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Closed-cell spray foam in new commercial construction

TobiasW | Posted in Energy Efficiency and Durability on

Hi. I would appreciate some input on the wall and roof assembly of a new commercial building in the Buffalo, NY area (climate zone 5). The building is a simple 24’x40′ box with 10/12 gable roof.

Code requires R-38 for the roof and R-13 (wall cavity) plus R-3.8 continuous for the wall.

Wall: 2×6 wood studs @ 24″ o.c. with 5/8″ type X GWB each side (1 hour fire rating due to location on lot). Weather barrier and exterior siding (not defined yet). We were thinking of foaming the wall cavities with closed cell foam and applying an additional 1″ of EPS at the inside to the studs to suffice the continuous insulation requirement. The idea with the closed-cell foam would be that it works as a vapor barrier to the inside and prevents moist air from travelling through the wall and condensating at the sheathing. There would be ca. 3″ deep air pockets between the studs as we would not need to foam the whole cavity depth.

If we were to have batt insulation between the studs (to cut down on cost) would we still position our EPS at the inside as vapor barrier? Or would we put it at the exterior to keep our sheathing warm? It seems that you would penetrate the EPS in both cases with fasteners for the GWB respectively the siding.

Roof: 2×10 wood rafters @ 24″ o.c., 5/8″ GWB @ inside and 1/2″ plywood at exterior, underlayment and asphalt shingles. Closed cell foam between rafters to achieve R-38.

I read the threads about exterior insulation and cold plywood and was wondering if this would be an approach that makes sense.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "If we were to have batt insulation between the studs (to cut down on cost) would we still position our EPS at the inside as vapor barrier? Or would we put it at the exterior to keep our sheathing warm?"

    A. If you choose to install batt insulation between the studs, the best place to locate the rigid foam insulation is on the exterior of the sheathing. If you go this route, you do not want to include an interior vapor barrier. More information on these issues can be found in these two articles:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    How to Install Rigid Foam Sheathing

  2. Expert Member
    Dana Dorsett | | #2

    Any empty stud cavity is a waste of space, and leads to higher thermal bridging (due to the shorter distance through the wood from the warm side to the cool side.) If you were putting the cc foam on the sheathing and the EPS on the interior with a 3" gap, and leaks into that space would have a free-flowing thermal bypass channel to run in. (And should it ever light off, the EPS would burn and spread pretty nicely in that "flue space".)

    Even crummy kraft-faced R19s that fully filled the space with the R4 EPS on the exterior of the structural sheathing would outperform 3" of closed cell foam in the stud bays. Key to getting the performance out of it is air-tight studbays- caulk everywhere the sheathing meets framing and seal every penetration with gun-foam.

    But I'd still opt for blown cellulose or high-density blown fiberglass (1.8lbs cu/ft min) or a full cavity fill of open cell foam over batts. Open cell foam would be sufficiently self-sealing that you could skip caulking the sheathing/framing corners in every stud bay. If batts, high density R21 "cathedral ceiling" batts would be preferred, at which point you'd have to bump the exterior foam from R4 up to R5 to be able to get away without an interior vapor retarder in your climate.

    On the 2x10 rafters you'd hit R38 with a mid-density R4/inch open cell foam (eg 0.7lb Demilec), which would also be sufficiently vapor retardent to get away without interior vapor retarders unless you anticipate high humidity as the normal operating condition (say, over a heated swimming pool or sauna.) If possible, putting half or more of the R value above the roof deck as rigid foam or rigid rock-wool would keep the roof deck above the dew point and allow you to go with a more vapor-open approach on the interior (half-pound open cell foam, or batts.)

  3. TobiasW | | #3

    Thanks for all the valuable input. I appreciate it.

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