Smart Vapor Barrier For Ceiling in Cold Climate?
New to me historic home that we are updating. Zone 5 in Northern MI. We have a wing with TIG 1.5″ thick oak planking for the entire ceiling. It’s just TIG planks attached directly to the topside of exposed rafters. Very cool look. The living space is primarily heated with a high efficiency wood burning stove but there is also a mini split in that room.
The attic above this 30’x 15 room/wing had never been insulated (rest of house had) since prior owners only used this wing during summers and closed it off during winter. We are planning to have either cellulose or mineral wool blown in to get it to R49.
The TIG planking is not sealed. I don’t want insulation and dust drifting down into living area through where the tongues and grooves meet.
What can we put down on the attic side of the planks to keep dust out of living space before we blow in the insulation that will still allow vapor to pass when needed? I don’t want to put down something like poly or insulation board since I am afraid it will trap vapor.
I was looking into smart vapor retarders like Certainteed Membrane. Looks like just regular 2 nil painters plastic, though. Would like something more durable if possible.
Any thoughts? Thank you.
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User ...488,
The main function of any membrane you add to the top side of the T&G ceiling will be as an air-barrier. In that capacity it will stop not only a lot of interior air leaking into the attic, but also almost all the water-vapour that piggybacks on it. It's usefulness as a vapour-barrier or retarder is a very secondary concern.
The benefit of a variable-perm membrane over poly is that it won't act as a wrong sided vapour-barrier when the house is being cooled in the summer. However that assumes you are going to air-condition the space for a prolonged period, and that the attic will attract a lot of moist air from outside. If those two things don't occur, it's no more useful than poly there.
Poly would be my choice. Detail it carefully, especially at the perimeter, and try and tie it to the walls to make it as air-tight as possible.
If you don't want poly, CertainTeed MemBrain is tougher than it looks, but if you are want something less likely to tear, you might want to look at Pro Clima's IntelloPlus or Benjamin Obdyke's VaporWise.
If you have a vented attic above that ceiling, you don't need a vapor barrier at all, but you do need an air barrier. I'm assuming you want to leave the T+G planks in place, so you need something that you can install from above. I see two options for you: a thin(ish) layer of spray foam, probably around 2-3", which will air seal everything, and get you some R value too (around R12-R18 for 2-3 inches). If you have any larger gaps in the planks though, there is some risk of some "ooze-through", and that is something to think about if the gaps aren't reasonbly tight. "Reasonably tight" here means if you went up there (don't actually do this though! It's just a conceptual example!) poured a thick liquid like chocolate syrup, would it leak through? If it won't, you're fine, if it will, you have some risk.
The other option would be poly sheet. I don't think you really need a smart vapor retarder like MemBrain here, and poly is a lot cheaper. Get some 6 mil poly, or thicker if you want (8 or 10 mil, I wouldn't go over that because it will be harder to get thicker material to conform to structure), then lay it over the attic floor structure and try to keep it tight to the top of the T+G planks. You don't have to worry about trapping moisture, because the T+G is both vapor AND air open, i.e. air can circulate through it, so you're going to get plenty of inward drying. You can then blow in loose fill insulation (cellulose would be my choice too for this) above the poly layer, allowing the poly to act as your air barrier.
Note that the tricky part is that you really want to seal that poly sheet to the top plates and the entire perimeter of the area you're planning to insulate so that you keep a good air barrier above the entirety of the T+G area. Spray foam makes that easy, but with poly, you need to do it "manually" -- with tape, caulk, and plenty of awkward manual labor.
Bill
I have a cap nailer so securing poly tight and tape/caulking the perimeter a few inches up doesn't scare me too much. I opted not to use spray foam when the insulation contractor did the perimeter walls...and I was worried that poly would end up trapping humid air between the bottom side of the poly and the top side of the planks. I've gotten a lot of varying opinions in the field and I'm a builder. My client (my wife) will fire me if I get this wrong. lol
We don't really use AC much up here. Maybe 20 total days a year.
The planks are painted-forgot to add that. Where the planks connect is what I meant by it not being sealed. There is always going to be a bit of a gap at the TIG.