Can you please better describe why you think the space through which floor joints (and ceiling joints) are common elements as this is where insulation is needed? I'm not quite reading that in the description of the Unit as it mentions the lowest surface of the concrete slab or subfloor
It seems that you're unsure where the structure ends and your unit begins. If you already understand this I apologize; the subfloor is the sheathing that overlays the floor joists. It's what supports the carpet, tile, hardwood, etc. The structure beneath the subfloor is common and maintained by the Association. This also applies to the slab. The lowest portion of the surface (top) of slab, not the underside of the slab, is the unit and she slab is the Association's to maintain. It would be difficult to demonstrate deterioration of the insulationas well without butchering your exterior walls, which I don't think you own. I find the language to be less than clear as to which face of the sheet rock delineates the unit. If it's the side facing the interio you don't own it, if it's the exterior face then you can carve it up to see, but you'd still be on the hook for costs either directly or indirectly.