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Spray foam in cathedral ceiling

I'm building in zone 5 with 2x10 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?

Asked by Scott Schaub
Posted Mon, 12/30/2013 - 15:43

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3 Answers

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1.
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Scott,
Q. "Do you see a problem?"

A. No.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/30/2013 - 17:03

2.
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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.

Answered by Scott Schaub
Posted Mon, 12/30/2013 - 17:25

3.
Helpful? 0

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

A. Yes.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 12/30/2013 - 17:36

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