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Community and Q&A

Insulating the roof deck with both closed-cell and open-cell foam?

MParlee | Posted in GBA Pro Help on

I am consulting on a home where the HO wants to have the insulation at the roof deck. They are thinking 2” of closed cell followed by 6” of open cell.

The HO put forward this idea.

2″ closed cell at the roof first to provide the vapor barrier then followed by 6″ of open cell which will be enough to encapsulate the top chord of the roof truss and minimize thermal conduction.

Does anyone have any experience with this type system?

Is there any concern of the two layers of foam de-bonding from one another?

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  1. GBA Editor
    Martin Holladay | | #1

    Climate matters. I know that you live in Iowa, so I'm going to assume that we are talking about climate zone 5 or 6.

    First of all, I know that it's possible to spray open-cell foam on a layer of cured closed-cell spray foam. Joe Lstiburek sometimes recommends that approach. When I interviewed him for an article on insulating low-slope roofs, Lstiburek said, “We fix the problem roofs ... one of two ways. ... The usual way we repair them is from the inside. We take out the gypsum ceiling and the insulation, and we spray 2 or 3 inches of closed-cell spray foam on the inside of the roof sheathing and the inside of the short walls. ... Then we repair the ceiling and we blow the space full of cellulose. If the space is too deep to fill with cellulose, we sometimes blow low-density spray foam over the high-density spray foam, because low-density foam is cheaper than high-density.”

    The only tricky part of the solution you are suggesting is making sure that the ratio of low-permeance insulation (the closed-cell foam) to high-permeance insulation (the open-cell foam) is correct. In theory, if there is too much open-cell foam (or too little closed-cell foam), then the inner surface of the closed-cell foam might be cold enough in the winter to permit condensation. One consolation, however: if any condensation occurs, it will be on the cured closed-cell foam rather than vulnerable roof sheathing.

    For more information on doing the calculations to reassure you about the ratio between your closed-cell and open-cell foam, see these two articles:

    Calculating the Minimum Thickness of Rigid Foam Sheathing

    Are Dew-Point Calculations Really Necessary?

  2. davidmeiland | | #2

    Mark, can you get a foam manufacturer to state in writing that their open cell product will stick to another company's closed cell product, or does one manufacturer make both products and agree that they work together?

    I'm sure it would cost more, but you could go with 6"+ of closed cell and get better R value overall and covering the trusses, eliminate any dewpoint issues, and eliminate any bonding issues. Of course you need to make sure they don't spray too much at once and cause problems that way.

    Seems like a manufacturer could provide a solution for you on this one.

  3. MParlee | | #3

    The foam is from the same manufacturer and the letter is a good point.
    I am early to this one and the insulation is a side issue as part of the consulting. I went to review the framers work or lack thereof.
    The report is in preliminary stages and is 17 pages long at this point but I am sure will balloon to 30 plus pages; of course most of this is captioned pictures.
    Martin, Thanks for the link, Will let you guys know what happens as we proceed.
    If you think of anything else please do not hesitate.

  4. kvconnors | | #4

    I've got a project where a new garage attic roof is spec'd to have 6" closed cell on the roof deck, then open cell for the balance of 2x12 depth (Z5). The garage will be heated to 45-50 degrees in winter (radiant slab), no conditioning in summer.

    One insulation contractor said he wouldn't touch it. I'm still waiting to talk to him about why. I've been reading about high humidity levels and "buoyancy" but don't know the physics behind it. It would be ambient temperature in summer (close to 100F and high humidity), but I don't see what would cause humidity to collect under the hip roof. Do you have any suggestions?

    1. kvconnors | | #5

      To be clear, closed cell is on underside of the sheathing.

  5. kbentley57 | | #6

    That would have to be accomplished in several layers, unless something has changed in the last decade that I'm not aware of. He may view it as not worth the time to come back over several passes, else the bid would be too high to make it a viable option, depending on which product he uses.

    As far as insulation goes, that would provide something like an R50, so not outside the norm of well insulated attics, but certainly something that's outside what his normal operations are.

    The links below should get you started on the right things to google.

    1. kvconnors | | #9

      Excellent Kyle! Yes, what we're showing now would be 3 passes. I think it could be done in 2.
      Appreciate your input and the links!

  6. kvconnors | | #7

    BTW, I spoke to someone from SPFA about the detail and he had no objections - said the closed cell could be less than the 6" CCPF (they have a document - SPFA-147 - on Hybrid systems for cold climates) that might interest folks. Based on closed cell + fiberglass, but swapping open cell would be similar. They charge for it on-line, but he emailed me a copy.

  7. Expert Member
    Akos | | #8

    Your suggested assembly will work great. You can read about it here:

    Also look at Table2 which shows how much closed cell foam you actually need, you can probably go less than 6".

    Closed cell foam with batts for the rest of the cavity also work very well and might be less cost than the open cell top up.

    1. kvconnors | | #10

      Akos, TY! Yes, that's what the SPFA guy suggested and their literature is based on FG batts. I'll suggest that to save cost.

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