Expansion around wood joists supported by slate?
My husband and I are soundproofing our townhouse basement, constructed in 1939. Between the houses is a concrete foundation with wood joists supporting the first floor. Under each joist is a small pile of slate. The joists and the slate seem to be where the brick wall separating the houses begins, but the actual border of what material starts where is difficult to see.
My question is, as one of many steps to reduce noise between the buildings, can I put cement in these gaps in the materials around the joists? Would foam allow for more expansion in the building material through the seasons? Or would it be better just to leave it and put our double ceiling in below the joists with no gap-filling at all?