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Do unvented cathedral plank ceilings require spray foam?

I am designing the envelope of a 600 sq. ft. house to be built later this year. It has a cathedral ceiling with a few irregularities that would prevent proper venting. I was planning to use cellulose between the rafter bays, insulate on top of the roof sheathing with rigid insulation, and use the airtight drywall approach to keep interior house moisture out of the cellulose. However, my wife and I want to build the house in a cottage style with wood planks on the walls and ceiling. If I cover the drywall with planks, I have added lots of holes which worries me that moist air is going to enter the cellulose. Does this mean that to avoid moisture issues, I must use spray foam instead of cellulose?

Asked by Eric Matsuzawa
Posted Feb 10, 2013 1:32 AM ET

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2 Answers

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1.

Eric,
Even if you have a thick layer of rigid foam above your roof sheathing -- which is the usual way to safely install cellulose in an unvented cathedral ceiling -- it's still a good idea to have an air barrier under the cellulose. Most builders use either gypsum drywall or rigid polyurethane foam as the air barrier under the rafters. Once the air barrier is tight and the cellulose is installed, you can then install a layer of boards as your finish ceiling. The nails won't ruin the air barrier.

Another option, as you suggest, is simply to use spray foam.

All of these options are explained in this article: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted Feb 10, 2013 6:22 AM ET

2.

Martin,
Thank you for clearing up my options for me. I didn't know that nails wouldn't harm the air barrier. Does rigid polyurethane foam have any benefits over gypsum drywall under the rafters other than increased R-value?

Answered by Eric Matsuzawa
Posted Feb 10, 2013 7:47 AM ET

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