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Spraying foam on the ceiling between the rafters.

TKKELLEY | Posted in Energy Efficiency and Durability on

The question is when going with foam installation on the attic CEILING between the rafters only not on the attic floor, which is recommended for attic CEILING open or closed cell.

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  1. Expert Member
    Dana Dorsett | | #1

    It can be either in some applications/climates, but in unvented roof assemblies going with closed cell is safer. The total R doesn't need to be as closed cell foam (which is pretty expensive at 17-18 cents per R per square foot, 2-4 time the cost of fiber insulation), but when splitting the R between foam & fiber insulation, the foam has to be tight to the roof deck, and sufficient R to limit wintertime condensation potential at the foam/fiber boundary. The minimum foam-R required (at the code required total R) is spelled out in TABLE R806.5 in the IRC 2012 by climate zone:

    The ratio of the foam R to the total R is important, since that is what determines the average temperature at the foam/fiber boundary. If you insulate to better than code, the foam-R has to increase proportionally. eg:

    The prescriptive foam-R for climate zone 5 is R20, but code-minimum for attics in zone 5 is R49. That makes the foam-R about 40% of the total R. If you are going for an R75 attic, you need to bump the foam up to R30 (40% of R75) or higher.

    Conversely, if you don't have sufficient rafter depth to hit IRC 2012 code min for a total R you can use proportionally less foam. In a zone 5 location if you have only 2x6 rafters, 2" of closed cell foam (R12-R13), and 3.5" of rock wool (R15) works, since R12/(R12+R15) gives it at least a 44% foam-R / total-R fraction, with a center cavity value of R27-R28.

    When insulating at the roof deck you must not use an interior side vapor barrier as tight as polyethylene- either half inch OSB (painted or unpainted) or latex-painted wallboard is about the right balance of vapor openness / vapor tightness.

    In climate zones 1-2 open cell foam works just fine. In zones 3 &4 it's iffy- in zone 3 it's advisable to at least spray=paint the foam with "vapor barrier" latex, or apply a "smart" membrane typevapor retarder (eg Intello Plus, Certanteed MemBrain) on the interior side. In zone 5 you can get away with open cell if you have plank roof sheathing and dark roofing (which uses the sun to speed drying) but it's risky with OSB or plywood roofing.

    Closed cell foam is blown with HFC245fa, which is a powerful greenhouse gas (~1000x greater than CO2), whereas open cell foam is blown with water, which has effectively zero climate impact. When using closed cell foam for roof deck insulation it's greener to go with a foam/fiber stackup for , and keep it to the minimum necessary for dew point control. If you can insulate ABOVE the roof deck, use rigid foam either EPS or polyiso (or a combination), both of which are blown with pentane (about 7x CO2 warming impact), but not XPS, which is blown with a combination of HFCs, the greatest fraction of which is HFC134a (the same as automotive air conditioning refrigerant), which is about 1400x CO2.

    When using rigid polyiso above the roof deck in climate zones 5 and up, derate it to R5/inch from a dew point control point of view which is less than the labeled R, since it's performance takes a dramatic hit when the average temp through the foam is below freezing. EPS performance increases at colder temps, and in multi-layers performance can be optimized by putting polyiso next to the roof deck, with layer of EPS over it, which keeps the polyiso at a warmer and higher-performing temperature. Eg: In zone 5 the prescriptive exterior R is R20, which would take 5" if all EPS, but only 3" if using polyiso at it's labeled-R. But since that 3" only performs at ~R15 during the season that matters it's not really enough. But if you went with 2" of polyiso (R12-R13), and 2" of EPS (R8) you have a fairly stable R20+, since when it's 0F outside the 2" EPS is performing at about R9.5, and keeps the polyiso layer warm enough to provide at least R11.

  2. GBA Editor
    Martin Holladay | | #2

    Dana is giving you good advice. Believe it or not, the last two times that you asked this question -- in this thread on 7/22/14 and on this thread on 7/23/14 -- I also gave you good advice. In both of those threads, I discussed which type of spray foam to choose when installing spray foam under roof sheathing (or, as you put it, on the attic ceiling). I also provided links to relevant articles which you can read if you want more information on these topics.

    In case you somehow missed my advice, here it is again.

    If you want to install the insulation between your rafters (along the sloping roof line), then spray foam makes sense. In most climates, closed-cell spray foam has proven to be less problematic than open-cell spray foam in this location, although open-cell spray foam can work if the details are right. Open-cell spray foam has a lower R-value per inch than closed-cell spray foam, and is more vapor-permeable.

    For more information on these issues, see:

    GBA Encyclopedia: Spray Foam Insulation

    Creating a Conditioned Attic

    Open-Cell Spray Foam and Damp Roof Sheathing

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