Plan for air sealing attic floor.
Long time reader, but first post. I have an 1890s house in Ohio. The attic is uninsulated due to some knob and tube wiring which will be gone this year. I plan to air seal the attic floor, then blow 18″ cellulose. My dilemma is that I have possible future plans to remodel the 2nd floor below the attic. Obviously I don’t want to have to deal with the cellulose at that time. I was thinking about airsealing the attic floor using cut and cobble 1.5″ foam board installed between the attic floor joists about 2″ above the plaster and lath ceiling below. I would attach ledger strips to the joists at the proper height to support the foamboard before sealing the seams with spray foam. This would accomplish the airsealing I need, but support the cellulose so that in the future I could remove the plaster and lath ceiling below without affecting the attic insulation. Is there a reason not to do it this way? If not, I have 2 questions.
1. Will 1.5″ foamboard support the weight of 18″ cellulose without sagging?
2. What type of foamboard would be best? Would polyiso serve the purpose?
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Replies
That sounds like a lot of work. Are the bays relatively unobstructed? My thought would be to drape something like Tyvek over the joists. Fasten it at the ends with a 1x2 screwed to the top of a joist.
The joists are held up by 1/4 bracing to the roof rafters, sort of like a built in place truss. I think it would be difficult to get effective air sealing with the tyvek.
When remodeling older places, I've had no problem installing new drywall over old damaged plaster. Takes a bit more effort to locate the joists and need long drywall screws but saves having to demo. Plus you can't beat the sound attenuation of a plaster ceiling. You do need to extend the existing ceiling boxes which is pretty easy. The only thing that sucks is cutting through that much stuff if you need to install any new lights.
This would let you air seal the attic floor as is and not have to cut and cobble which is a pain.
I'd go with Akos suggestion of a new layer of drywall right over whatever you have now. If you don't have that option, I see no reason you couldn't use 1"+ polyiso, but I wouldn't trust canned foam to hold it in place in a semi-structural way to support insulation without anything below the polyiso to offer physical support. What I would do in this case is tack 1x2 furring strips along the inside edges of the floor joists (a finish nailer works great for this), then lay some 1/4" wafer board, or thin plywood/OSB, on top of the furring strips. I'd air seal the attic floor the usual way so that the wafer board doesn't have to be sealed, since I think it is easier to build things out that way. In this assembly, that wafer board supports the insulation and the furring strips support the wafer board, so you could completely remove the plaster/drywall and not worry about the insulation above.
Bill