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Does roofing tar paper act as a vapor barrier and how many layers?

I am re-roofing a cathedral ceiling house. Looking up it has 2" tongue/grove pine resting on 4x6 joist beams (5') centers running to a ridge beam. I have done the tear off - and the 2" pine is the ceiling on the bottom side and existing decking on the top side. The pine is the only thing up there now. It's a tight fit. 3/12 pitch - Southern Michigan.
I want to insulate, ventilate if necessary, and re-roof.
What materials should I use?
What is the order of the layers needed?
Can I lay down a layer of roofing paper on the top of the pine to protect it while I make the decisions
(or will this create a vapor barrier in the wrong order and cause problems)?
734-678-0280 - Craig
craig@lanegainer.com

Asked by craig askins
Posted Tue, 10/11/2011 - 11:36

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1 Answer

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Craig,
Q. "What materials should I use?"

A. It will probably be easiest for you to install rigid foam insulation on top of the existing roof sheathing. I recommend that you use polyisocyanurate. You can stack several layers of foam, as necessary, until you achieve the R-value you want. Don't install less than minimum code requirements.

Q. "What is the order of the layers needed?"

A. Asphalt felt, several layers of 2 inch polyiso with staggered seams, 2x4 sleepers laid flat on the foam, eaves to ridge (to provide a ventilation channel), plywood, asphalt felt, roofing.

Q. "Can I lay down a layer of roofing paper on the top of the pine to protect it while I make the decisions?"

A. Yes.

Q. "Or will this create a vapor barrier in the wrong order and cause problems?"

A. #30 asphalt felt has a permeance of 1.75 perm. It's a vapor retarder, not a vapor barrier. It won't cause any problems in your roof assembly.

Answered by Martin Holladay, GBA Advisor
Posted Tue, 10/11/2011 - 13:26

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