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Community and Q&A

Rigid Foam Instead of Spray Foam for Unvented Attic

badmechanic | Posted in Energy Efficiency and Durability on

The awesome GBA article, “Insulating Low-Slope Residential Roofs” and PNNL both advise to use spray foam directly below the sheathing for an unvented attic if above the sheathing is uninsulated on top. I want to use rigid foam instead of spray foam below the sheathing and then somehow make up for the lack of a perfect fit, like using a small amount of spray foam or “great stuff” foam to seal the edges. Would it work? I don’t want to use spray foam because it’s expensive and blowing agents. Polyiso I can live with. This is a thin, flat roof (9.5 inch joists) and for every inch of thickness we lose an inch in ceiling height due to zoning. Ceiling height is already 8′. We are in the bay area 1 mile between climate zone 3 and 4. If it were not for the height issue I would put foam board on top and call it a day. “IRC R806.5 Unvented attic and unvented enclosed rafter assemblies” seems to allow for using rigid foam board as the “air impermeable insulation” that goes directly below the sheathing in an unvented attic:

5.3.Where preformed insulation board is used as the air-impermeable insulation layer, it shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.

That makes me feel like this would be allowed if we spray, tape, or otherwise air seal along the edges of the foam, presumably where it meets the joists.

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  1. Expert Member
    BILL WICHERS | | #1

    We usually call that "use canned foam to glue in rigid foam panels" method "cut'n'cobble", and it's not a good idea to use in unvented attics as you described because the canned foam cannot be trusted long term, and any air leaks will allow moisture to accumulate behind the rigid foam.

    The reason spray foam is recommended here is because it is what is called "fully adhered insulation", which means there is no gap -- it is quite literally glued to the sheathing. There are no gaps behind it where moisture can accumulate, so no risk of moisture problems. In your particular application, closed cell spray foam is really the only option.

    If you really want to avoid the use of spray foam, see if you can make a vented roof assembly instead. You may be able to put small furring strips in the rafter pays to hold your polyiso 1" or more away from the roof sheathing, then use that as a vent channel from eave to ridge. If you have soffit and ridge vents, this vent channel will take care of any moisture that might get up there, so you can now put in polyiso with canned foam the way you describe and not worry. You can also use batts for cheaper R value between the polyiso and the interior of the attic if you want (although batts really need an air barrier on the interior side to be effective insulation).


  2. badmechanic | | #2

    Thanks, I'll look for cut n cobble GBA articles.

    For what it's worth, I was assuming we would use adhesive to glue the rigid foam board to the sheathing. And maybe mechanical fasteners, too. I'm not sure what adhesive people use for gluing polyiso to sheathing.

    Also I don't mind using small amounts of the spray foam the big girls and boys use rather than the great stuff. I'm trying to avoid using 1400 SQ ft * X inches of spray foam.

    1. Expert Member
      BILL WICHERS | | #3

      Adhesive behind the polyiso isn't enough. Spray foam IS an adhesive, absically -- there will be NO voids. You can't achieve that by gluing something else up.

      My recommendation would be to try to make a vented assembly, otherwise use closed cell spray foam. You don't have to use only closed cell spray foam though -- you can use enough to get vapor control (there are ratios to find out how much you need for this, those ratios are listed in some of the articles), then you can use "other stuff" for the rest -- batts, etc.

      Vented assemblies tend to be the least prone to problems down the road. Vented assemblies are simple and robust, which is why they're usually recommended over other options unless you really have no choice.


  3. Expert Member
    Dana Dorsett | | #4

    >"We are in the bay area 1 mile between climate zone 3 and 4.

    CA state climate zone numbers have no correlation to IECC climate zones, though in this case they are similar enough to be confusing. Most of the SF Bay area is within IECC climate zone 3C (marine) though parts of the east bay are zone 3B (dry). See:

    Compare that to the CA climate zone map used by CA building codes:

    In IECC zones 3B or 3C it's safe to use (cheaper and more environmentally friendly) open cell foam rather than closed cell foam directly on the underside of the roof deck. In the wetter more foggy 3C side it may be beneficial to use a "smart" vapor retarder on the interior facing side of the foam to limit moisture transfer rates which in some cases may result in rapid swings in attic air humidity levels over the course of a day- an issue much more common in the eastern "-A" climate zones. At 5" or more of depth it probably won't much matter in zone 3C.

    Cut'n' cobbled foam board is more likely to cause rather than prevent roof deck rot issues in your climate, but it is not an extreme hazard. If you go that route do NOT use foam with foil facers, which would block the roof deck from being able to dry toward the interior. If it's a tile or slate roof with inherently good drying capacity toward the exterior a foil facer may not be much of an issue in your climate. Fiber faced roofing polyiso would still have some drying capacity, but not foil faced goods.

    If it's possible to establish the 1" air gap (as recommended by Bill) it would meet code, if there is soffit to ridge venting in each rafter bay, but 1.5-2" is better, especially on roof pitches lower than 4:12.

    1. Greenbuild1 | | #8

      Hi Badmechanic. What did you decide to do with this? Hope it went well. To others on the thread, wouldn't taping the perimeter of each layer of foam to the rafters (ie tape on the rafter and foam, folded 90 degrees, likely from the bottom so the roof decking can be installed first) prevent the passage of air? Thanks much

  4. Expert Member
    BILL WICHERS | | #5

    >"If you go that route do NOT use foam with foil facers,"

    It's probably worth mentioning that a lot of EPS in the box stores has a poly facer, which is just as much of a problem in terms of vapor blocking as the foil facer on foil faced polyiso. I agree kraft/fiberglass faced polyiso would be best here, with UNfaced EPS a second choice. I would not use XPS here, since it is least vapor open in these thicknesses compared to the other two.

    I like to use 1x2s to make an approx 1.5" air gap when doing this sort of thing. I agree 1" is code minimum, but is really not enough. 1x2s go up fast with a finish nailer, and make it easy to establish a consistent vent channel quickly for minimal cost.


    1. brendanalbano | | #6

      If you end up using closed-cell spray foam, the blowing-agent concern can be addressed by using a spray-foam that uses HFO as its blowing agent. Demilec Heatlok HFO, Lapolla Foam-lok 2000 4G, Huntsman Proseal HFO are all examples that use the HFO blowing agent.

      It's still expensive, and the foam is still a petroleum product (but so is rigid board), but at least it's not the greenhouse gas catastrophe that the old blowing agents were!

  5. Deleted | | #7


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