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Foil faced ISO over 3″ closed-cell spray foam?

dc429scj | Posted in Energy Efficiency and Durability on

Looking for an answer before I start this project. I live in northern MN. I had my attic spray foamed 4″ behind knee walls, 3″ in the living space. This is a 1 1/2 story with a non-vented roof. I want to know if its O.K. to use foil faced 1″ rigid sheets to cover studs [Editor’s note: I think David means “rafters”], with the foil facing down between the knee walls. I did plan on filling all voids and foil taping the joints. Then 1/2″ sheetrock over the foam. Just want to make sure this isn’t considered a double vapor barrier, and if this would be considered good construction.


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  1. GBA Editor
    Martin Holladay | | #1

    I thought you were talking about a kneewall, until you said that the "foil is facing down." Is this a wall, or a floor, or a flat ceiling, or a sloping ceiling?

  2. GBA Editor
    Martin Holladay | | #2

    Although I don't fully understand your question, I'll provide some general information that might be helpful.

    If any GBA readers are confused by the term "ISO," it is one manufacturer's brand name for polyisocyanurate, a type of rigid foam.

    It sounds as if you want to fill some framing cavities (perhaps studs, joists, or rafters -- I'm not sure which) with closed-cell spray polyurethane foam. It also sounds as if you want to install a layer of polyisocyanurate (rigid foam) on one side of the assembly.

    There's no reason you can't do that, as long as you aren't sandwiching any plywood or OSB between the two types of foam.

  3. dc429scj | | #3

    This is for the pitched roof in between the knee wall. 2x4 rafters with 3"-3 1/2" closed cell spray foam already sprayed in. Behind the knee walls I had them spray right over the rafters for R-28. Between the knee walls its considered R-21 minimum and I was looking to save an inch of ceiling height by using 1" foil faced sheets instead of 2" pink foam. If my math was right my whole roof would be right around R-28 when done. I could have gotten R-31 with the pink foam, but space is limited. I started reading on vapor, air, and thermal barriers and got a little lost. If this information changes your advise, let me know.

  4. GBA Editor
    Martin Holladay | | #4

    Since your house is in northern Minnesota (Climate Zone 7), you really want to strive for a higher R-value than you are planning. The 2009 IRC requires a minimum of R-49 of ceiling insulation or roof insulation in your climate zone, and you have only R-28.

    If there really is no way to install more insulation from the interior of your house, you should consider adding more insulation above your roof sheathing the next time that your re-roof your house.

    Your use of the phrases "behind the kneewall" and "between the kneewall" is still confusing. But, to answer your question, it is permissible to combine closed-cell spray foam between the rafters with foil-faced polyisocyanurate under the rafters. Remember that your local code inspector may require any spray foam to be covered with drywall.

  5. dc429scj | | #5

    Thanks for the answer. Behind the knee wall would be from the knee wall to the edge of the soffit. Between the knee walls is the living space from knee wall to peak. Since this is a 1 1/2 story house I have about 11' between the knee walls. Without tearing off the roof, R-49 would not be possible. I would lose all usefulness of the space. All the exposed spray foam is behind the knee walls. I will sheetrock between up to the peak of the roof. The roof when I started was R-11 fiberglass from 1950's completely covered in cardboard. I even had some rafters missing insulation. I had an ice dam last year, but only in the spot that was missing insulation. I will definitely go for more R-value when I can afford to do the roof and hopefully only when it needs replacing. I will be tearing the attic floor off, would it help anything to insulate the floor at all? Thanks for your help.

  6. Expert Member
    Dana Dorsett | | #6

    While 3.5" of closed cell foam is semi-impermeable at about 0.35 perms, it is still an adequated drying path for the roof deck. But putting foil-faced polyiso on the interior would reduce that to about 0.02 perms, locking both the rafters and the roof deck into a "moisture trap", between the ~0.1 perm (and often wet) and the even lower permeance interior side.

    You could get away with an inch of unfaced Type-II EPS on the interior side, which would only bring the total permance down to about 0.032 perms.

  7. dc429scj | | #7

    That's where I get a little lost. What's more important, R-value or permeability. With the one inch foil foam I can get to R-28, with 2" pink foam I get R- 31. Both of those would lose permeability. If I do the 1" foam like suggested I can get R-26 and keep some permeability. So my next question would be, which of these three is the best option? Let me know. THANKS!

  8. GBA Editor
    Martin Holladay | | #8

    If you have already hired a spray-foam contractor to install closed-cell spray foam on the underside of your roof sheathing, than you are beyond the point of caring about vapor permeance. Your closed-cell spray foam is a vapor barrier, so your roof sheathing isn't going to be drying inward.

    If you want to install rigid foam on the underside of your rafters, then I think that the vapor permeance of the rigid foam is almost irrelevant. So, to answer your question: in your case, the R-value of the rigid foam matters more than the vapor permeance of the rigid foam.

    That said, you should know that EPS and polyisocyanurate are considered to be more environmentally friendly than XPS.

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