GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Closed-cell foam in attic (interior roof deck) – Zone 4 (borderline Zone 5)

mack0911 | Posted in General Questions on

Would the use of closed cell foam on the interior attic roof deck prevent inward drying potential?  The roof has 15# felt, and the water/ice membrane is only at the eves.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Stockwell | | #1

    Yes(at any significant thickness). Asphalt roofing prevents outward drying potential as well. However, this is a roof. The goal is no need for drying of any sort. It's one of the times you can sandwich sheathing between two impermeable surfaces and be fine.

  2. mack0911 | | #2

    Thank you very much Kevin. What would you recommend for thickness? Contractor is proposing 4.25" closed cell (I have 2x8 rafter). Is that thick enough to address dew point control? Also, I don't know what to do to address thermal bridging since rafters won't be covered entirely. Probably only 3 inches left after the foam is sprayed.

    1. Expert Member
      Dana Dorsett | | #4

      4.25" of closed cell foam is nowhere near code minimum performance on either an R value or U-factor basis.

      With 2x8 rafters and R30 rock wool in the cavities you can get there with 3.5" of rigid polyiso above the roof deck and have reasonable dew point control at the roof deck (enough for zone 5) no interior side vapor retarder needed and would beat code on a U-factor basis by quite a bit, if only slightly ahead of code on an R value basis.

      1. mack0911 | | #6

        Dana - in my situation, I am doing a retrofit of my existing attic so applying rigid foam above the roof deck wouldn't be an option. So I am left with 4.25" of closed cell between rafters or 8-10" of open cell. My HVAC equipment is located in the attic so based on what I've read it seems the best option is to insulate/air seal the interior roof deck. I am a bit confused since the spray foam contractors I've spoken with have different opinions. One says you should only use open cell because you can find roof leaks, and the others say use closed cell because it stops/minimizes water vapor infusion. None have addressed the issue of thermal bridging other than to over spray the open cell to 10". Should I go with the closed cell at 4" inches, fill the rest of the cavity with R15 rockwool (would stick out about 1/4 of inch assuming 2x8 rafters are 7 1/4 deep) for thermal barrier? What would you recommend for thermal bridging?

        1. Expert Member
          Dana Dorsett | | #7

          Do you have sufficient room to add insulation strips to the rafter edges? If yes...

          1> At 3" of HFO blown closed cell foam you have sufficient dew point control for up to R30 of fiber insulation, and that would leave 4.25" of remaining depth in the existing rafters. Installing 1.5" wide strips of 1.5" foil faced polyiso cap-nailed and glued to the rafter edges you'd have 5.75", enough depth at center cavity for friction-fitted R23 rock wool, and would have an R9 thermal break over the rafters, bring the framing fraction up to about R17-R18. Even though the center-cavity R would be about R43-R44 it would probably still meet code-min performance on a U-factor basis, depending on your actual framing fraction.

          If the batts are to be left exposed, applying a layer of PERFORATED aluminized fabric type radiant barrier on the underside to keep them in place would add another ~R1 of performance without creating a moisture trap. Even though radiant barrier isn't necessarily "worth it" for the additional R1 on an energy savings basis it's cheaper and easier to install than gypsum board, which is only good for R0.5. Most perforated RB is in the ~5 perm range, comparable to the vapor permenace of interior latex paint on wallboard.

          2> If you need to be able to mount gypsum board for a finished ceiling, 1.5" wide strips 3/4" polyiso bonded to 1x furring would give you that depth along with a nailing surface. These have been dubbed Bonfiglioli strips, named for a proponent of this method. (See: ) With a the 1.5" thick Bonfiglioli strip the framing fraction is about R14.5, (which is still higher-R than a milled 2 x 12), and wouldn't meet code min on a U-factor basis unless the rafters are 24" o.c. and fairly long, for a lower than average framing fraction.

          Even at box store pricing R23 rock wool is about $1.10 per square foot (material cost), whereas an inch of HFO blown foam runs north of $1.3o per square foot (installed cost). Backing off to 3" rather than 4" pretty much pays for the material cost of the rock wool + edge strip polyiso + perforated RB. But the performance of 3" foam + 5.5" of rock wool + edge strips is substantially higher than that 4" of HFO blown foam, or even 6".

