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Open cell vs closed cell spray foam insulation in my attic

user-7032327 | Posted in General Questions on

I am insulation an old brick house with closed cell on the exterior walls and I am tying to decide between open or closed cell under the roof deck in the unvented attic. Most insulation contractors are recommending open because it’s cheaper and also because if I go with closed cell and the roof leaks it will not penetrate the insulation and could remain stuck between the sheathing and the insulation and could rot the wood and or travel to a more hidden spot of the roof or attic causing worse problems that are harder to notice than if the roof leaked and water penetrated through the open cell which I would be able to see and fix if necessary. I’m putting on a new roof so assuming the roof is water tight and not leaking should I go with open cell? Or maybe a hybrid of a thin layer of closed cell on the roof deck and the rest of my r value in open cell? Anyone have thoughts on this? I read Martins article which makes total sense but it doesn’t address the notion of closed cell being problematic if the roof leaks and can’t penetrate the insulation. I guess the question is whether closed cell is worth the extra money etc. thanks!

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Replies

  1. Brendan Albano | | #1

    The answer is somewhat climate specific, so if you tell folks what climate zone you're in you will get better advice.

    But the general wisdom I've seen thrown around is that the roof leak issue with closed-cell is overstated and the best choice is usually a hybrid approach of an appropriate thickness of closed-cell foam for your climate zone, plus open-cell foam, fiberglass, or cellulose to make up the rest of the R-value you are targeting.

    Also, if you do use closed-cell, take a look at the new closed-cell foams that use a low-global-warming-potential blowing agent, as the blowing agents in conventional closed-cell foams are real bad from a global warming perspective. Two products to look at are Lapolla Foam-lok 2000 4G and Demilec Heatlok HFO High Lift

  2. Steve Knapp CZ 3A Georgia | | #2

    Closed cell foam is less risky. See this article for additional information: https://www.greenbuildingadvisor.com/blogs/dept/musings/open-cell-spray-foam-and-damp-roof-sheathing

    You may be able to safely apply a combination of closed cell foam and air permeable insulation, as Brendan suggested. See this article for additional information: https://www.greenbuildingadvisor.com/articles/dept/musings/flash-and-batt-insulation

    Insulating an old brick building can be challenging. See this article for more information: https://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-old-brick-buildings

  3. John Clark | | #3

    You could do ocSPF (open cell) IF you installed diffusion vents* and/or a small dedicated supply duct (if code allows), in any case the link below explains the theory behind why using only ocSPF might cause issues with regards to humidity.

    https://buildingscience.com/documents/building-science-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer

    The “diffusion vent” is an open slot at the ridge and hips, covered with a water-resistant but vapor open (500+ perm) air barrier membrane.

  4. Expert Member
    Michael Maines | | #4

    John, I'm excited about BSC's research into diffusion vents, and just saw Kohta Euno present at the NESEA conference. It's important to note that they tested standard high-perm membranes such as Typar, at ±40 perms, and found that they did not work--it really needs to be a super-high perm product, as you note, which is not as easy to find. Of course diffusion vents are not allowed by code, with air-permeable insulation, but that may change with enough research.

  5. user-7032327 | | #5

    Thanks guys - the house is in upstate NY.

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