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?
Posted Jul 14, 2014 12:47 PM ET
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