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

Should closed-cell spray foam cover rafters to be vapor barrier?

J Star | Posted in General Questions on

Can Closed Cell spray foam, sprayed against the plywood roof sheathing (from the inside) between the rafters (2-3″ thick) make for an adequate vapor barrier? Or will the rafters allow for vapor-bridging to occur?

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

    J.,
    First of all, why do you want a vapor barrier in this location? Most types of roofing -- including asphalt shingles, EPDM, and standing-seam metal roofing -- are vapor barriers, so roof sheathing generally doesn't dry to the exterior.

    In the U.S., there aren't any building codes that require an interior vapor barrier in this location (although building codes in some climates require a vapor retarder, which is a less stringent barrier).

    Two inches of closed-cell spray foam has an R-value of about R-12, while 3 inches has an R-value of about R-18. You need a much higher R-value to meet minimum code requirements. If you are planning a flash-and-batt installation, R-12 of closed-cell foam will work in Climate Zones 1-3 or Marine Zone 4, but not in anywhere colder. R-18 of closed-cell spray foam will work in Climate Zones 1-4, but not in anywhere colder. Of course, the flash-and-batt approach requires additional insulation to meet minimum code requirements (R-38 for Zones 2-3, and R-49 for colder zones).

  2. Expert Member
    Dana Dorsett | | #2

    The desirability of a vapor barrier at that location aside...

    The vapor permeance per inch of thickness of dry softwoods is comparable to that of 2lb foam. When the moisture content of the wood is high it's more vapor permeable than closed cell foam.

    Covering the rafters with foam would only trap moisture in the wood, making it take longer to dry. It would improve the thermal performance by reducing the magnitude of the thermal bridge, but it's not really the "right" thing to do. There are lower cost, higher resilience ways to build in that much additional performance.

  3. GBA Editor
    Martin Holladay | | #3

    J,
    To expand on my first response (prompted by Dana's first sentence): Yes, if you plan to install spray foam in this location, it's a good idea to choose closed-cell spray foam rather than open-cell spray foam, precisely because closed-cell spray foam doesn't allow as much vapor transport as open-cell spray foam. Isolating the roof sheathing from the indoor air, which may be humid, is a good thing. But you don't need to cover the rafters to achieve that benefit.

    For more information on this issue, see High Humidity in Unvented Conditioned Attics.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |