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Spray foam for Outsulation on roof + compression strength

jberks | Posted in Energy Efficiency and Durability on

Hey GBA’ers,

I have a flat roof, where I am thinking about what materials to insulate it with. After looking at different foams, and reading Martins posts on the matter, I have a few questions regarding my specific scenario.

For instance, does anyone have any experience with spray foaming a flat roof? Can the foam come out relatively flat? I plan to put a vegetative live roof and patio decking on top of the roof membrane, and wondering if the compression strength of the foam will be ok with the weight. Are there any other complications associated with doing the spray foam that I’m not thinking about.

Below is my logic:

My roof assembly is as follows:

-Roofing membrane
-6″ outsulation
-3/4″ Plywood (taped seams)
-9.5″ I joists with X” of rockwool batting insulation

So being in Zone 6, The exterior R value needs to be at least 51% to prevent rot issues. The max depth of outsulation I have to play with is 6″, going higher will cause further work and complications. Thus the Outsulation material will determine the thickness of the Insulation and the total R value of the roof.

I could go with 6″ of 20psi EPS, which is the cheaper option, but I would have to cut back on the interior batting to meet the recommended ratios and it would lessen the total R value of the roof.

So, In the interest of getting as much R performance at the roof, I was thinking 6″ of closed cell Sprayfoam at R-6 per inch (R36), and thus I could have 8.5″ of batting at R-4 per inch (R34) creating a total of R70.

What do you think of all this?


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  1. Expert Member
    Dana Dorsett | | #1

    People use 3lb polyurethane as both the insulation and roofing in many locations, but it's expensive and not all that green due to both the HFC245fa blowing agent (an extremely powerful greenhouse gas) and the high polymer weight per R.

    You don't need 20psi EPS to have a walkable roof. Most roofing EPS in commercial flat roofs is Type-VII (1.25lbs nominal density)_ or Type-II (1.5lbs nominal density), rated at 15psi. If overtopped with decking the weight will be even more distributed. Buying reclaimed roofing EPS makes it even cheaper (and greener) than batts.

    When the insulation is continuous, without thermally bridging rafters or joists you only need to duck under U0.026 or
    R38.5 "whole assembly, with all materials factored in to meet code minimum. With credit for roofing, air films, roof deck etc you can usually get there with ~R35 of continuous foam. With 6" of reclaimed 2lb roofing polyiso derated to R5/inch for climate you'd have R30 above the roof deck, and even R10 under the roof deck would be enough to meet code min, but you could safely install up to R30.

    Using 7" of half-pound open cell spray foam on the under side of the roof deck instead of rock wool batts would be R26-ish, giving you a more perfect fit, plenty of dew point margin at the roof deck, using about the same amount of polymer than 1" of 3lb foam, and less than 2" of 2lb foam. Open cell foam uses water, not HFCs as the blowing agent.

  2. jberks | | #2

    Thanks Dana!

  3. jberks | | #3

    Hi Dana,

    Given your advice above, I agree that 6" polyiso on top is my best bet. Thank you.

    Naturally, I started doing a bit of research on polyiso and its whole derating issue. Reading a bunch of articles on here I came across the graph that I know you're familiar with which shows the R-temperature curves of various unnamed brands showing that there is a huge variance of performance across brands. So then I start trying to find out which are some better ones.

    Then I came across Dow's THERMAX, which they claim they have the magic formula where sub 50F temperatures don't derate its R-value. And they show their test results. Certainly interesting if its true. take a look:

    But what's more important is what the cost of this thermax is, value engineering is important (to me at least). I'll call the Dow rep on Monday.

    What do you make of all this? Any experience on which would be a better brand?


  4. Expert Member
    Dana Dorsett | | #4

    The tech note from DOW was news to me- thank you for that!

    With Thermax and a presumptive R6.5/inch mean performance at outdoor temperatures that matter you could back off to 5" (R32.5), with 7" of fluff or open cell foam under the roof deck.

    Five inches of Thermax is unlikely to be cost effective against six inches of RECLAIMED roofing foam though, given that you'll pay a premium for Thermax over brand-X virgin stock foam, and reclaimed foam is usually no more than a third the cost of virgin stock.

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