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Low-Slope Roof in a Cold Climate

austinharris | Posted in General Questions on


Mostly looking for confirmation/advise on insulating a low slope roof (with a parapet) in cold climates.

I am in Vermont, zone 6, with a rubber membrane roof 1/4″ over 12″

Ive been doing some research on best techniques for insulating (unvented) and feel an 8″-9″ layer of closed cell SPF on the underside of the roof sheathing will best fit my needs. Is this thick enough to halt any potential dew point concerns?

Please reference my detail attached for a visual. I should also note that this is new construction, nothing has been built yet.

Thanks in advance,


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  1. Jon_R | | #1

    Thermal bridging negates too much of your R value. So consider some foam above the sheathing or some kind of thermal break on the interior side. Also consider a mix of closed cell SPF and fluffy insulation.

    1. austinharris | | #2

      Thanks for the reply Jon, is bridging still a concern with a trussed roof system?

  2. Expert Member
    ARMANDO COBO | | #3

    I haven’t design a flat roof in CZ6, but have designed many in CZ4-5 in the SW. I agree with Jon, using rigid foam on top of the roof decking is your best bet, and fill the cavity below with ocSPF.
    I prefer the ocSPF under the roof deck in this application because it expands to all corners of the truss webs, and behind the truss steel plates, where other insulations leave voids.

  3. walta100 | | #4

    I have to ask why spray foam? Being as spray foam is the most expensive, riskiest and least green way to get an R of insulation.

    The drawing look like new construction if so it seems to me that flat roof could be cover in several layers of used/ reclaimed foam at a much lower cost making a much less risky assembly.


  4. Expert Member
    ARMANDO COBO | | #5

    I agree to a point. I'm assuming if he was going to spend money on 8"-9" ccSPF, price was not the main factor. Most folks don't think of reclaimed insulation, even though is a great option, as it requires finding large supplies, all the same, and in good condition. If you have to create a puzzle of reclaimed rigid foam, it may not be worth it. Since I work in new construction, I perhaps have an inhered appreciation for new foam boards.
    Installing all new rigid foam on top of the roof decking is a great option, and almost exclusively done in commercial jobs with tall parapets, but in residential is almost aesthetically unwanted, and again, perhaps I'm biased to install ocSPF in flat roof trusses, for the above reasons. Perhaps it maybe the fact that we do things in different regions.

  5. Expert Member
    BILL WICHERS | | #6

    It's worth mentioning that the suppliest of reclaimed materials are much tighter than they were a year or two ago. I agree reclaimed material would be great here if you can find enough of the right stuff (note that a lot of the reclaimed polyiso is roofing polyiso which seems like a perfect fit here :-).

    If rigid foam isn't an option, then I agree with Armando that open cell will better fill things, but you'll probably need a few inch thick layer of closed cell against the sheathing for vapor control. You could fill the rest with open cell to save money.

    Thermal bridging is going to be less with an open truss system, but you'll still have some. The advantage to spray foam here is that it's easy to install wall around all the parts of the trusses. It would be a nightmare to try to do that with batts. You might consider spray applied loose fill insulation that has a binder in it too (the "sticky stuff") as another option that might be cheaper compared to spray foam. Note that you'd still need to air seal if you go that route though.


  6. austinharris | | #7

    I would LOVE to use a recycled product here. However, with low sloped roofs already being risky in snow/rain heavy climates I don’t think the risk-to-reward of imperfect product works out here.

    Believe it or not I hate using spray foam! I generally vent, net and strap dense pack cellulose when it comes to cathedral space. This low slope assembly has a bit more going on. The pitch is so shallow it does not create the stack effect that you would see with a steeper roof pitch. Venting this mechanically opens a whole other can of worms when it comes to detailing that I don’t want to get into.

    The main factors driving a ccSPF cavity are air and moisture control. In cold climates, air flow plays a huge roll in unvented low-slope roof assemblies. From my understanding, I need to stop air flow entirely. Warm moist air cannot make its way to a cold dry surface (roof sheathing) because it will condense and rot the sheathing. The ccSPF insulation also must be thick enough to keep the dew point away from roof sheathing AND underside of the lower ccSPF surface. The minimum R-value for the foam layer in a hybrid insulation cavity (ccSPF & fluffy) is R-25 for zone 6.

    It sounds like I would be safe with the following:

    Option 1: +/- 5” of ccSPF and then fill the rest of the cavity with cellulose to save to $$ and environment :). The 5” would take care of dew point concerns and the cellulose would increase total R-value.

    Option 2: 6” of XPS or Polyiso on top of the roof sheathing, NO fluffy insulation below. Insulating with some sort of fluffy insulation below would pull that dew point ratio back towards the sheathing and that’s a no no.

    Thank you in advance

    1. BuildByMD | | #9

      I’m currently building similar home . 3000sqft if 1/4:12 flat roof.

      I’m planing on above deck poly iso to R49 and vapor barrier under the polyiso and TPO above it.

      I’m running into struggle how to insulate the Rims, I want to about spraying It


    2. Expert Member
      ARMANDO COBO | | #10

      @Lorant – If you are using dimensional lumber or TJI, you could use cut-and-cobble or a flash-and-batt, but quality installation is key, and there is a lot of labor involved.
      If you are using roof trusses, I only recommend ocSPF, since it’s the easiest way to fill and insulate between the truss webs and metal connectors.

      1. BuildByMD | | #11


        I got couple quotes to spray my roof to r49 and it around 28,000-30,000 with. 8” of polyiso over deck o can get it to 12000-14000 and I could spray the rims for about 3000.

        Also I’m trying to put a wood ceiling and having spray foam I would have to drywall the ceiling but above deck insulation I’m reading I don’t need a firewall. Is that right?

      2. Expert Member
        ARMANDO COBO | | #12

        I'm sorry, I should 've been clear. I read your post as you made the decision to install polyiso and TPO above the roof decking, and I think that's great.

        I read one question for the rim joists insulation only, and that's what I recommend ocSPF.

        You can install your wood ceiling, but you may want to ask your local building official on weather they consider that as a conditioned attic, in which case you may need to install drywall and ventilation. I've done that in the past.

  7. DanCK | | #8

    Hey austin,

    I believe your on the right track here. I'm currently experiencing issues with a similar roof assembly that we will be taking apart and switching to closed cell spray foam.
    I've constructed this a few times in the last couple of years and encourage closed cell foam for the other projects. The project that has failed had budget restrictions that influenced our decision at the time. The condition of the roof has deteriorated very quickly and it was detailed fairly well.

    "with low sloped roofs already being risky in snow/rain heavy climates I don’t think the risk-to-reward of imperfect product works out here"

    Your correct here. Although the closed cell spray foam isn't an environmentally friendly product, having an assembly fail and having to discard of all the building products that went into it certainly offsets the negative effects pretty quickly.

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