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Spray Foam Insulation Over Roof Decking With Metal Roofing

TexasTammy | Posted in Energy Efficiency and Durability on

I live in Central Texas. We have extreme weather at times. Triple digit heat, hail, drought, torrential rain, and occasionally power outages due to our weather.
Due to recent hail damage we are considering going with a metal roof. My question is…can we put spray foam insulation on top of our decking and then install metal roofing on top of the foam? If so would it require the acrylic coating mentioned in an article l read?
We would keep several turtle back vents for ventilation. I have read that metal roofs can actually cause mold if not properly vented. Would the SPF pose problems is sandwiched between decking and the metal? The ribs of the metal would  not have foam. The overhangs could be made more “open” if that would be beneficial. Also, will the ridge row require any special consideration? Will the SPF plus the metal be lighter weight than asphalt shingles? The home is a double wide manufactured home that had an addition added to each end. One end contains two bedrooms and 1 bathroom. The other has a double car garage attached to the house via a enclosed breezeway type room. Is SPF a fire hazard? Does insurance companies have any bias towards SPF? Are there government/tax rebates/incentives for utilizing this “green product”? Could my home be used to show local contractors how they could help homeowners become more energy efficient?
I apologize for so many questions and look forward to any answers you may offer. Thank you.

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Replies

  1. Expert Member
    Zephyr7 | | #1

    I don't see how you could make this work. It's very difficult to get a flat surface (pretty much impossible, actually) with closed cell spray foam, and open cell needs some sort of structure to be trimmed flush with (usually studs).

    SPF has some requirements for fire barriers, but it's not considered to be a hazard if installed properly.

    The usual material used to insulate on top of the roof is rigid foam panels, usually polyiso. I would insulate this way and not try to make spray foam work here. Commerical roofs are commonly insulated this way, so roofers are familiar with the product. It's not uncommon to leave a ventilation space between the rigid foam and the metal roof to help with drying.

    Bill

    1. TexasTammy | | #3

      I have heard how fabulous the SPF was with cutting utility costs that l thought l should go that route if feasible and had not considered the foam panels yet... but now will. Thanks for your help.

  2. Joshua Van Tol | | #2

    Attaching the metal roof thorough the uneven spray foam would be a nightmare.

    Why not just use rigid foam, such as PolyIso or EPS? It's cheaper per R, and easier to install. Also it's nice and flat.

    1. TexasTammy | | #5

      I have heard how fabulous the SPF was with cutting utility costs that l thought l should go that route if feasible and had not considered the foam panels yet... but now will. Thanks for your help.

      Reply | Edit Reply | Delete Reply

  3. Deleted | | #4

    Deleted

  4. Expert Member
    Zephyr7 | | #6

    You probably heard that in marketing materials from spray foam people. Yes, SPF is a very good performing insulating material, but that doesn't mean there aren't things that can perform just as well, or even better, in the right situations. Insulating a roof from the interior, such as a non-vented catherdral ceiling, is a great place for SPF. Insulating a roof from the exterior is NOT a good place for SPF. Insulating a roof from the exterior is a great place for rigid foam, which will actually outperform SPF since it will be a continuous layer without thermal bridges.

    Another place where SPF works, but really isn't the best option, is in typical studwalls.

    BTW, with all that "don't use SPF here" stuff I just said, keep in mind that I'm one of the more pro-SPF people on this site. SPF should really be thought of as a niche product, and the big niches are UNvented roof assemblies and uneven foundation walls (cut stone, etc). When you're insulating on the exterior, that's the place to use either rigid foam or something like Rockwool Comfortboard, not SPF.

    Bill

  5. TexasTammy | | #7

    Thanks 👍

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