2018 IRC – ceiling vapor retarder
DBars
| Posted in Building Code Questions on
If my wall assembly requires a class II vapor retarder per R702.7, does my ceiling assembly also require a class II vr by virtue of R805? If not, where are the requirements for ceiling vapor retarders?
Thanks, Daniel
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R805.1 says yes: https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction#IRC2018_Pt03_Ch08_SecR805.1. That's assuming you have a vented roof. If you have an unvented roof, these are the rules: https://codes.iccsafe.org/content/IRC2018/chapter-8-roof-ceiling-construction#IRC2018_Pt03_Ch08_SecR806.5
Awesome, thanks.
I am unvented, with rigid over sheathing and permeable under, in zone 4C. What I read in 806.5 is that class 1 vapor retarders are not permitted at the ceiling. Since it says nothing else about vapor retarders, it's OK to put either a class II, or class III, or no vr at the ceiling, is that correct?
Yes I believe that is the case. I would be cautious, though, if your air-sealing details are anything but perfect; a lot of ridges rot out due to warm, moist air floating up there and doing its best to get out when it's cold outside, or trying to get in when it's hot and humid outside. I would also not want indoor air circulating freely into the rafter cavities, so I would use either painted drywall or a variable permeance membrane.
Thanks for the caution. We'll have to be meticulous about keeping out the air.
To give you a better picture of what I'm planning, please see the detail. I noted a class III vr (latex paint) for the ceiling, and class II vr (kraft faced insulation) for the wall. How does this look to you?
Daniel
That looks pretty good to me. I like to see a more robust rain screen but your climate is pretty balanced so the drainwrap is probably enough. Making the drywall into a real air control layer is challenging; I recommend moving it to the sheathing at both the walls and roof.
Thanks for checking it. Ideally, should there be an exterior air barrier (red line), and also an interior air barrier (blue line)? - or would this be considered a double air barrier?