My original design for my cathedral ceiling involved 2×12 rafters filled with fiberglass batts or cellulose. Then with 1″ xps foam installed directly on top of the rafters. I planned on installing purlins on top of the foam board with metal roofing screwed to the panels.
To comply with the code for unvented cathedral in Indiana, I am supposed to have r-15 of foam above the sheathing. I don’t intend to have conventional plywood or osb sheathing. It seams like all the code is made to vent the moisture off the sheathing.
What if I just considered the space between the purlins to be the “vent”?
Thanks for any advice.
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First of all, you need an air barrier to enclose the top side of your fiberglass or cellulose insulation.
Second, you need a surface on which to install your asphalt felt underlayment. (The asphalt felt catches dripping condensation, and holds the water long enough to evaporate, while helping keep your fluffy insulation dry.)
The best layer to accomplish this is plywood or OSB sheathing. Above the asphalt felt, you can install purlins.
Rake-to-rake venting in the air spaces between the purlins works just fine, but it may not be acceptable to your local building department. Talk to your inspector to find out.
For more information, see How to Build an Insulated Cathedral Ceiling.
The R15 exterior foam prescriptive in the IRC presumes an R49 total R. It's the exterior R to total R ratio that determines the temperature of the roof deck, which is what's relevant. If the cavity fill is 11.25" deep with pretty good R3.7/inch cellulose, the cavity fill is then R42-ish, in which case you would need MORE than R15 above the roof deck. (The cavity fill has to be no more than 70% of the total. R42/0.7= R60, so you'd need at least R18, not R15 to fully protect the roof deck.
You can still get there without exterior insulation if you use a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus between the gypsum & fiber, and take pains to make it air tight to the interior. These materials are class-II vapor retarders when the proximate air is dry (like it would be in winter, as long as you ventilate adequately and don't run humidifiers), but become vapor open when the proximate air is humid (like it would be during the warmer spring days, when the roof deck is releasing it's stored moisture burden into the rafter bays.) The code is silent on the use of these materirals, but they work better than fixed permeance class-II vapor retarders such as "vapor barrier latex".
Even if it didn't have #30 felt or other vapor retardent materials on the top side, the vent space between the purlins is still not good enough, since the wintertime moisture drive is from the other side of the roof deck, which is itself fairly vapor retardent. (Half inch OSB is less than 1 perm when dry). When the vent space is below the roof deck, the diffusing moisture enters the vent space and is constantly being purged t the exterior by convection, rather than encountering the llow permeance wood first. You can sort-of get away with venting it to the exterior using #15 felt in a zone 4A climate but that would be less water proof to the drips and wind-blown moisture that gets by the metal roofing. It would be straying a bit further outside of code than smart vapor retarders would be, and a bit riskier.
I had a cathedral ceiling in a rent house many years ago and we tried all kinds of ways to add insulation to it to make the living space below more comfortable. We ended up having to replace the roof and went ahead and re did some of the decking that was bad especially over the cathedral ceiling with a radiant barrier decking. The difference was like night and day. We insulated with dense cellulose and they put in a continuous vent for air movement. So far the upgrade had been great. My guess is that the roof and interior ceiling are so close together, you need a way to either remove or reflect the heat from reaching the drywall so it does not transfer down into the living space below. Hope this helps. http://www.avalancheinsulation.com
I believe the best and fastest way to do a cathedral ceiling is with the use of SIPs. An 8" urethane SIP would have an R-50 and a 12" styrene SIP would have an R-48. Use a T&G SIP joints, tape and seal ALL interior joints (including ridge beams) and make no roof penetrations in the cathedral SIP. It will be easier, faster, better insulated and stronger than any rafter type of setup.