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Is closed cell spray foam between rafters in an old house a good idea?

ThriftyMom | Posted in GBA Pro Help on

To all, but especially to Dana Dorsett and Martin Holladay,

Thanks again for answering my question “Ok to put rigid foam between rafters…” on Nov 9, 2016. To repeat for everyone what our situation is: We live near Chicago, so in Climate Zone 5, I think. We need a new roof, and we also need to add more insulation to our attic. Right now there is rock wool between the joists of the attic floor, but it is insufficient. We get ice damming, and we are cold in winter, hot in summer. Ventilation is probably insufficient in our attic, because an addition we put on 4 years ago eliminated all the eave/soffit vents along one side of the house. I would very much like to convert our attic from vented to unvented so that it could be used for storage and keep a more consistent temperature for that storage. The highest height of the attic is 45″, so it would be preferable not to lose much height.

Thank you for telling me about the article “How to Install Rigid Foam On Top of Roof Sheathing.” I think this would be the ideal way to go if the cost could be minimized; but, I have contacted more than 15 reputable residential roofing contractors, and none of them are willing and able to help me do this. Many are not sure what I am asking them to do. Some that have done at least similar projects tell me that it would be extremely costly because the extra thickness would require them to build new fascia board or something like that, and it would dramatically increase the labor and materials costs for re shingling our roof. It wouldn’t be worth the cost, since it is just a storage area you can’t even stand straight up in.

In your article “How to Build an Insulated Cathedral Ceiling,” you say one acceptable way is to “Install closed-cell spray foam against the underside of the roof sheathing, and no other type of insulation.” I contacted our city (Wheaton, IL) and someone there said the code requirement would be R38. Several spray foam contractors have said that for around $5,000 they could spray closed-cell foam to fill the ~5.5″ deep cavities between our rafters, which would meet the R38 code because (R7/ inch)(5.5 inches) = R38.5. To me, this might be worth the cost since a storage area with somewhat comfortable temperatures would be very helpful to us. Do you think this would be a smart thing for us to do? What depth of closed cell foam do you advise? Should I attach a layer of rigid foam board to the bottom edge of the rafters? How thick? I know I have to cover the foam with a layer of drywall or attach a foil rated fire barrier.

Two last questions: several of the spray foam contractors have suggested to me that open-cell foam would be a better solution for us because it would enable us to tell if a leak in the roof occurs, plus it would be easier to repair the roof if needed, plus it would save us some money. They said that with closed cell if a roof leak develops your roof may rot and you won’t know, plus if roof repair is needed it is very difficult b/c the closed cell is like concrete. In your “How to Build and Insulated Cathedral Ceiling” article, you say that open-cell spray foam is risky in all climate zones. Why would open cell be risky? Is closed cell truly risky in the ways they told me it is? (I am bewildered by these conflicting opinions, and I am unsure where to turn for wise advice.)

Thanks so much for your time.

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  1. Expert Member
    Dana Dorsett | | #1

    To do it safely and with high quality the 5.5" of closed cell foam would have to be applied in a minimum of 2 passes, and with some foam products 3 passes, which takes time, and costs more than $5K.

    R7/inch is a myth, no matter what the spec says. Reality is R6 maybe, but R5+ for sure.

    R38 that is thermally broken by 5.5" deep rafter in no way matches the performance of R38 broken by 11.25" rafters, since the framing fraction is only about R6 at 5.5" but is about R12 at 11.25". The performance of high R/inch foam is essentially wasted by the low-R thermal bridging over 7-10% (depending on rafter spacing and blocking details) of the surface.

    Open cell foam is risky to the roof deck since it is far more vapor permeable, and the roof deck (particularly on north facing pitches) loads up with enough moisture over a winter to develop rot/mold issues over the long term. But as little as 2" of closed cell foam followed by open cell foam or fiber insulation to fatten out the R value is more than sufficient for protecting the roof deck due to it's lower vapor permeance.

    Alternatively, the open cell foam can be covered on the interior side by a Class-II vapor retarder (or a "smart" vapor retarder). Since open cell foam is air tight, there is no risk of air-transported moisture- it's all vapor diffusion. Using "vapor barrier latex" primer on wallboard, or 2-mil nylon (Certainteed MemBrain) would be protective of the roof deck with an open cell solution.

    But at 5.5" (about R20) and no thermal break over the rafters there would still be some ice damming potential. If you pre-installed some 1.5" wide strips of 2" thick rigid polyiso to just the rafter edges, held in place with 1x3s through-screwed to the rafters it adds 2.75" of depth to the cavity, for a total of 7.75". Filling it with open cell foam would deliver ~R29 in the middle, but the R12 polyiso edge strips triple the R-value of the thermally bridging rafters, which lowers the ice damming potential considerably compared to the 5.5" closed cell solution. To do ~8" of open cell foam safely and with high quality requires 2 passes.

    You would still need to use vapor barrier latex or 2-mil nylon, but it should be quite a bit cheaper (and a heluva lot greener) than 5.5" of closed cell foam. It's less than half the polymer, and uses water instead of HFC245fa (an extremely powerful greenhouse gas) as the blowing agent. With the furring through-screwed to the rafters the ceiling gypsum can be fastened to the 1x3s

    If you can find a roofing contractor capable & willing to install a 2" polyiso nailbase panel (not a huge facia board detailing problem) that would add another R8-R9 to the stackup (including over the rafters), and it would be ~R38 between the rafters, but it would outperform R38 fluff between 2x12s with good measure due to the thermal breaks over the rafters.

  2. GBA Editor
    Martin Holladay | | #2

    Lots of issues here. You can trust the advice you get on GBA. Unfortunately, contractor ignorance is a big problem, and many roofers just want to continue to work the way they were taught by their grandfathers.

    It's true, though, that a well insulated roof is expensive. There's no getting around that fact.

    For more information on the risks of open-cell spray foam installed on the underside of roof sheathing, see these two articles:

    High Humidity in Unvented Conditioned Attics

    Open-Cell Spray Foam and Damp Roof Sheathing

    Dana gave you several options to consider. Good luck finding a contractor who will perform the suggested work.

  3. ThriftyMom | | #3

    Thanks, Dana and Martin! I will ponder all of your good advice while I go and attempt to further de-clutter our home (and hopefully reduce our need for storage). If it ends up we still decide to get the storage in the attic, now I am confident we won't make any foolish mistakes. Thank you for being there. I hope lots of other people get some good ideas from reading this.

  4. Expert Member
    Dana Dorsett | | #4

    I meant to mention: Standard 3" drip edge flashing would be good enough to deal with 2" nailbase foam panels without needing to re-work/replace the facia. The exposed 3" of metal may present aesthetic issues to some people, but it's not really such a big deal. Only when you go fatter with the foam do you end up with a significant extra expense on the facia details.

  5. ThriftyMom | | #5

    Thanks, Dana! I will tell this to the contractor who is coming by today to give us an estimate.

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