          >3 With 2" of HFO blown foam there would be 5.25" of depth remaining. Installing an R23 batt in there and even compressing it would deliver at least R22 on the batt (retored to R23-ish if covered with perforated RB), R14 on the foam for R36-37 total. That's still a huge dew point margin for zone 4, and almost enough for zone 5, and would outperform 4.25" of closed cell foam at dramatically less money. But it would still be well below code.

          >3a Adding 1" polyiso + 1x furring (1.75" total) Bonfilioli strips would bring the cavity depth up to 7.0". Compressing an R30 rock wool batt into that 7" cavity would perform at about R29, restored to R30 if held in place by perforated RB. With R14/R44= 32% of the total it would still be just fine in zone 4. The framing fraction would run about R15.5. The ~5 perm RB would slow down wintertime moisture accumulation in the fiber by quite a bit compared to exposed batts.

          I think I'd run with #3a, betting on climate change keeping it in zone 4, perhaps deeper into zone 4 over the lifecycle of the house.

          1. mack0911 | | #10

            Dana - great information. See attached pictures of my attic to give you sense of what I’m working with.

  3. Stockwell | | #3 If Martin was here, that is the article he would link you to. I am building in zone 4 and using 2.5" of closed cell(roughly R-15) and R-34 worth of open cell over that to get to the recommended R-49. Your 4.25" of closed cell is plenty for dew point control. You could use 3" of closed cell and fill the rest with open cell for about the same cost. Some people have used strips of foam board on the rafters to combat thermal bridging.

  4. jackofalltrades777 | | #5

    I would add that once the underside is sprayed and then when they are about to install the roof. Make sure the OSB sheathing is dry. Problem is if it is wet and they install the asphalt roof and the underside is spray foamed. The sheathing has nowhere to dry and can start to rot. This happened on a commercial project and resulted in a rotted & failing roof a few years later.

  5. Jon_R | | #8

    See below where BSC writes "... roofing system using ccSPF on plywood sheathing with cellulose insulation on the interior has the capability according to WUFI to safely dry a leak up to 0.6% of the rainfall ..."

    In other words, this roof design does allow some inward drying. Outward drying wasn't considered, but IMO, felt can allow a relatively significant increase in total drying ability (perhaps to 1%). Low perms aren't zero perms - they make a difference.

    1. Jon_R | | #21

      Data on outward drying of a asphalt shingled/permeable membrane roof (figure 20). It helps.

  6. Jon_R | | #9

    A question - does the use of closed cell foam on the interior attic roof deck prevent reasonable repairs when the roof eventually leaks?

  7. GBA Editor
    Martin Holladay | | #11

    Q. "Does the use of closed-cell foam on the interior attic roof deck prevent reasonable repairs when the roof eventually leaks?"

    A. If there is sheathing rot in the future, I wouldn't say that closed-cell spray foam "prevents" repairs -- but it certainly complicates the repairs. I can imagine that the roofers might use a few words that small children shouldn't hear. Here are the steps:
    1. Locate and mark the rafters.
    2. Cut out the rotten sheathing -- a rectangular cut, back to solid sheathing -- with a circular saw set to 1/2 inch or 5/8 inch.
    3. Use a Sawzall to cut the spray foam by inserting the Sawzall through the circular saw kerfs, and taking care not to cut the rafters.

    1. mack0911 | | #12

      Martin/Dana - considering the complications of roof repair caused by the use of closed cell foam on interior roof deck, would it make more sense to spray foam the attic floor instead? I should also mention that I’m getting solar panels which will cover most of the roof. Not sure if this changes the choice of insulation options. Let me know your thoughts.

      Thanks again for all the suggestions thus far (from everyone).

    2. Jon_R | | #15

      So the foam is removed along with the sheathing. How does one put this back together in a way that prevents interior air from reaching the sheathing? Cut and cobble rigid + canned spray foam + tape the new sheathing joints? A DIY two part foam kit from above?

