Why don’t we use plastic as a barrier between the ceiling and a vented attic?
I have been thinking about this. Why couldn’t a builder staple a 2′ x the length of the top plate piece of plastic to the top plates of the walls before the rafters are set? This piece could then hang down a foot on either side of the wall and still leave the ceiling open for electrical. Then, once its time to close up the ceiling, you could poly the whole ceiling and have an ample piece next to every wall to tie into and gett a good tape seal on. It seems like this could be much more efficient than using plywood/osb for an air barrier. You could still use the sheetrock on the walls as an air barrier, and the attic insulation could breathe through the roof vents.
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Replies
Ben,
Every house needs an air barrier. Your wall needs an air barrier, and your ceiling also needs an air barrier. You also need to do a good job joining your wall air barrier to your ceiling air barrier.
It sounds like you are aware of these facts, and are thinking ahead. That's good.
In most U.S. climates, you don't need interior polyethylene as a vapor barrier. If you live in the coldest sections of Montana, Minnesota, Vermont, or Maine, you might want to use interior polyethylene as an air barrier. But in other areas of the country, it isn't a good choice -- especially if your home will be air conditioned. Interior polyethylene will prevent your wall and ceiling assemblies from drying to the interior.
In most cases, the easiest material to use as your ceiling air barrier is gypsum drywall.
Its funny, you skipped right over my area of North Dakota. I understand that we are trying to get away from poly on the walls, however, it seems like it could be easier to detail the ceiling with my method. I also could be completely wrong, but having tried the airtight drywall approach, it can get a little messy and takes a lot of caulking to do the job. If I wanted to try my method, is there any reason I shouldn't?
Ben,
You can certainly install interior polyethylene in North Dakota if you want, especially on your ceiling (under a vented attic).
If you plan to air condition your house, interior poly can still cause problems if it is installed on your walls, even in North Dakota, especially if you install so-called "reservoir" cladding and vapor-permeable wall sheathing.
Decades ago ceiling poly was a no no. Martin?
AJ,
It depends on the climate. Ben lives in North Dakota, so he should be fine -- especially if the insulation is installed on the floor of a vented attic.
I have considered almost the same thing. What if he used house wrap instead of polyethylene? That way he could establish an air barrier that is vapor permeable. I'm guessing that taping up tyvek that is stapled to the bottom of the roof rafters is even more trouble than detailing air tight drywall? It is still an intriguing idea to me.
Ben's proposal to drape strips of poly over the top plates, for later joining to a sheet material for air barrier purposes, is what I did. However, I used MemBrain for the air barrier, for the reasons Martin cited (vs poly). Being able to piece in a sheet material on the ceiling and on the walls separately and join them easily and reliably struck me as a good way to address the wall-ceiling joint. I did get a good blower door result, 0.65 ACH/50 on the first pass.