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I have a friend who is remodeling his attic in Columbia, SC and his builder is telling him that all he needs for insulation

nerdbuilder | Posted in Energy Efficiency and Durability on

is 5.5 inches ofopen cell spray foam and that this will perform at R49. Of course his existing rafters happen to 2x6s. I recomende furring down and blow in netted cellulose or R30C. Any comments

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  1. jklingel | | #1

    R 8.9/inch, huh. Pretty wild. I don't recall ever reading a claim that high. Maybe so, but I'd get a second opinion. Will the open cell foam be vapor retarded, like w/ paint? I think it is pretty vapor open as is, and I'd check w/ an experienced person locally to see if it should be painted or left alone in SC.

  2. Expert Member
    ARMANDO COBO | | #2

    5.5” of OC Foam is R20, PERIOD!!! It does NOT perform as an R49, that’s pure BS; R-value is R-value. Your builder friend needs to learn the code, see 2009 IRC R806.4. To install OC Foam under the roof decking of an UNVENTED ATTIC you need 7” AND 1” R5 rigid foam above the roof decking, for a total of R30 in Climate Zone 3, where SC is located.
    You can not use netted cellulose either, as it is not code to use PERMEABLE insulation only on an unvented attic.

  3. nerdbuilder | | #3

    Thanks guys,
    I guess I should have clarified my solution. In the cellulose application I was recommending that it be furred down enough to have an airspace and then of course continuous ridge venting. I am extremely leery of the OC foam for the exact reason you say- the roof decking insulation requirement. My friend does not want to spend the money taking off the existing roof in order to get the required vapor barrier / R value on top of the structural deck. That is why I see no choice but to fur down to get the necessary depth for R30 and venting.
    Thanks again for your research on this.

  4. nerdbuilder | | #4

    More food for thought,
    The first solution I had was fur down to get 2" airspace, then R30 batts, then foamboard taped and sealed. This is pretty much a direct spec from Joseph Lstiburek "Builder's Guide to Mixed Humid Climates". The only problem is my copy is the Revised 2000 edition and I worry that I might be telling my buddy to use a system that is no longer recommended. I tried looking on, but just haven't had the time to read all the data in those studies. Also I've been reading the thread on this website about the poor guy who is having all the trouble with his 5.5" icynene application and that really makes me worry about not having the insulation on top of the roof sheathing.

  5. Expert Member
    ARMANDO COBO | | #5

    Your friend has two other options for an unvented attic, again see R806.4: 1. Install CC Foam under the roof decking and OC Foam under the CC Foam, in all to R30, or 2. Install all R30 CC Foam under the roof decking. CC Foam needs to be installed at no more than 2” thickness at a time.
    For a vented attic, he can install baffles at the perimeter, vents at the top, and blow FG or Cellulose insulation to an R30 depth. It’ll be wise to air-seal the attic floor and all openings with calking, can foam or spray foams like

  6. homedesign | | #6

    Armando, I am not exactly a fan of Foam......

    However, my understanding is that open cell spray foam is NOT air-permeable.
    In other words it would be possible to meet the Code with "enough" Open Cell Foam and NO Rigid insulation above

    Do you have a source that says Open Cell foam is Air-Permeable?

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