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Community and Q&A

Exterior insulation

Eric Mailhot | Posted in GBA Pro Help on

I was reading the GBA article on how to design a wall and for exterior insulation it was mentionned that a good technic was to use rigid foam.

Instead can we use closed – cell spray foam which have a higher r-value and can also act as a WRB ?

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  1. BrianVarick | | #1

    The biggest reason people like to use exterior insulation is to keep the studs from being thermal bridges. Polyiso will have around the same R value as closed cell spray foam and be much cheaper if you can find a good source for it.

  2. Expert Member
    Rick Evans | | #2


    I guess it might work in theory, but I would hate to be your siding contractor!

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Eric,

    You can. I've seen it done on energy-retrofit projects on both walls and roofs, but there are better options. As Brian points out, you can get good R-value from polyiso, which is more environmentally-friendly than most closed cell foam, and can be used as a WRB if detailed properly and if your goal is to eliminate the need for a separate WRB.

  4. GBA Editor
    Brian Pontolilo | | #4

    I'm sure we published the house from the photo below, but I can't find the article right now. I'll keep looking. Here's an example of where it was done on a roof:

    Still, I think there are better options in most cases.

  5. Eric Mailhot | | #5

    For a Canadian cold climate, polyiso (PIC) was not appropriate for exterior insulation. I think I readed here also. Hence, my question on closed -cell foam which, as far as I know, its not affected by our stringent winter.

    1. Expert Member
      Zephyr7 | | #8

      Polyiso will still work in colder weather, you just need to derate the R value a little. Instead of the usual R6 per inch given for polyiso, it’s safer to use R5 per inch in colder climates. Aside from that, polyiso will work just fine.

      There are also some types of polyiso that don’t suffer from the “reduced performance in cold weather” problem. Dow Thermax is one such type of polyiso that is readily available. There are some newer blowing agents that don’t suffer from the reduced performance problem too, but it’s difficult to know which manufacturer is using what in their polyiso unless they specifically say (like Dow does for their Thermax product).

      You could use EPS instead of polyiso and avoid the issue completely. XPS is another option, but less green in terms of the blowing agents used. Neither of those types of foam have any need for R value derating in colder weather.


      1. Expert Member
        Dana Dorsett | | #11

        >"You could use EPS instead of polyiso and avoid the issue completely. XPS is another option, but less green in terms of the blowing agents used."

        XPS needs to be derated for degradation in performance over time from the loss of those blowing agents over any reasonable lifecycle for a house. XPS is warranteed to (only) 90% of the labeled R-value, but at full depletion (after several decades) would be performing at about 85% of labeled R, which would be the same performance as EPS of similar density (which doesn't vary with time, and is not dependent upon blowing agents for performance.)

        There is a large variation in what could be considered a "...Canadian cold climate...". The actual climate and the fraction of the R-value that is exterior polyiso will affect what (if any) derating level would be appropriate.

        1. Eric Mailhot | | #13

          Thx and Canadian climate is Montreal, cold with high humidity. Now considering Dow thermax or Rockwool for the build. Even if Rockwool is rated as a lower R, It has a lot of qualities (constant r value, water repellent, fire resistant and vapor perm) but complicated the installation because of the weight and thickness required.

          1. Expert Member
            Dana Dorsett | | #15

            Thermax requires NO derating for temperature at Montreal's fair-to-middlin' climate zone 6 type outdoor temps (unlike 2lb roofing polyiso.) Even in January the mean outdoor temp is north of -10C/+14F.


            Even if one went with as little as 2" of a cheap brand-X 1lb foil faced polyiso there would still be adequate dew point control at the sheathing layer on a 2x6/R23 type of wall if the siding is installed with a rainscreen gap next to a foil facer in Montreal's climate (but more would still be better, of course.)

            With the straw-man low-end roofing polyiso it would still only take 3". See figure 2:


            When it's -10C/+14F outside and 20C indoors with only 2" of the lower performance roofing polyiso and R23 the warm side of the polyiso would be about 0C/32F, and the mean temp through the foam about -5C/+23F. At that temp the performance of the 2" foam at only about R8.8.

            With 3" of roofing foam the warm side would run about +2C/36F, the mean temp through the foam about -4C/+25F, with the foam performing at about R13.5. That's the worst-week's average performance. The average performance over the coldest weeks of winter would be north of R14. Interpolating from the IRC's tables it would only takes about R13 for dew point control on R23 using only a Class-III vapor retarder on the interior side.

            At 2" Thermax would hit that point any time the mean temp through the foam was under +24C/+75F:


            But so would cheaper brands of foil faced polyiso, if the foil facer was next to an air gap.

            At layers thick enough for achieving R40 whole-wall the amount of derating of roofing polyiso even less, since the mean temperature through the foam layer averaged over the full heating season would put it even higher on the curve. Assuming the 2x6 wall is ~R15 "whole wall", derating even 2lb roofing polyiso to R5/inch (or R25 @ 5" ) would be conservative. That's only an inch thicker than what it would take with Thermax, and more than an inch thinner than it would take with rigid rock wool.

  6. Eric Mailhot | | #6

    Looking for a R40 wall without the fat exterior insulation of rigid foam or mineral wool. 2’1/2 closed-cell foam will do it + this will eliminate the need of applying a WRB on the plywood sheathing, in theory.

  7. Zdesign | | #7

    For a R40 wall without Closed Cell spray foam, 2x6s 24 OC, R23 Rockwool, 7/16" Zip, and then 2" Kooltherm K12 Framing board will net you 39.7 give or take a little.

    1. Eric Mailhot | | #9

      Kooltherm k12 at r8 per inch is interesting , but is it available in Canada ? Zip is a bit difficult to find up here and what I heard a bit pricey in Canada. Leaning more to 1/2 cdx plywood with a delta sa wrb

      1. Sofiane Azzi | | #10

        Kooltherm is officially unavailable in Canada. They source it on a case by case basis with approval from relevant authorities at the moment , but I wouldn’t count on it being available for purchase. I could elaborate further, but I don’t think it would add much to the discussion.

        1. Eric Mailhot | | #12

          Ok thx

          1. Sofiane Azzi | | #14

            On a more useful note, my suggestion would be to go with an eps or graphite polystyrene foam rated about R5 per inch such as this one :

            With 3 inches, you would be at R15 and performance improves in colder weather when you need it most. Further, performance doesn’t degrade over time like it would with polyiso or XPS.

            I hope someone else with more experience and knowledge than me on this forum chimes in, but I think this would make more sense than going with outside sprayfoam.

            Bonne chance dans tes recherches!

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