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Community and Q&A

Closed-cell foam thickness vs. R-value

Ross Williams | Posted in Energy Efficiency and Durability on

We are in the Asheville area in the mountains of Western North Carolina (Zone 4A) where I am renovating a small 300 sq ft shed roofed cottage. I intend to use closed cell foam in the roof, walls, and floor. I want a well insulated tight envelope as we are going to try to heat with an electric space heater.

I have now had two different contractors recommending only 3″ of foam in the roof (R-18-21) and tell me I don’t have to worry too much about the R values, as there is a steep diminishing return curve after 3″. Is this correct? Should I insist on 5 or 6″?

I love saving money, but don’t want to be kicking myself once the drywall is up if it takes more of a heater to keep it comfortable in the winter.


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  1. GBA Editor
    Martin Holladay | | #1

    In Climate Zone 4, building codes require (in the prescriptive code requirements) that ceilings or roofs be insulated at a minimum to R-49.

    There are performance paths to code compliance that may allow you to install less insulation than R-49, but R-49 is still a good minimum goal.

    It's possible that you live in a rural area without code enforcement. It's also likely that your local building department doesn't require renovations to comply with energy code requirements.

    All of that said, your spray foam contractors are giving you bad advice. If you install R-18 or R-21 insulation in your roof, one thing is for sure: your roof will leak heat at more than twice the rate of a roof that barely meets the current code.

    For more information on this issue, see It’s OK to Skimp On Insulation, Icynene Says.

  2. Expert Member
    Dana Dorsett | | #2

    If you " saving money...", don't use closed cell foam between framing- it's a waste!

    When closed cell foam is installed between framing the thermally bridging framing SUBSTANTIALLY robs it of it's potential performance. A 2x6 wall with 3" (R20) closed cell foam conducts about 45% more heat than a 2x6 wall with 5.5 (R20) of open cell foam or cellulose, despite having the same R value insulation. See:

    The hit isn't quite as bad in roofs as it is in walls (due to the smaller framing fraction of roofs compared to walls), but it's still substantial. With closed cell foam at half the code requirement you're paying a premium price for insulation that underperforms the standard.

    The size of the electric space heater won't be higher by going sub-code in thermal performance, and the total amount of power use might still be "affordable" in cheap electricity country. But insulating it to code-min for the same or less money using fiber insulation (or open cell foam) and heating it with a half-ton point terminal heat pump (PTHP) with self-contained backup resistance heating rather than an 1800W space heater would cut the heating bill by significantly more than half, and would be able to cool the place as well. Or even if staying sub-code, but using cheaper insulation and going with a PTHP rather than electric space heater could be cheaper up front and use half the power:

    A 20 x 15 house framed with 2x4 walls & 2x6 rafters, with 9' walls, a 15% window/floor area ratio and a door or two has about 500 square feet of wall to insulate and maybe 400 square feet of cathedralized ceiling. To do that all with 3" of closed cell foam runs about $2500-$3K. To do a full cavity fill with open cell foam would run about $1500-2K. Applying the $1000 savings to a PTHP rather than taking it to code-min. would use less electricity than a fully code-min house heated with an electric space heater. With open cell foam you'd have to use a class-II vapor retarder (or "smart" vapor retarder) on the ceiling to protect the roof deck from winter moisture uptake, but the walls wouldn't need it unless keeping indoor humidity very high all winter.

  3. Ross Williams | | #3

    Martin & Dana.
    Thanks for the detailed responses. I get most of that, however, this is a 300sq ft guest cottage. There is no room in the framing to meet code with mineral wool, fiberglass, dense pack cellulose, or spray and batt.

    The walls are old rough cut full dimension 2x4" 14" to 20"o.c. Ceiling 2x6 24" o.c. and floor 2x6 12"o.c.. The sheathing is a full 1" diagonal oak and pine with old tar paper and covered with aluminum siding. There is no budget for removing the siding and no interior space to lose on making the walls thicker. So I am trying to seal and insulate as best I can within those restrictions. I have also replaced the 4 windows and the door with fairly decent vinyl and fiberglass modern units.

    Would a thermal break of say 1/2" foam over the interior studs before drywall be worthwhile, or cause a condensation problem?

    We are thinking to eventually replace the space heater with a small split system.

    Would it make sense to spray 2" closed cell in the roof then fill the roof, walls, and floor with open cell?

    Any other thoughts?
    Thanks again.

  4. Steve Knapp CZ 3A Georgia | | #4

    You could combine rigid foam with fluffy insulation. (See If you purchased reclaimed rigid foam it probably would be a third to half as expensive as new materials. (And much cheaper than contractor installed closed cell foam.)

