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Can I use rigid foam above roof sheathing and closed-cell spray foam below?

user-5811309 | Posted in Energy Efficiency and Durability on

I’m going to be re-roofing my house (when the weather gets a bit warmer), and feel this is a good time to upgrade the insulation. Here are the particulars of my house:

I live in climate zone 7.
Cathedral ceilings in most of the house.
True 2×8 rafters.
Low slope roof – modbit.

I know I have to go with an unvented assembly. I like the idea of rigid foam above the roof sheathing, to help with things like thermal bridging. However, I don’t like the idea of increasing the thickness of my roof at least 6″ to get the minimum R30 for condensation control. So I’m wondering, can I use rigid foam above the roof sheathing, or another product, in conjunction with closed cell spray foam beneath the roof sheathing in the rafter bays, to get a high R-value?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Edward,
    I don't recommend that approach, because you would end up sandwiching the roof sheathing between two vapor-impermeable layers. That means that if the sheathing every gets damp, it can't dry out. Adding R-30 above the roof sheathing is the preferable approach.

  2. user-5811309 | | #2

    Thanks for the reply, Martin!

    I have a couple more questions in regards to insulating from underneath.

    First, is there any concern for the roof sheathing if insulated using closed cell spray foam underneath and having a mod-bit membrane on top?

    Next, I'm considering insulating using the flash and batt method to bring down costs: I'm looking at 5 inches of foam and filling the rest of the rafter bay with roxul. I'm wondering if I can install rigid foam panels underneath (before putting up drywall), or if it would be better to use rigid wool insulation (for the same reason you listed above)? Or, would it be better to fill the entire void with closed cell spray foam if I were to install rigid foam in this manner?

  3. GBA Editor
    Martin Holladay | | #3

    Edward,
    Q. "Is there any concern for the roof sheathing if insulated using closed-cell spray foam underneath and having a modified-bitumen membrane on top?"

    A. In a word, yes -- although it is certainly true that many builders install closed-cell spray foam on the underside of roof sheathing, even when the sheathing is topped by membrane roofing that makes outward drying impossible.

    Most (but not all) types of roofing make outward drying of damp roof sheathing impossible. That's why the best approach, by far, for insulating a low-slope roof like yours is to install an adequate thickness of rigid foam insulation above the roof sheathing. The rigid foam insulation keeps the roof sheathing warm and dry. Here is a link to an article with more information on this topic: Insulating Low-Slope Residential Roofs.

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