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Can you use open-cell foam against OSB and provide a 3″ air gap behind it?

Sturgill12 | Posted in GBA Pro Help on

Building a new construction home in a Mixed humid KY climate. Question is referring to insulating the attic space of a 4/12 pitch roof system with a standard roofline. Is it a good idea, logical or crazy to try to insulate the roofline of our 4/12 roof instead of the back of drywall ceiling deck. Plan is to form a 3″ air shute from the vented eave between the 2ft on center rafter trusses all the way to the vented ridge using 7/16 osb or stapled in place insulwebbing fabric, applying 6″ of open cell directly against the osb/insulwebbing fabric following the roofline. The drywall ceiling deck in this format would not be air sealed and no careful attention would be made to airsealing the can lights and or other drywall openings, letting the air seal layer live at the roofline level.

Reasons for wanting to do this-
1. Easier access for future wiring needs not digging thru foam to run new lines and or access to attic in general.
2. Less cost in labor and or products to try to seal every penetration in ceiling drywall, not buying insulation rated can lights and or other products that will touch insulation.
3. Make electricians job little easier and or dry waller .
4. Afraid to let open cell foam or closed cell foam touch underside of osb roof deck , for fear or roof sheathing rot /failure. Also afraid of moisture buildup in main house or above ceiling drywall if the humidity cant freely pass thru the open cell foam and be carried out the ridge via the 3″ air shute living below the cold roof deck.
5. If the open cell foam touches the roof deck the vapor will drive thru the foam , thru the osb decking and then hit the vapor impermeable synthetic roofing felt under the standing seam metal roofing and get trapped in that space , leading to osb getting wet staying wet or rotting, if I leave the air gap under the osb roof deck the ridge vent should carry any vapor drive out the roof envelope to the outside. This is my solution that I’ve created in my mind .
6. Did I mention i really like the ability to have full access crawl space and full access attic space to work in.
7. Mitsubishi Hyper heat VRF heat pump with ducted air handler will live in crawlspace with a dedicated
sante fe style dehumidifier ducted in to the system. Also plan to use panasonic ceiling mounted erv in main house for fresh air exchange .

I’m unsure if this approach is energy efficiency suicide to not control heat loss at ceiling drywall level but I would reasonably assume the leaky drywall ceiling would allow air to move in both directions balancing out overall temps between the semi conditioned attic zone and main house. Wasn’t planning to put dedicated supply vents in the semi-conditioned attic space.

Just FYI the structure is 2×4 walls with open cell sprayed in the walls, zip sheathing, conditioned crawlspace with 3″ closed cell on block walls, 4″ open cell on rim area, Rhino roof synthetic roof felt with Standing seam galvalume metal roof .

WHAT DO THE PRO’S THINK?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Cody,
    Your plan to install site-built ventilation chutes before installing open-cell spray foam under your sloped roof assembly is a good one. Your approach is far less risky than the ordinary approach of installing open-cell spray foam directly against the roof sheathing. For more information on your approach, see Site-Built Ventilation Baffles for Roofs.

    The main problem with your plan is that it doesn't meet minimum code requirements for R-value. In your climate zone (Zone 4), the IRC calls for a minimum of R-49 ceiling insulation. If you install 6 inches of open-cell spray foam, you'll have only R-22.

    For more information on this issue, see Creating a Conditioned Attic.

  2. Sturgill12 | | #2

    Thanks Martin for your ensite , looks like we should just spray the back side of the drywall ceiling for our air seal and then beef up the depth of cellulose on top of the open cell to hit the R-49 that I wasnt even focusing on, to busy clouding my head with sheathing rot with open cell.
    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    Cody,
    You could take another approach.

    1. You could install the ventilation baffles as planned.

    2. You could install R-15 of closed-cell spray foam (about 2 1/2 inches) or open-cell spray foam (a little more than 4 inches).

    3. You could install R-34 of fiberglass, mineral wool, or cellulose on the interior side of the spray foam (up against the cured spray foam, following the roof assembly).

    For more on this approach, see Combining Exterior Rigid Foam With Fluffy Insulation. Although this article discusses exterior rigid foam, the same principles apply to interior spray foam installed parallel to the roof.

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