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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