Building wrap as an air barrier above a cathedral ceiling?
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Hello,
I’m building a small house in Alaska with a 12/12 cathedral ceiling. I’m planning on a 1 1/2″ vent channel above the fiberglass batts I will be using. Venting will be the usual eave to ridge vent design.
I’ve been advised to create an air barrier above the batts so they aren’t wind washed by air traveling up the vent channel.
I haven’t been able to find much information on this approach, and none of the builders in my area are familiar with it. Can anyone supply me with some “do’s and don’t’s” one this technique?
Also, as the building is already dried-in, I would need to run the building wrap air barrier into the rafter bays one at a time. I was hoping I could cut the roll of building wrap into small sections that I can secure in each bay by wrapping one end around a 2×2, securing with screws against a rafter, stretching tight to the next rafter, and securing with another 2×2. Would this provide enough resistance to keep the batts from squeezing into the vent channel?
thanks for any information.
JS
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Replies
Use the premade vent channels. Your approach sounds time consuming and tedious. You still need an air barrier. May want to rethink fiberglass. May also want to address thermal bridging through the rafters. I hear Alaska is pretty warm lately, but can't really count on that forever. 10" of fiberglass will be well under code. Lots of discussions here about how to insulate a cathedral ceiling.
Justin,
For information on how to insulate a cathedral ceiling, as well as a discussion of different materials you can use for site-built ventilation baffles, see this article: How to Build an Insulated Cathedral Ceiling.
Thanks for the reply. I've found that article helpful already. It mentions the need for an air barrier on both sides of the insulation when using fiberglass, but doesn't specifically go into the idea of installing a membrane of housewrap as the separation between between the insulation and the under-sheathing vent channel. That's what I'm hoping for tips on!
And, as for the pre-made vent channels, they make me worry that I would be creating a secondary vapor barrier. Is this an unreasonable fear?
Because I'll have a vapor barrier on the warm side consisting of a sheet of poly and then 1 1/2" rigid foam, then the batts, putting styrofoam or plastic vent channels on the cold side makes me think I would be tapping any moisture that managed to get into the rafter cavity. No?
Justin,
I agree with Stephen -- I think that installing housewrap in an airtight manner in this location, while maintaining a consistent air gap between the housewrap and the roof sheathing, is difficult and fussy work.
I would use thin plywood, rigid foam, or commercial ventilation baffles for this purpose.
Justin,
If you prefer a vapor-permeable ventilation baffle, choose fiberboard, thin plywood, or EPS -- all of which are vapor-permeable enough to address your worries.
Hi Martin, I'm re-opening this thread for a moment....
I am planning on using thin plywood as you recommended as an air barrier above the batting insulation in my cathedral ceiling. My supplier doesn't stock 1/8" luan plywood, but does have 1/4". Would 1/4" plywood be vapor permeable enough for this application?
Justin,
Q. "Would 1/4-inch plywood be vapor permeable enough for this application?"
A. Yes. For more information on this topic, see Site-Built Ventilation Baffles for Roofs.
In that article, I wrote, "What if interior moisture is able to reach the underside of a ventilation baffle — isn’t it possible that the moisture might condense against the baffle (especially if the baffle is cold)? If so, isn’t this a good argument in favor of using vapor-permeable materials (for example, fiberboard, cardboard, or thin EPS) for ventilation baffles?
"The answers to both questions is a qualified yes. Anyone worried about this possibility should probably make their ventilation baffles out of a vapor-permeable material.
"That said, there really aren’t any reports of failures or problems resulting from the use of vapor-impermeable materials — for example, polypropylene, vinyl, or foil-faced polyiso — to make ventilation baffles. The main reasons:
Not much moisture manages to make its way to the ventilation baffles (especially in homes that pay attention to airtightness);
The air in the ventilation channels is often warmer than outdoor air, a fact which limits condensation; and
Any moisture that does make its way there seems to be incorporated into the rafters via sorption. The ventilation channels are able to remove a limited amount of moisture from the rafters, and it appears that the rate of drying exceeds the rate of wetting."
The more vapor open and the more water proof your venting channel is, the more benefit you get out of it. It is definitely possible to build a vent baffle that is very airtight in between the rafters. You can see a good example in Alex Wilson's house <a
Thanks for all the info and the links! Justin