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Community and Q&A

Spray foam in cathedral ceiling

Scott Schaub | Posted in Energy Efficiency and Durability on

I’m building in zone 5 with 2×10 roof rafters. Do you see a problem using 2″ closed cell on the underside of the roof sheathing with the remainder of the cavity filled with open cell in order to achieve the code required R-38?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Do you see a problem?"

    A. No.

  2. Scott Schaub | | #2

    I should know better than asking a yes/no question of a Vermonter. Can you help me understand why, in the case of a typical flash and batt you say to follow the tables for exterior foam in establishing the thickness of the spray foam (flash) where in the scenario I describe, you think ~R-13 closed cell is sufficient? Is it because open cell foam is air impermeable where fiberglass batt is air permeable? If the answer is a single syllable word other than yes, please clarify. Thanks.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Is it because open-cell foam is air-impermeable where fiberglass batt is air-permeable?"

    A. Yes.

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