder; because it has a variable permeance, it is often called a "smart" vapor retarder. It is usually used on the interior side of walls or ceilings that are insulated with fiberglass batts, cellulose, or blown-in fiberglass.

    Since you plan to use spray-polyurethane foam, which is already a vapor retarder, you don't need MemBrain.

  4. mFK35Pqseu | | #4

    If the building inspector insists would this be the product of choice

  5. wjrobinson | | #5

    Jim, membrain should be fine, the makers of products are the ones to truly ask though. Get your sprayfoam company together with the inspector. Last point, a sprayfoam company worth hiring would have done many installs like yours and know how to handle the barrier/inspector/membrain issue for your particular home and climate. My last sprayfoam project had a vapor barrier paint sprayed on it to comply with inspector.

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