Possible attic floor insulation with vermiculite/fiberglass, then Foamglas resting on joists, then cellulose?
I’m in Maryland, climate 4A, mixed-humid. I am thinking of a retrofit attic insulation in my 1930’s 2-floor brick house. The attic floor currently has 2″ vermiculite, then covered under my direction a few years ago with R-19 + R-11 fiberglass batts. I did not know about air sealing at the time, but must assume that there is air leakage. After reading at GBA and BSC, I worry that air leakage will accumulate water on the underside of the attic decking. I’d like to fix that.
Because of the vermiculite, which I must assume has asbestos, I don’t want to disturb the attic floor to seal properly. I initially thought of converting to a non-vented attic, with insulation boards to rest on top of the roof deck (the roof needs replacing anyway), but I believe this would be very expensive because this is a complex roof. I also cannot have a ventilated cathedralized attic because the attic has no soffits, with instead gable-end windows.
But before considering insulation between the rafters in a non-ventilated roof (e.g. rigid foam attached to rafters with cellulose blown behind it), I’d like to ask your reaction to this idea to essentially transfer the attic deck insulation (e.g. R-15 above and remainder below the deck) to do the same on the attic floor. From inside to outside:
1) existing plaster ceiling
2) 6″ floor joist space filled with 2″ vermiculite then R-19 fiberglass batts
3) remove the batts that cover the joists, and instead lay down and seal a layer of Foamglas insulation, resting on top of the joists. This would make a walkable floor on top of the joists, as an air and vapor barrier (perm 0.0). Construct Foamglas boxes over the ceiling light and plumbing penetrations to allow for easier future access from above. (No HVAC in attic). At periphery, seal between Foamglas and top of brick walls.
4) R-40 cellulose on top of Foamglas
In sum: ~R60 attic floor with ~R-23 below air/vapor barrier and R-40 above.
Weaknesses or concerns?
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