Air Sealing an Old Attic Prior to Adding Insulation
I’m working on an air sealing plan for the attic of an old house. The blown-in insulation has been removed for the air sealing work. The ceiling of the room below has wood planks nailed to the bottom of the attic joists so there are lots of spaces that air can leak through to the attic. Of course, we will eventually install drywall on the ceiling but I think it would greatly improve the overall performance of this old leaky house if we capped the attic floor. I want to add an air barrier on the attic side by putting something down between the joists. I believe I have two options, (1) spray foam the entire floor of the attic (either between the joist, or including the joists) or (2) install rigid foam or reflective foil insulation between the joists and caulk the edges that touch the joists. Does anyone have any feedback or ideas for the easiest and most affordable method to seal up the attic?
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Just another option. How about a permeable peel and stick membrane, like Henry’s Blueskin? You could stick it in the bays, and then wrap right over the joists to the next bay.
Thanks David, that's an idea however the attic would have to be super clean so the paper would stick and there are places where there are posts or other obstructions that might make installation difficult. I like the idea of being able to just lay something down though without have too much cutting. Makes me think that spray foam, though expensive, might be the easiest to install. Maybe it would be easier to install something like the Blueskin to the ceiling of the room below the attic and caulk the perimeter, and then air seal the perimeter of the attic with spray foam including the top of the wall bays where there may or may not be a wall plate.
I suspect that if you plan on putting in taped drywall below, air sealing efforts above the ceiling will not be value added.
I'm personally not a fan of the spray foam idea. In addition to not liking spray foam for most situations generally, I think that attics are subject to high variations in both temperature and humidity, and movement of the wood means that the foam will crack and eventually leak. If you're doing drywall, I'd skip the foam on the top side, and only do the Blueskin if you have a large amount of money to spare. Probably better (imo) to spend it on air sealing elsewhere, or for hiring a competent HVAC consultant and installer (probably two separate people).
Andrew, that’s true! If sheetrock is part of this project, she can use the airtight drywall approach. Then maybe foam the top of the balloon framed walls if there’s no top plates. I would also second spending some time and money on a competent HVAC designer, that does accurate, real, load calcs, and then a good installer, especially if a duct system is needed
If the ceiling is open, I would install stapled on air barrier, tape the edges and corners, and then install 1x furring strips to protect the air barrier from drywall screws. It's much easier than working from the top, since it's one flat plane.
You could also remove all the existing strips/planks and do the same procedure if head height is an issue.
Another possibility, one with lots of debate, is to remove the strips/planks, and install a layer of OSB. You'd again tape the edges and seams, but would provide adequate air sealing.
Thank you Andrew and Aaron. It does seem a whole lot easier to install the air barrier from below. Aaron, why would the furring strip nails not affect the air barrier like the drywall screws? Guess what I'm saying is, are the furring strips really needed?