cathedral ceiling vapor barrier
How many different answers can I get here?
I’ve been reading about condensation, relative humidity, dewpoint, ridge vents, etc, and I am wondering about the choices my contractor and architect have made as far as my roof goes. I gather that you’ve all had variations on this discussion already, but I’m hoping to address my specific circumstance. Thanks in advance for advice.
I have in progress a second-floor addition with cathedral ceilings. Overall 24′ wide. 2×12 rafters, 6×12 structural ridge, with rafters abutting the sides of the ridge beam, then blocked with 2×12 in each bay.
No, I did not build this myself, as I am merely the curious homeowner. However, if I had built it myself, I would have set the upper ends of the rafters atop the ridge beam so that there was at least the possibility of a ridge vent. But that’s another story.
Anyway. The roof is on, R-30 FG insulation is in, kraft face down (with a nominal 2″ airspace between it and the roof deck, I suppose). There is no ridge vent, but there are 2″ holes bored at the top of each piece of blocking between rafters atop the exterior walls. So, is it wise to install a poly vapor barrier on the underside of the rafters before the drywall goes in? Maybe just in the bathroom at least? Or is this just asking for trouble..? What would be the proper installation method– construction adhesive and 6 mil visqueen or something like that? Or do I plug the 2″ holes in the blocking, seal it up tight and cross my fingers?
BTW, I’m in “sunny” southern California, generally hot & dry. Fabulous January this year.
Replies
Given your climate, you're trying really hard to find something to worry about. First off, I doubt you're at much risk for condensation there as the vapor drive should be minimal. Second, the kraft paper is a vapor retarder. Do not use plastic, too. Two vapor barriers in one assembly is a recipe for rot anywhere. I wouldn't lose any sleep over venting the roof, either. Just make sure that any penetrations of the drywall ceiling such as recessed lights are completely air sealed. Then relax and enjoy your new room.
Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom
Thanks for that, Andy. I can always use fewer things to lose sleep over. The roof thing is just such a crucial part of the house... And there's so much chatter to read regarding the venting and the moisture and this and that... I'll be more than happy not to spend time or money doing some unnecessary vapor barrier.
A lot of the chatter about vbs here is from those of us in the north. The vapor drive here is much stronger, so it's more of an issue. In all cases though, air sealing is far more important than vb.Andy
"Never try to teach a pig to sing. It wastes your time and annoys the pig." Robert A. Heinlein
"Get off your dead #### and on your dying feet." Mom