I completely agree. The very old buildings in Montreal are soundproof. But the new “optimized” condos coming up have thin concrete floors(the bare minimum code) and the common walls are also soundproofed to the bare minimum code I think STC 50 if I remember.
Concrete floors with multiple thin layers of foam embedded in the concrete end up stronger and lighter than concrete without foam, and are way better at sound insulation. The downside is it's logistically harder to arrange everything in the mould.
When someone figures out a process to do it without so much labor, it'll become standard practice, because it'll save concrete and weight.
Can’t know this by looking at it from the outside. A renter or even a buyer gets so little time to analyze what’s underneath the flooring or inside the common walls. I do not know if one can request the materials and blueprint of a condo before buying it.
Typically the foam is only in the middle of the concrete slab, so there are still a few inches of attachment all around all the edges with contact. There is rebar in those edges too.
