Poly Sheeting for Air Barrier
The attic in the house I’m building is vented and entirely out of the conditioned space. As such, I need to be very careful about the ceiling and how well it is air-sealed.
The given part of the ceiling is that I will run 1×3 strapping 16″ O.C. perpendicular to the joists and attach 5/8″ Type X drywall to said strapping. I will minimize the number of protrusions through the drywall but will not eliminate them entirely. At a minimum, there will be several ceiling fans, a few select recessed lights (judiciously placed — most lights will be wall-mounted) and maybe an exhaust fan and each will be carefully sealed. I plan on using the 3/4″ gap between drywall and joists to run select electrical, where it makes sense.
What I’m also wanting to do is first staple 6mil poly sheeting (typically used as a vapor barrier) to the bottom of the joists before the strapping, sealing all of the edges with Tyvek tape equivalent. I see this concept being done in many photos of “smart vapor barriers” like Intello Plus and similar. I have zero need for a vapor barrier in my neck of the woods (Phoenix, AZ) so using inexpensive poly sheets would be every bit the air barrier without any “smarts” needed.
My thinking for adding this poly, first, is mostly as a “belt and suspenders” method of air sealing. That is, while the ceiling fan boxes are going all the way through regardless (and would thus be sealed twice), I might be able to reduce the intrusions of the recessed lighting through the poly to just a few individual wires (ultra slim canless lights), which is easier to seal.
As a side benefit, there will be an empty gap of 3/4″ between the drywall and attic insulation, making it easier in some potential future to fish more electrical through it, possibly. That’s not a super compelling reason at the moment but who knows later?
It’s just that I never see this done and when the equivalent is done with Intello Plus and such, the focus is on the vapor part and not the air sealing part, usually.
What are your thoughts on my “belt-and-suspenders” idea of poly + drywall?
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I wouldn't bother. The poly doesn't really add much if any benefit to the drywall itself as an air barrier, and it's tricky to seal any electrical penetrations to the poly (it's actually easier to seal them to the drywall). I think you're making a lot of extra work for yourself if you try this, for little if any real benefit.
You won't be able to fish wire in the gap between the drywall and the poly without damaging the poly. Any wire fishing tools or the wire itself will snag the poly, probably in multiple places, and make big tears. This will snag your fished cable, making it a bear to complete the job, and destroy your redundant air barrier at the same time. I've had this happen myself (in walls), as I used to be a cable installer (datacom stuff, paid for college that way), and have fished untold thousands of runs and hundreds of miles of many kinds of cable.
Unfortunately, although you may only want to use the poly as an air-barrier, but it will still act as a vapour-barrier too. You don't need one in Phoenix, but if there is one installed it will be on the wrong side of your assembly in a cooling climate, and may become a condensing surface.
Thanks, Bill and Malcom. I am sufficiently convinced that it's not only not worth the effort but will be counter-productive!
Glad to find this article as it describes a strategy I was also looking to employ. Exactly the same, except I am in 4C Marine climate, north of Seattle, WA.
Bill, I hear your concern over the fishing of wires under the poly (not something I am looking to do). If I spec a thicker (10 mil, 15 mil?) plastic, would this still be a futile attempt? I am concerned of poor craftsmanship creating a poor air barrier at the ceilings, and do not want to spend the extra money to create a service chase beneath the drywall (avoiding all penetrations).
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In temperate climates that don't require prolonged periods of cooling, poly works well as an air barrier. It is what is used as a combined air/vapour barrier in a good 95% of Canadian homes largely without problems.
While I can see the advantage of smart vapour-retarders on walls, many of which benefit from being able to dry to the inside, ceilings under attics are different and that advantage isn't useful.
I've never had problems either fishing or sealing wires using 6 mil poly. I'm not sure anything thicker is warranted.