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Community and Q&A

Rigid foam for hot roof

ABradford_1 | Posted in Energy Efficiency and Durability on

I am interested in converting our bungalow attic to finished space, and it was not designed for venting. I’ve heard I can do a hot roof insulation method using spray foam. However, I am a DIY guy by nature, and was interested in methods I’ve seen using closed-cell polyiso rigid foam and spray foam cans to achieve the same result, at least in walls. I am curious about using this method on a hot roof. Surely it would cause head-scratching with code officers. Would it work, though? The rafters are only 2×6”, so I could theoretically fill the cavities with rigid foam, seal them up with spray cans, and then apply more rigid foam in the opposite direction for extra air sealing.
I live in a cold climate (Maine) so it looks like 8+ inches of closed cell foam gets me to code.

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  1. Expert Member
    ARMANDO COBO | | #1

    Some manufacturers do not sell closed cell foam to untrained installers, as there are too many issues you need to deal with to do it correctly. Sprayed CC foams need to be installed in 2" increments, or there is a fire risk. This is a job for a pro. Read on:,

  2. davidmeiland | | #2

    I think the question is about cutting strips of rigid foam to fit between rafters, and then foaming them in at the edges with one-part. I have done a small-ish roof in this manner and it works well, as far as I can tell. I ripped all of the material on the tablesaw in my shop before carting it to the job, and then we cross-cut it with a knife. Everything was cut with 3/8" gaps around all 4 edges. Everything was rectangular and there were no blocks or other obstacles such as can lights. There were a few wires following the framing. We put (3) 2" layers in a 2x8 rafter space with the leftover air-space to the inside, with drywall under the rafters, which meets the code required R-30 for cathedral ceilings here.

    I did it this way to avoid bringing a foam rig to a small job and would do it again in similar circumstances, although I would add a 1" layer of rigid continuous under the rafters, and then hang the drywall with long screws.

  3. GBA Editor
    Martin Holladay | | #3

    The method you describe is called "cut and cobble." Several people have reported failures when the cut-and-cobble method is used to insulate an unvented cathedral ceiling; the problems seem to arise when imperfect application of spray foam or building movement (due to changes in humidity, changes in temperature, or settling) allows cracks to develop that permit interior air to contact cold roof sheathing.

    To read about the method, and its risks, in more detail, see Cut-and-Cobble Insulation.

  4. ABradford_1 | | #4


    That is an excellent article, and explains EXACTLY what I wanted to do as well as the concerns I had about it. In regard to thermal bridging, however, would I get a similar effect by adding a continuous layer of foam to the inside once I have filled the rafter bays? I could do this perpendicular to the bays. I am also curious how spray foam is much better, given that it only fills the bays as well. The issue of thermal bridging in the rafters would remain, yes? Sounds like exterior rigid is the ONLY recommendation?

  5. GBA Editor
    Martin Holladay | | #5

    You are correct that if you fill the rafter bays with spray foam, you have not addressed thermal bridging through the rafters -- which is why installing rigid foam above the roof sheathing is always preferable to any approach that simply puts insulation between the rafters.

    Your plan to install a continuous layer of rigid foam on the interior side of the rafters also addresses thermal bridging.

    Just because an insulation method doesn't address thermal bridging, however, doesn't mean that the method is worthless or risky. It just means that it isn't as effective as methods that address thermal bridging.

    According to most experts, when closed-cell spray foam is installed on the underside of roof sheathing, the results are a fairly airtight installation -- assuming that the installer is conscientious, of course. This type of installation is less risky than cut-and-cobble, because it does a better job of stopping interior air from contacting the cold roof sheathing.

    For a complete run-down of all of your options, see How to Build an Insulated Cathedral Ceiling.

  6. Expert Member
    Dana Dorsett | | #6

    Foil faced polyiso should NOT be used in a roof deck cut'n'cobble, since the foil facers form a moisture trap, placing the roof deck between the shingle & felt layup (about 0.1 perms, a minimal class-I vapor retarder) and the foil (about 0.01 perms a very powerful class-I vapor retarder), which makes it impossible for the roof deck to dry at any reasonable rate.

    Putting foil faced iso above the roof deck is fine, as long as there aren't any class-I vapor retarders on the interior side of the roof deck.

    When DIY spray-foaming the interior of a rafter (or stud) bay, you get better air sealing if you start by "picture framing" it, first sealing the decking to the framing, then filling in the rest.

