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Community and Q&A

Open cell spray foam risk in sloped ceiling application

dwk020 | Posted in General Questions on

Would anyone have any reservations in climate zone 5 to use open cell spray foam in a sloped ceiling application? One contractor is telling me that closed cell is not an option because: the closed cell spray foam has a more intense exothermic reaction while being sprayed. More heat is produced and because the cells are closed there is no way for the heat to dissipate correctly if you overspray/fill a section. In this event the foam with trap heat in its core and it can start a fire. This does not happen with the open cell spray foam because its cells allow the heat to dissipate easier so you fill a sloped sheetrocked cathedral ceiling without concern of trapping heat. The only way to spray foam the sloped sheetrocked cathedral ceiling sections with closed cell spray foam is to remove the sheetrock so that you have direct access to the roof deck and rafters like you do in the kneewalls and unfinished attic space.

I have never read or heard of this before. Any comments would be really apprecaited.

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  1. Expert Member
    Dana Dorsett | | #1

    HFO1234ze- blown closed cell foam can be applied in very high lifts without much fire risk, and is substantially greener than the high-exothermic stuff blown with HFC245fa that can catch on fire when installed in lifts of more than 2" per pass.

    HFC245fa has a very high lifecycle global warming potential (on the order of 1000x CO2), HFO1234ze has practically none.

    There are (HFC245fa blown) 2lb pours that might work, but half-pound open cell pours aren't particularly less risky.

    Any type of poured or sprayed solution into a closed cavity has lots of quality risks. Shooting open cell foam several feet into a rafter bay from one end invariably has voids created by droplets that catch then ends of nail points then expands, blocking spray from passing in to the not-yet filled cavity. Half pound or 2lb "slow rise" pours will fill the space, but can easily overfill, bow-out the ceiling gypsum (or BLOW it out), and adheres to it, making fixing it more difficult.

    Compare the costs of blowing the rafter bays full of cellulose then re-roofing with enough exterior polyiso to be at least 40% of the total R to stripping the ceiling gypsum and installing HFO blown closed cell foam. The HFO blown goods run about R7/inch, and can be blown in lifts of at least 5", usually as much as 8" without fire risk. Demilec HeatlokHFO High Lift is one such product, Lapolla Foam-Lok 2000-4G is another, but there are others.

  2. dwk020 | | #2

    Thanks for your response Dana - I don't think re-reroofing is an option at this point. What about just blowing in the cellulose in those portions and leaving that as the only solution. I know there is some vapor risk, but isn't it mitigated by the ceiling if air sealing is done well?

  3. Expert Member
    Dana Dorsett | | #3

    With dark shingles on sunnier-side roof pitches dense packed cellulose (or open cell foam) might work. On north facing or shadier pitches it's pretty risky in a zone 5 location. But as little as 2" of closed cell foam with the rest being cellulose is protective of the roof deck, even on north facing pitches. It's worth reading this short set of north-side WUFI simulations with different insulation stackups performed by the folks at Building Science Corporation before doing any unvented solution outside the realm of what the IRC code prescribes:

    Cutting to the chase, in Table 3 you'll see in the Chicago 5A row, with dark asphalt roofing the all cellulose columns still shows some risk on the north facing pitches, above 28% moisture content for more than 4 weeks in a high indoor humidity profile, still above 28% for one week in the lower humidity profile. But with even 1" of closed cell foam (the 1" ccSPF + spray fiberglass column) the risk drops considerably, to pretty much zero risk for the roof deck, with some risk if the roofing is light reflective metal. At 2" ccSPF (the next column to the right) the roof deck is safe even with the reflective roofing.

    How deep are your rafters?

  4. dwk020 | | #4

    Thanks again for the response. I had read this once before and it all seems like a calculated set of risks when you are trying to retrofit. I don't know off the top of my head the depth of the rafters, so I'll have to get back to you.

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