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Community and Q&A

To open cell foam attic or not?

Travldalot | Posted in Energy Efficiency and Durability on

We live in NC (climate zone 3A)  in a 10 yr old 2 story home, with walk up stairway to attic (2nd floor is insulated, so attic floor is T&G, 3/4” plywood, that runs all the way behind the knee walls to the soffit area, and is glued and screwed down.  Attic venting is ridge and soffit vents. Attic ceiling is 10’ with 600+ sq ft of storage space, which could easily be retrofitted for another bedroom.  Heat pump mechanicals are located in attic knee wall space, as well as insulated flexible reflective duct work going from mechanicals to ceiling vents of 2nd floor bedrooms, for heating /cooling.
Like most attics, the temperature in the attic varies from the 20’s in winter to 110+ F in the summer.  So I was thinking of having the attic open cell foamed to keep the temperature more consistent.  In that way the mechanicals wouldn’t have to work so hard, to include temperature variations in the air in the insulated flexible duct work.  So I had an estimate from a local, reputable, professional icynene installer.  The company advised they would totally foam seal the attic, ceiling joists, ridge and soffit vents etc., and thus the attic temperature would not have the extreme temperature variations. I asked the contractor about moisture, mold, leaks and “overcooking” the asphalt shingles, and got a reasonable explanation re all four, which I won’t discuss here. However, in speaking to some friends who are engineers and/or contractors, they expressed concern the asphalt shingles would wear out a year or two earlier than they would, and how the attic would “breathe” and / or acclimate to humidity changes.
So my question is two fold. Would Green Build advisor members recommend I proceed with open cell foam, and if not, what other options do I have to achieve my goal of minimizing temperature variations?  I’m not really finding other options.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Travldalot.

    It sounds like you are considering an unvented roof assembly with open-cell spray foam. This is not generally recommended. You have other options, though the challenge in an existing home like yours is working with the available rafter depth, though you can make an improvement without reaching code levels of insulation, particularly if you do an excellent job air sealing. If you will be re-roofing in the near future, you have even more options. Here are some articles I would recommend for you to start with: How to Build an Insulated Cathedral Ceiling, Open Cell Spray Foam and Damp Roof Sheathing, and Insulating Behind Kneewalls.

  2. Travldalot | | #2

    Brian, thanks for your response. The links to the articles you forwarded are very informational, and based on reading them, open cell potentially not a good idea. It does follow that the humidity in a closed (open cell foam) attic has no place to go (unless you would run a dehumdifier to try and compensate for humidity levels. To retrofit each rafter space with a baffle and then insulate each space with fiberglass would be incredibly time consuming for a homeowner and most likely cost prohibitive, or impossible to find a contractor who would perform such work. Is there any options I'm missing, short of leaving well enough alone and tolerating the extreme attic heat in summer and cold attic temps in winter?? FYI, no plans to re-roof, shingles have another 10-15 years left in the same. ~ Thanks again for any and all comments.

  3. Expert Member
    BILL WICHERS | | #3

    You can safely use closed cell spray foam against the underside of the roof in an attic, you just don’t want to use open cell spray foam in this application.

    Once you’ve spray foamed your attic, the humidity levels in your attic should track the humidity levels inside your home since the attic becomes part of the conditioned space. It shouldn’t be necessary to run a dehumidifier in the attic if the attic is sealed properly from the outdoors.


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