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Community and Q&A

Rooftop rigid insulation and solar panels

jeffodonnell | Posted in Green Building Techniques on

We are in the design phase of a modest story and a half house in SE Minnesota, zone 6. My builder suggested room in attic trusses for our upper floor, as well as a few inches of closed cell foam on the interior against the roof deck, followed by fiberglass or cellulose. I realized that may not be enough to control moisture in a cold climate and am debating getting a quote on 4 inches vs looking into rigid foam on the outside of the roof deck. One detail that makes it interesting is we are looking at a bonus room above the garage as well. We plan on using metal roofing of some sort.

My questions are: Can someone help me cost compare the two options for my area as well as help me understand the right way to install the exterior foam so that I can install solar panels in the near future? (Sheathing vs strapping) A polyiso/esp combo seems appropriate to me, but expensive. (I would love to find reclaimed) Also, can someone direct me to some pictures of what a thick roof like that will look like when done?

Thanks for the help.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can someone help me cost compare the two options [spray foam on the underside of the roof sheathing versus rigid foam above the roof sheathing] for my area?"

    A. Costs are intensely local, so you will have to do your own research by getting bids. In most cases, the least expensive way to proceed (and the most environmentally friendly way to proceed) is to install used (reclaimed) rigid foam above the roof sheathing.

    Q. "Can someone help me as well to understand the right way to install the exterior foam so that I can install solar panels in the near future?"

    A. PV installers have access to a huge number of options when it comes to rack attachment hardware. A good PV installer should be able to select fastening hardware that works for a roof with rigid foam above the sheathing.

    For more information on this issue, see Solar on SIPs.

    Q. Also, can someone direct me to some pictures of what a thick roof like that will look like when done?

    A. You can look at photos of Joe Lstiburek's house -- he installed rigid foam above his roof sheathing as part of a retrofit project. Read the article here:

  2. Stockwell | | #2


    I was looking at similar circumstances as you in my new yet-to-start build(foam, metal roof, solar etc.). If you are doing foam outside the first layer of sheathing, I would suggest laying nailbase over that, screwed down into the framing below. That will provide a firm foundation for the metal roofing and, in turn, your solar panels. The downside? $$$$ that's the only online place I have seen that directly lists pricing. It quickly adds up. I opted to use trusses, spray 2 1/2" closed cell foam against the sheathing(zone 4), then open cell foam below that to get to R-49(open cell is cheaper than cellulose in our area). There is very little thermal bridging because the truss webs get mostly covered in foam. I will have plywood sheathing, then underlayment, metal roof, solar panels. Either way works, but one way is cheaper.

  3. jeffodonnell | | #3

    I can find 4x8 sheets of polyiso with black felt in sizes of 1/2, 1, 3, 4, and 4.5 inches and cost from $5/sheet to $26/sheet. Knowing that multiple layers are best, and the 4.5 is cheapest per r, is there a good combination to be thinking of buying when it comes to rvalue as well as the other details of finishing the roof? Should I still consider eps on top because of my climate?
    Kevin, I still plan on looking into the nail base option, just need to check with my supplier.

  4. GBA Editor
    Martin Holladay | | #4

    I think it's a good idea to have two layers of rigid foam with overlapping (staggered) seams. Of course, that's probably going to cost more per R than a single layer of rigid foam. Only you can decide whether your budget can handle the upcharge.

    When it comes to evaluating the cold-weather performance of polyiso, you can either de-rate the polyiso R-value to account for cold weather, or add an exterior layer of EPS to keep the poly warmer. Either strategy works. If you don't know how to proceed, price out the EPS and base your decision on price.

  5. jeffodonnell | | #5

    Thanks Martin and Kevin for your answers. Joe Lstibureks articles(and pictures) have helped quite a bit in deciding on at least two layers. I need to review the articles here to decide on derating vs eps layer.It's just time to get all the numbers and have the conversation with my builder. He's a big fan of spray foam.....

  6. Expert Member
    Dana Dorsett | | #6

    In zone 6 you need to have at least half the R on the exterior for dew point control, and to be conservative you'll have to derate the polyiso to R5/inch or a bit less, so you'll need at LEAST 5" to be able to use R25 fiber insulation or open cell foam on the under side of the roof deck for an R49-ish roof. With 2 layers of 3" polyiso derated to R5/inch for climate you can go to R28-R30 or so on the interior.

    If going with EPS + polyiso you'll still want 3" polyiso + 3" EPS (top layer), though 3" + 2" would still make it with R23-R24 under the roof deck:

    The 2lb polyiso typically use for roofing polyiso has a labeled R of R5.5/inch, so at 3" it's R16.5 (labeled), R15(derated). Type-II EPS is R8.4 (labeled), but will perform at about R9.5 during the coldest weeks of winter. So from a labeled R point of view you'd be at R24.9, and R24 would be needed under the roof deck to hit code on an R-value basis. (It would beat code even at R22 under the roof deck on a U-factor basis, due to the exterior foam thermal break over the rafters.) From a derated & uprated point of view, you'd be at about R24.5, so you're still fine if you limit the interior side R to R24., but would be pushing your luck if taking it to R28 or higher with only 2" of EPS.

    But R28-R30 fiber would be fine if it's 3" EPS (R12.6 labeled , but ~R14+ during the coldest weeks.)

    Think about the amount & type of insulation you intend to install on the interior- it matters.

  7. Stockwell | | #7

    You can get the polyiso for a reasonable cost, but there are far more costs. You will definitely need another layer of plywood sheathing over the foam. The long screws you will need to anchor that down through the foam are not cheap. Labor will be a significant factor. Then you have to get everyone to sign off on that combination being able to hold the metal roof and solar panels. It's not an easy project.

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