GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Hot roof in Massachusetts?

Jim Cassidy | 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.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jim,
    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. Jim Cassidy | | #2

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

  3. GBA Editor
    Martin Holladay | | #3

    Jim,
    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. Jim Cassidy | | #4

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

  5. Aj Builder, Upstate NY Zone 6a | | #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.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |