Why upstairs floors are the ones you notice
An upstairs floor is not just a floor. It is also the ceiling of the room underneath, and that dual role is why it gets noticed while a ground-floor squeak goes unremarked for years. When somebody walks across a bedroom, the subfloor flexes between the joists, and any connection that has loosened rubs. On a ground floor over a basement that noise dissipates into an empty space. On a second storey it radiates directly into the living room where somebody is sitting. The problem is compounded by house geometry. The dominant older house forms across Ontario, the two-storey semi, the Foursquare, the postwar two-storey, all place bedrooms directly over living space with a staircase running up between them. There is nothing structurally wrong with that, but it means the noisiest floor in the house is also the one you sit under every evening. Access is the other half of the story: if the ceiling below is finished drywall or plaster, we cannot simply reach the joists, so the approach has to be chosen accordingly.