    Using 8" of spray applied closed cell foam to "meet code" is expensive, and underperforms thermally due to the R9.5 worth 8" of wood thermally bridging the R49 foam, cutting the average performance about in half the center-bay value. In a Maine US climate zone 6 climate if you have 2x6 rafters you'll do just fine with 2" of closed cell foam (R12-13) filling the other 3.5" with cellulose (R12-R13) for a 50/50 foam/fiber ratio. While the R25 ish center-cavity R value is only half of the code prescriptive R49, at a ~15% framing fraction the average R is only about R4 less than what it would be with 5.5" of closed cell foam, something that can be made up by another inch of EPS (or 3/4" of polyiso) above the roof deck when it's time to re-roof, at a much lower environmental impact.

    An R49 high density batt perfectly installed between 2x12 framing at 15% framing fraction has a "whole-assembly" R of about R35 after factoring in the thermal bridging (R36 if you add in the attic floor decking and ceiling gypsum) . With 8" closed cell cavity fill comes in at about R31 "whole-assembly", more than 10% lower performance. The 5.5" close cell option comes in at R21.5 , and the 2" foam + 3.5" cellulose becomes R17.5 after thermal bridging. If taking the 2" foam/3.5" fiber approach, to get up to code-min performance would then take about 4" of EPS above the roof deck, or a stackup of 1.5" of polyiso on the roof deck + 2" of EPS the exterior for a total exterior foam depth of 3.5". (For the latter stackup it's important to put the polyiso between the EPS and roof deck, to keep the polyiso at a temp where it performs better. The 2-foam stackup will outperform 3.5" of polyiso when it's on the exterior of an insulated rafter bay.)

    Closed cell foam is blown with a VERY high global warming potential agent (HFC245fa, at about 1000x CO2), whereas EPS & polyiso are blown with pentane (7x CO2). To avoid wintertime moisture accumulation in the fiber layer (prior to adding exterior insulation) you'll need a 50/50 or higher foam/fiber ratio for your climate, but anything beyond that is likely to end up having a higher lifecycle environmental hit than the energy use it's marginally higher performance than the fiber insulation it is displacing. If you add interior side rigid foam to deal with the thermal bridging you have to increase the spray foam R by the same amount- it's really better to save the spray foam money and apply it to the exterior (and more benign) rigid foam later.

    Closed cell polyurethane foam is also more expensive, at about 17-18 cents per square foot per R, compared to about 10 cents per R-foot for polyiso & EPS. Using any more than the minimum necessary for air-sealing & dew-point control on the fiber layers is almost as bad for the pocketbook as it is for the environment.

  7. ABradford_1 | | #7

    Wow that's excellent information, thank you! I hadn't thought of working in stages. Certainly a lower R value in the beginning is better than nothing (we currently have little to no attic floor insulation, so this place breathes like an Olympian swimmer).
    So it really sounds like if you want to have an interior application before adding exterior rigid foam, you are limited to are sealing by spray foam, to some degree. It is the only method that guarantees an air seal then? Are there no examples where a highly carefully applied layer of NON foil faced rigid foam spray sealed at the edges that prevent air leakage? Sorry to push the subject to it's death. I am very interested in limiting spray foam application.

    So given your vast array of solutions, let's put together my favorite scenario. Then you can shoot it down for good and I will face the spray.

    -We use NON foil polyiso rigid on the interior, with spray foam cans picture-framing the rafter bays. -We fill the rest of the interior bays with cellulose (is wet, dry, or dense pack the best option here?). -We apply a final interior rigid layer to stop thermal thermal bridging (or not, and save money for later? Would love to not deepen rafters, as it's already a very shallow roof) -When appropriate, we re-roof and add the necessary exterior rigid layers to meet and/or exceed code.

    I should emphasize our need to do this cheaply. I am also very green-conscious and very capable of executing the work myself. I make violins, so I may have more trouble managing workflow due to interest in perfect fit than anything.

    Thanks everyone for your help! Looking for the perfect combo of performance, economy, and enviromental impact! Want my cake and eat it too.

  8. GBA Editor
    Martin Holladay | | #8

    I don't recommend your approach. Sensible approaches are explained in this article: How to Build an Insulated Cathedral Ceiling.

  9. Expert Member
    Dana Dorsett | | #9

    2" of spray polyurethane using a DIY 600 board-foot kits costs between $2.25-2:50 per square foot. A pro will charge between $2.00-2.25 per square foot if it's all opened up and easy to shoot. Either way it's about the same cost for the spray foam layer.