  5. Kevin Spellman | | #5

    I am also in Asheville. Code is a bit behind as they don't require R-49 yet. I am using a combo of 2 1/2" of closed cell and fill the rest with open cell. Open cell foam is cheaper than cellulose in this area.

  6. GBA Editor
    Martin Holladay | | #6

    If you can't achieve R-49, or don't want to, or feel you don't have to, then you can install insulation with a lower R-value, of course. Just do it with your eyes open.

    The rule in your climate zone (Zone 4A) for flash-and-batt roof assemblies (that is, roof assemblies that combine spray foam with a fluffy insulation like fiberglass or mineral wool) is that a minimum of 31% of the roof assembly's R-value has to come from the spray foam layer.

    For more information on this type of roof assembly, see these articles:

    Flash-and-Batt Insulation

    Combining Exterior Rigid Foam With Fluffy Insulation

  7. Ross Williams | | #7

    I like your approach of closed cell followed by open cell, as my rafter and stud bays are highly irregular and batting them would likely drive me batty. It is also why I dismissed placing rigid foam in the bays.

    Did you hire a contractor, or do it yourself? If a contractor would you share the name?

    Tell me if I'm crazy. Regarding thermal bridging, I believe my roof will perform better than the model test roofs mentioned in the studies, assuming the test roofs have the sheathing in full contact with the rafters.

    Our roof is 29g metal on 1x8 purlins, so there is about 65% less contact between the roofing and rafters. The space between the purlins will allow 1" of foam to cover 65% of the rafter tops.

    Also, the rafters are 24" o.c. which should also help lower total bridging.

    The drywall will be hung on 2x4 furring under the rafters, so the rafters will not contact the drywall directly. Would it make sense to foam all the rafter sides and bottoms between the 2x4 furring?

    Thanks again, everyone. Great information. I really appreciate the help and the community. It can get lonely out here.

  8. GBA Editor
    Martin Holladay | | #8

    It's never a good idea to spray foam directly against the underside of metal roofing. That creates a nightmare for the roofer who has to remove a roofing panel in the future to make repairs. (Closed-cell spray foam is a type of glue.)

    Of course, if your roof has roofing underlayment (like asphalt felt), you could spray against that if you wanted.

  9. Ross Williams | | #9

    It is bare metal and of course, the contractors told me "No problem. We do it on commercial buildings all the time". You must see a lot of this dubious "professional" advice.

    I'll have to think of what I can apply to keep the foam from sticking to the metal without it falling out of the bays as it's sprayed.
    Thanks again. It's so great to have a trusted source of information.

  10. GBA Editor
    Martin Holladay | | #10

    You can install narrow strips of asphalt felt in each rafter bay from the interior if you want, leaving tabs at the sides to staple the felt to the sides of the rafters.

    Almost anything will work -- rectangles of friction-fit rigid foam are another possibility. You just need something to prevent the spray foam from contacting the metal roofing.

  11. Ross Williams | | #11

    Perfect! Your the best.

  12. Expert Member
    Dana Dorsett | | #12

    Fill the wall cavities with half-pound foam, which will run about R15 @ 4.0", and should cost less than $1.50 per square foot. That will outperform 3" of closed cell foam (at more than twice the installed cost.

    Fill the rafter bays with half pound open cell foam, which should run about $2 per square foot. Leave the space under the purlins to give the roof deck at least some capacity to dry toward the exterior.

    Install 2-mil nylon (Certainteed MemBrain) behind the gypsum board on both the walls and ceiling. A 9' x 100' roll costs $160 ordered through box stores, a 10' x 100' roll about $200 from other online vendors but there may be better deals through local distributors. The open cell foam is already a good air barrier, so excessive detailing as an air barrier isn't necessary, except at the edges , the top & bottom plates of the studwalls, etc. Even with a LOT of scrap you're adding at most 35 cents per square foot to the cost of insulation, still well under the cost of 3" of closed cell foam. A guest house with intermittent use doesn't have much indoor humidity sources, but 2-mil nylon will behave like a vapor barrier whenever the roof deck is cold enough to have a moisture accumulation problem, but becomes vapor open when ever the moisture in the open cell foam reaches levels high enough to support mold. With open cell foam there is no convective path to bring higher rates of moisture accumulation to the roof deck. With even 2" of closed cell foam drying toward the interior is severely impeded, but with open cell foam + 2-mil nylon the (fairly vapor open) ceiling paint becomes the limiting factor for drying, and the nylon is the limiting factor for moisture uptake when the roof deck is cold.

