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Vented Cathedral Ceiling - Questions for Martin Holladay

I have read your comments extensively and find the information very helpful. However, I have several concerns about my proposed construction.

I plan to construct a vented cathedral ceiling using pre-engineered wood trusses at 24" on center with 2x12 top chords in a Zone 4A location. The IRC 2012 allows R-30 for cathedral ceilings less than 500 SF or 20% of the total roof and my project qualifies for this exception.

My proposed construction includes:
1. Composite polymer shingles over Zip-system 5/8" sheathing and ice dam protection at the eaves
2. AccuVent ventilation barrier on the underside of the sheathing caulked and taped
3. R-30 7-1/4" Roxuul ComfortBatt rock wool insulation (which will leave about a 2" air gap at the bottom
4. 1" XPS R-5 bridging insulation on the understide of the top chord, Styrofoam Cladmate XL or equal
5. 5/8" drywall with vapor retarder paint

My questions are:
1. Is it better to seal the AccuVent with ssSPF closed cell spray than rely on caulking and taping?
2. Will I need straping at 16" or 24" on center on the underside of the top chord or will the combination of the XPS and the 5/8" drywall be sufficient to minimize bending?
3. Do I have any problems with vapor transfer with this system?

Thanks, Jim

Asked by James Enloe
Posted Mon, 07/14/2014 - 11:47

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2 Answers

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Jim,
I don't think it's a good idea to leave a 2 inch gap between the underside of the mineral wool insulation and the top side of the rigid foam. If I were you, I would fill this gap with insulation. (You could easily compress 3.5-inch-thick fiberglass batts down to 2 inches to fill this gap if you wanted.)

Q. "Is it better to seal the AccuVent with closed-cell spray foam than rely on caulking and taping?"

A. Not really. I would just use caulk (and tape at the overlaps) if I were you.

Q. "Will I need straping at 16 in. or 24 in. on center on the underside of the top chord or will the combination of the XPS and the 5/8-in. drywall be sufficient to minimize bending?"

A. Here in New England, we often strap our ceilings with 1x3 or 1x4 strapping, so the usual practice here would be to install strapping under the rigid foam if the ceiling included rigid foam. However, in many areas of the country, drywallers have never heard of the practice of strapping ceilings. Talk to your drywall contractor; either way (with or without strapping) will work.

Q. "Do I have any problems with vapor transfer with this system?"

A. No.

Answered by Martin Holladay, GBA Advisor
Posted Mon, 07/14/2014 - 14:06
Edited Mon, 07/14/2014 - 14:08.

2.
Helpful? 0

Thanks. You have been most helpful. Jim

Answered by James Enloe
Posted Mon, 07/14/2014 - 15:13

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