[agm413]

In my basement which is partially below grade/unfinished ceiling the plywood subfloor does not completely go into CMU wall that is between the next townhouse and mine. picture from below.

The basement does have some conditioning but is typically cooler than the rest of the house because of fewer vents + the garage and a walkout to the back. Can I just use some great stuff between the cement blocks and the plywood to give more air sealing between the floors, or would that not be a good idea?

 

[SW Dweller]

In my basement which is partially below grade/unfinished ceiling the plywood subfloor does not completely go into CMU wall that is between the next townhouse and mine. picture from below.

The basement does have some conditioning but is typically cooler than the rest of the house because of fewer vents + the garage and a walkout to the back. Can I just use some great stuff between the cement blocks and the plywood to give more air sealing between the floors, or would that not be a good idea?

That would work

The spray stuff is really messy, you could try a non drying caulk.

 

[carpdad]

Maybe get small roll of fiberglass insulation, make a big wad and stuff into the gap. Higher r value rockwool insulation is said to be a good air sealer. I haven't used it so don't know if true. Search. Rockwool is firestop as well and that gap could need such.

 

[agm413]

That would work

The spray stuff is really messy, you could try a non drying caulk.

Caulk could be good too - might use a backer rod as well in that case since the gap is larger in some boards vs others.