    A half ton PTHP + wall sleeve runs about $100, and is a fairly straightforward DIY for those with electrician skills. These are pretty cheap & reliable, made in massive numbers for the hotel/motel biz, and many/most come with wall thermostats or hand held remote options (similar to those for mini-split). It's a lot more money than a $50 space heater and a $100 window shaker AC, but it's a lot nicer too, and will use less than half the amount of electricity for heating.

    A thermal break on the framing is "worth it" only if you're keeping it fully heated/cooled all the time. A half-ton PTHP is a bit overkill for your actual heating & cooling loads, and will have plenty of capacity for even a sub-code insulation levels.

    Do you have 240V power to this building, or only 120V?

  13. Kevin Spellman | | #13


    USI Allied Insulation gets the majority of insulation business in the Asheville area. Rough numbers from any local contractors should be in the neighborhood of $.30/in/sf for open cell and $1/in/sf for closed cell. Dana's advice is good. My situation is different in that I have thick staggered stud walls and I am spooked by possible moisture issues at the plywood sheathing, thus my layer of closed cell foam. Same for my roof--theory would tell me I would have a problem with condensation if I used only open cell.Locals insist they have never seen an issue with a roof full of open cell, but I err on the side of "could happen" and opted for a layer of closed cell to make sure it never happens. In your case, as Dana described, leaving an air space for venting above a layer of open cell in your roof would work great. It should be able to move vapor if need be.

  14. Ross Williams | | #14

    Thanks. Great discussion.
    To be clear, I have no moisture barrier of any type under the metal roofing. The metal sits directly on the purlins. I was thinking we needed a layer of Closed Cell to seal against any potential leaks and to up the R-value in the limited 7.5" rafter bays (full 6" rafters + 2x4 furring).

    I think what you are saying is to put the moisture barrier between the frame and the gyp board so that any water that gets in will not get through to the gyp board and is then free to dry through the Open cell foam to the exterior.

    However, using all Open Cell, we only get the roof to R26.5 (7.5"x R3.5).

    2" of Closed cell followed by Open cell get us to R33 or 3" of Closed cell to R48. Are you concerned that the layer of Closed Cell will trap moisture and cause problems?

    The cottage has a new 60A subpanel off the 200A main house panel. The subpanel has only one 15A for lights and three 20A circuits for receptacles, space heater, mini frig, 7gal 120v Bosch mini water heater, two burner induction hot plate and toaster oven.

    The subpanel still has lots of room for a 240V breaker if the load calculations will allow it. Should be ok with a 15A, or 20A 240V breaker.

    Thanks again for all your time and help.

    and Kevin,
    Thank you as well. I am done with old houses, after this project I'm going new from the ground up. :)

  15. Expert Member
    Dana Dorsett | | #15

    Re-reading your description, aluminum siding is inherently back ventilated, which means you'd only need the vapor retarder for the cathedralized ceiling.

    To be clear, the 2 mil nylon vapor retarder "... moisture barrier..." is not a weather resistant barrier designed for managing liquid water. It's a vapor retarder, which only manages moisture movement as water vapor.

    Are you saying that there is no roof deck and that the purlins are resting directly on the rafters, and that there is no roof sheathing (plank or whatever) of any kind?

    All halt ton PTHPs are just fine on a 15A/240V breaker. In heat pump mode it only pulls about 2-2.5A of 240V when running steady-state, a bit more on spin-up. Those with resistance heat backup will pull about 10-13A when running on the heat strips. This space won't need more than 1.5kw of heat strip, but most come with 2.4kw or more.

    The heating and cooling loads of this space will be under a half-ton. That makes almost all split systems automatically ridiculously oversized, which is lousy for both comfort & efficiency. A 3/4 ton mini-split that can modulate below 2000 BTU/hr is probably going to work better than any split system heat pump out there. (LG Art Cool LAN/LAU 090HYV1, Mitsubishi MSZ/MUZ FH09NA. There are others.)

  16. Ross Williams | | #16

    That's correct. No roof sheathing. Only metal roofing attached to purlins attached to rafters, with the 2x4 furring under the rafters to hang the gyp board on.

    You have convinced me a small PTHP is a better way to go. Thanks, they were not even on my radar.

  17. Expert Member
    Peter Engle | | #17

    The earlier answers had it right: install some sort of dam material in the rafters and under the purlins. Felt paper, cardboard, rigid foam insulation - pretty much anything will work, so long as it is fastened well enough to spray the foam against. For your climate, I like the solution of using open cell with an interior smart vapor retarder. And, you can't get much smaller equipment than a 1/2 ton PTHP. The combination seems like a good fit for the space.