    If you can't find a damp-spray cellulose contractor to fill the other 3.5" at a reasonable cost, you can DIY unfaced (or kraft faced, if you must, but NOT foil-faced) R13 fiberglass into the remaining 3.5" - just take extreme care to install it as perfectly as possible, no gaps or voids/depressions and you can get the performance out of it. Do NOT install rigid foam on the interior side- the mold/rot risk is higher than the marginal energy savings reward. Save it for the exterior.

    An air-tight R17.5 whole-assembly value (R 18.5-R19 when you include the R-values of the interior side gypsum & roof decking) is a pretty good performer compared to most "R38" attics the way most attic insulation has historically been installed. Adding another R17-R20 above the roof deck later will be worth it in the long run, but it's a long payout in strictly energy cost savings terms. If you are less than 10 years from re-roofing the energy cost hit you're taking in the mean time is pretty small. The ice-damming risk is higher without the thermal break, but it's not a disaster- it's unlikely that you have the headroom to add R17-R20 of rigid foam inside the rafters any way, even if it WAS a good idea (which it isn't, in an unvented roof.)

  10. ABradford_1 | | #10

    Excellent and excellent. I really appreciate the responses on this. The real-world solutions provided help us determine what projects are worth contracting out and what ones we can tackle ourselves.
    This may be a thread-jack question, but is the air/moisture sealing advantage you get from doing spray foam in the roof the same for the basement rim joist area? I ask, because the local insulation contractors are very spray foam-centric, and I want to know if it's really the best solution.
    We are working with a 100% uninsulated 2-unit bungalow from 1910, and it's impeccable state of preservation is unfortunately likely due to how well it breathes. We don't want to start insulating to find we ruin it all.

  11. frasca | | #11

    I keep coming across this thread and am curious what the community thinks of this comment:

    "We are working with a 100% uninsulated 2-unit bungalow from 1910, and it's impeccable state of preservation is unfortunately likely due to how well it breathes. We don't want to start insulating to find we ruin it all."

    Is that a common thought, that more breathability helps preserve the construction materials of an old house?

    Is there any truth to it?

  12. user-2310254 | | #12


    I suggest you start a new thread to avoid pulling this one off-topic.

  13. GBA Editor
    Martin Holladay | | #13

    I'm willing to answer your question.

    Q. "Is that a common thought, that more breathability helps preserve the construction materials of an old house?"

    A. "Breathability" is a tricky word, because it means different things to different people. But in general, if an old wood-framed house has uninsulated walls, it stays dry. When the house is heated during the winter, the house is pumping tremendous amounts of heat through the walls, and the heat keeps the wood dry.

    Once the walls are insulated, there are more opportunities for moisture to accumulate on the cold sheathing -- especially if the remodeling contractors don't know what they are doing.

    The solution, of course, is to choose an insulating method that prevents moisture accumulation. Few of us can afford to pay the heating bills for an uninuslated house.

  14. frasca | | #14

    Thank you Martin. That makes sense to me. Apologies if I took the thread off topic.

  15. frasca | | #15

    "The solution, of course, is to choose an insulating method that prevents moisture accumulation."

    Martin - Hate to ask, but do you have any links handy to pieces you've written on this? I've searched the site for 'insulation' and read the first few hits. I was hoping to find something like a 'definitive guide to insulating stud cavities' that would help me take my climate, my house construction, and my hvac systems, and pick a wall insulation that would avoid moisture accumulation. I may have missed it...

  16. GBA Editor
    Martin Holladay | | #16

    Start by reading this article: "How to Design a Wall."

    To design a wall that works, it's helpful to have a good understanding of air barriers, vapor barriers, water-resistive barriers, sheathing options, and rainscreens. It's also a good idea for you to know why walls rot. That's why I suggest that you also read the articles listed below.

    For information on air barriers, see "Questions and Answers About Air Barriers."

    For information on vapor barriers, see these two articles: "Vapor Retarders and Vapor Barriers" and "Do I Need a Vapor Retarder?"

    For information on water-resistive barriers, see "All About Water-Resistive Barriers."

    For information on sheathings, see "Wall Sheathing Options."

    For information on rainscreens, see "All About Rainscreens."

    For information on wall rot, see "All About Wall Rot."

    Finally, if you want to delve deeply into any of these issues, you might want to scan the titles in this comprehensive list: "How To Do Everything."

  17. frasca | | #17

    Thanks Martin, I'll get started!

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