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Community and Q&A

Cathedral ceiling R-value boost

user-901114 | Posted in General Questions on

Zone 6. Layers from the outside in:
Galvalume steel roof
Grace advantec
Plywood deck
1 inch air space
1 inch foil faced tuff r
5.5 inch roxul
Rafters are filled at 7.5 inches.. 24oc

Question; what can I put under the roxul and rafters to get r38?

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  1. GBA Editor
    Martin Holladay | | #1

    I would recommend that you install a layer of rigid foam.

  2. user-901114 | | #2

    What kind would you recommended? Xps?what are the pros and cons of this assembly? Thanks.

  3. GBA Editor
    Martin Holladay | | #3

    The pros and cons of the assembly? You're going to end up with a "foam sandwich," which is not ideal. It's usually better if the foam is only on one side of the assembly, not both. But in this case, you'll probably be alright.

    I would recommend foil-faced polyiso -- as much as you need to get the required R-value. Tape the seams with a high-quality tape, and install strapping. The foil facing plus the air space give you a slight boost in your R-value.

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