  18. Expert Member
    Dana Dorsett | | #18

    If the purlins & roofing are already up (probably are) you're kind of between a rock and a hard place, since disassembly to put a continuous sheathing of any type probably isn't in the cards.

    You may have to glue some strips of 1"-1.5" thick x 1" wide foam board (any type) to the steel roofing about 1/4" way from the rafter on each side of the rafters and another mid-way between the rafters as spacers, then use 3/8" fan fold XPS siding underlayment (non-perforated, with a vapor permeance greater than 1 perm) as the weather resistant semi-permeable weather resistant drain material. XPS is probably the "right' material (HFC blowing agents notwithstanding.) Fan-fold siding underlayment comes in 50' long sheets, so there won't be any issues with reliably water sealing any seams- it will be continuous from eaves to ridge. With 4x8 - 4x10 sheets of cut'n'cobbled board material it's not really possible to get a good water-seal at butted up seams.

    Cut the fan-fold XPS about 1/4-3/8" narrower than the space between the rafters, and glue it to your spacers with foam-board construction adhesive or tack a few blocks into the rafter to keep it in place, then use can-foam to seal the edges in place all the way up to the 1/4" gap between the spacer strip to seal the exterior sides of the rafters from any liquid moisture that gets by the roofing and onto the XPS. It's pretty important that water running down the weather resistant barrier (in this case, the fan-fold XPS) NOT be able to run over onto the exterior sides of the rafters, since that is the most likely leak point. The rafter edges don't have to be perfectly covered- incidental splashes can still dry toward the exterior through spacer foam, but you don't want it to become a drain channel, with a blockage at every purlin crossing.

  19. Ross Williams | | #19

    Good description for the moisture barrier, but how do I get a high enough R-value within the remaining roughly 6" of rafter space without using closed cell foam?

    For the roof deck, I am currently thinking to spray 3" closed cell to seal and waterproof, followed by an R19 batt compressed to fill the remain 4 1/2" of space. I expect with the increased R-value of compressing the glass, plus the decrease inches filled (2x6 R19 batt compressed to 4 1/2") that I would end up with maybe R 14 giving me a total around R36.

    What is your concern with this approach?
    Thanks again

  20. GBA Editor
    Martin Holladay | | #20

    Your plan to go ahead with flash-and-batt -- using 3 inches of closed-cell spray foam, and about 4.5 inches of compressed fiberglass batts -- will work.

    The resulting R-value will be less than minimum code requirements, but the assembly shouldn't have any moisture problems.

  21. Ross Williams | | #21

    Your tar paper idea to protect the metal was quick and easy. The moisture issue is what I was concerned about. I think it's a go. Thanks again for all your help.

  22. Expert Member
    Dana Dorsett | | #22

    Basically won't ever be able to meet code for R-value, so what is your working definition of "... a high enough R-value..."? High enough for comfort? High enough to keep the energy use sufficiently bounded?

    Even an R22-R27-ish open cell foam roof compared to your R36-ish stackup isn't going to change the size of the heating/cooling equipment (= smallest available), and the difference in peak ceiling temperature (comfort) and energy use is also quite small, a small number that is already reduced by more than half if using a PTHP rather than an electric space heater for those days when you're actually heating the place.

    Unless you're fully conditioning the space 365 days out of the year ( or even if you are), there is no "payback" on bumping it up to R36-ish using expensive closed cell foam + batts. (From net energy & annual energy cost point of view you would be better off applying the cost difference toward rooftop solar at $3/watt less subsidies to offset heating & cooling energy use/cost.) The ~$3/per square foot cost of the 3" closed cell foam would cost at least 1.3x more than 7.25" of open cell foam.

    If sticking with a flash & batt approach, a mere 2" of closed cell foam is more than sufficient dew point control on 5.25" of rock wool or fiberglass (of any density) in your neighborhood. An R19 compressed to 5.5" performs at about R18, so compressed to 5.25" it would be about R17, which would put the center-cavity R at about R30, and a foam-R ratio approaching 40% (45% if HFO blown foam). That would be about $400 cheaper than the 3" of closed cell foam.

    With full dimension 2x framing batts designed for 1.5" wide milled lumber are going to be a bit squished from the sides, but at a regular 24" o.c. spacing it should be do-able.

    BTW: Compressed to 4.5" an R19 will perform at about R15, so you cheated yourself about R1 on your estimate, not that it matters.

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