How do I make an airtight ceiling if it’s rough sawn exposed beam construction?
mcooney| Posted inGreen Building Techniqueson
I’ve got a camp in the woods that I’m adding a room onto. I have dug the foundations and constructed the walls. I just watch Dr. Joe’s video on roof venting and he says that the number one rule is to make an airtight ceiling. The trouble is that I have a rough sawn hemlock ceiling with exposed joists. I am intending on leaving it unfinished to fit the more rustic look of the camp. Can I lay poly on the top of the ceiling deck (the attic floor) to make the ceiling airtight or will that create vapor issues?
Thanks,
Mike in Maine
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Replies
If the attic or cathedralized ceiling is vented above the ceiling plane (eg soffit-to-ridge venting), it's OK do use polyethylene sheeting as the air barrier. It's easier to damage than using 1/2" gypsum board as the air barrier though. If going with a gypsum air-barrier you may have to use flexible sheet goods over the beams, glue & tape sealed to the gypsum air-barrier for continuity. A layer of OSB could work as well, though not quite as air-tight as gypsum board, it's has variable permeance to water vapor (its a "smart" vapor retarder), which may be preferable to gypsum board (which is very vapor open) or polyethylene (which is extremely vapor-tight.)
Thanks Dana. Are you suggesting that I lay gypsum or OSB on top of the ceiling deck in the attic space? The roof rafters have not been installed yet so that would be relatively simple to do. I'm guessing that the sheets would have to be sealed at the seams. BTW, the roof will be vented soffit to ridge.
Mike,
If this is a level (horizontal) ceiling, and if you have a vented, unconditioned attic above the ceiling, then you definitely need to install an air barrier above the ceiling. If I were you, I would probably use 1/2-inch plywood or OSB with taped seams. (Use a high-quality tape for this purpose.) Once the air barrier has been installed, you can install insulation on your attic floor.