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Community and Q&A

Open cell vs closed cell

stanglx | Posted in GBA Pro Help on

I am renovating my daughter’s room located on the second floor of the house. I got a estimate from a spray foam company. They recommended 6 inch open cell in between my ceiling 2×8 rafters. On the west side exterior wall with a brick wall in the center of the wood frame (chimney) they want to apply 3 inch closed spray foam in 2×4 wall. I believe I am in zone 4a. My question is: is the open cell the right product and do I need a vapor barrier?

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  1. GBA Editor
    Martin Holladay | | #1

    According to the 2009 International Residential Code, the minimum R-value for ceiling insulation in Zone 4A is R-38. If you intend to use open-cell spray polyurethane foam, you need a minimum of 10 1/2 inches of spray foam to meet the code requirement; 6 inches would be insufficient.

    Three inches of closed-cell spray foam between your wall studs would provide about R-19 in the center of each stud bay. (The code requirement in your climate zone is for a minimum of R-13 for walls.)

    In your climate zone, either open-cell spray foam or closed-cell spray foam can be installed in walls or ceilings without the need of an interior-side vapor retarder.

    Your insulation contractor is suggesting that you install ceiling insulation that is less than minimum code requirements. However, in most jurisdictions, there is no requirement that you follow the building code when retrofitting insulation in one room of an older house. So installing skimpy insulation isn't illegal.

    Of course, it would be better if you added more ceiling insulation -- at least enough to meet minimum code requirements.

  2. stanglx | | #2

    Thank you for youre very timely response. Im not quite sure why he chose to spray only 6inches and not spray at least 8 inches. Now depending on how much he spras assuming it's 6 can I fill in the rest of thespace with batt? Also if I redo that section of the roof I can add rigid foam to get closer to the goal? Or is that going to interfere with the open cell system? He prefers the open over the closed because he said if water seeps through the open youll know where and will be able to repair where as the closed the water just colects or will travel elsewhere causing more problems. To me that makes sense. What do you thinkm

  3. Expert Member
    Dana Dorsett | | #3

    The reason they go for 6" is that it's the maximum you can spray in one lift without running into shrinkage & or fire-hazard-while-installing issues. (Some manufacturers spec 5" max, per lift, others 6", but none allow 7".) In 2x8 framing you have 7-1/4" nominal depth- not 8", so if they gave you a full cavity fill you'd be at about R27 for foam that tests at R3.7/inch, but at 6" it's only R22. To fill in the "extra" 1-1/4" with batting would give you another R5 bringing it up to R27, but still well shy of code.

    On the 2x4 walls going with the much more expensive closed cell may buy you an R20-ish center cavity, R, but with a typical 25% framing fraction the difference between open vs. closed cell is pretty marginal:

    At R3.7/inch open cell would give you the code-minimum R13, but with a 25% framing fraction the "whole-wall" R is about R9.5, including siding & gypsum.

    Going with 3" of R6.5/inch closed cell you have 3" of thermal bridging on the wood instead of 3.5", and even if it's R19.5 center cavity, the "whole-wall" R has only been elevated to R10.3.

    That's less than an R1 difference.

    If you can suffer the loss of 1/4" of interior space that difference can be made up at lower cost with a layer of fan-fold XPS siding underlayment under the gypsum. Siding underlayment usually has thin polyolefin facers reducing it's permeance to ~0.7-0.8 perms, a minimalist class-II vapor retarder, which would be fine for your climate, and would be offer BETTER drying capacity than 3" of closed cell foam.

  4. wjrobinson | | #4

    Not to be the Antichrist but I have built a couple homes with 5 inches open cell spray foam in the ceiling and the walls and the homes use half the fuell of any house I ever built before with three times the insulation in fiberglass.

    Adding up every insulation strategy that is blogged about in one assembly is nuts.

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