      It seems to me that it would be ideal if a strong bond between the original foam and sheathing could be prevented - allowing the sheathing to be pried off the intact foam. Or I suppose one could proactively replace the shingles at 15 years, hoping to avoid sheathing rot.

      IMO, repair-ability and long life are part of green building.

  8. GBA Editor
    Martin Holladay | | #13

    In Comment #6, you wrote, "My HVAC equipment is located in the attic so based on what I've read it seems the best option is to insulate/air seal the interior roof deck." That reasoning was correct.

    If I were you, I would proceed with your plan to install closed-cell spray foam on the underside of the roof sheathing (since installing rigid foam on the exterior side of your roof sheathing is evidently impossible).

    1. mack0911 | | #18

      Martin/Dana - I recently had solar panels installed which pretty much cover the whole roof. Does the fact that there are much more roof penetrations due to solar panel install, change your thoughts on spraying the underside of roof deck?

      1. GBA Editor
        Martin Holladay | | #19

        It's hard to assess the situation without a site visit. In theory, your roofers knew what they were doing, and you have durable roofing with high-quality flashing. In theory, your solar installers knew what they were doing, and all of the solar panel mounts are properly flashed.

        If this is true, there is no reason that your roof should leak, and you should go ahead and install insulation under the roof sheathing.

        That said, if your roofing is old, or your roofers were unskilled, or your solar installers were sloppy, you may have problems. There is no way for me to guess the details of your situation.

        1. mack0911 | | #20

          Martin - the roof is approx 5 years old, however, last year I had a leak due to a nail pop. I only discovered it because the leak was directly over the attic hatch door so it leaked through. The leak was fixed but i am worried there may be other nail pops in the future. Should I just do the best I can adding more insulation to the attic floor and accept the lower efficiency (since HVAC is in the attic)? Or is spraying closed cell on the roof deck worth the risk?

  9. mack0911 | | #14

    Thank for the quick reply Martin. See attached pics of attic. Do you see anything that would complicate the plans to use closed cell? Dana's recommendations seem to be the best course of action.

  10. mack0911 | | #16

    Hello all - just wanted to share the following comment from a spray foam contractor here in NY (Long Island - Zone 4/borderline Zone 5). He was quite rude when I sent him links to current research on conditioned/unvented attics:

    "closed cell foam is not recommended for an attic from our 49 year real world experience. Open cell foam is a better option."

    1. Expert Member
      Dana Dorsett | | #17

      >"Martin/Dana - considering the complications of roof repair caused by the use of closed cell foam on interior roof deck, would it make more sense to spray foam the attic floor instead?"

      With the ducts in the attic, no it's not better to insulate at the attic floor instead.

      Regarding roofing repair, when it's time to re-roof adding a layer of foam board over the structural roof deck with a nailer deck & roofing atop that works just fine over the repaired section of structural roof as well as the rest of it that still retains spray foam on the underside of the roof deck. It's also possible to cut out & replace the bad section and install closed cell foam on the underside of the repair using a small closed-cell foam DIY kit rather than tying up a truck & crew to install less than 100 board feet of foam. This isn't some insurmountable expensive problem.

      When re-roofing, installing a self healing impermeable membrane such as Grace Ice & Water over the entire roof pretty much prevents the roof from exterior side moisture drives & bulk water leaks from reaching the roof deck.

      >"closed cell foam is not recommended for an attic from our 49 year real world experience. Open cell foam is a better option."

      To me that reads: "We have a much better gross margin on open cell compared to closed cell."

      It's BS.

      They were NOT spraying open or closed cell polyurethane on the underside of roof decks in 1969 (49 years ago) or even 1979 (the year it first became available as a commercial product) unless they were the very first installers of the original products sold in Europe- it didn't hit the US until later ( in most places a decade later.)

      So 49 year, 39 years, 29 years, whaddevah, when it comes down to "...real world experience..." does it really matter? We're living in the era of "alternate facts", after all. :-)

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |