from the lower surface of the concrete slab (which contrary to mark47n's statement, means the concrete is yours) or SUBFLOOR and then to the upper surface of the sheetrock (obvious statement it is the side facing upward) which means the joist space is common or limited common elements.Each Unit is measured horizontally from the exterior surface of the sheet rock of all opposite walls and vertically from the lower surface of the concrete slab or subfloor forming the lowest floor of the unit to the upper surface of the sheetrock forming the ceiling of the top floor of the unit.
a subfloor is the lowest level of "flooring" installed on the floor joists. The space available for insulation would be below that and part of the common or limited common elements.
so, now define the difference between repair and improvement.
also you need to check code requirements in place at the time of building.
If the current situation meets codes existing at the time it was built, added insulation is an improvement. If degradation has caused an issue, it would be repair but if it is not required to be repaired to meet the codes existing at the time of construction, it is not a required repair and it falls back onto; it meets codes applicable at the time it was built.

