Floor squeaks between upstairs and finished downstairs.
We have a 2 story house, the lower level is a finished den with sprayed ceiling (popcorn), one half of the above area is the living room and the other half is a covered porch. Between the living room and covered porch is a french door which sits over a enclosed I-Beam. The living room floor is laminate flooring. The porch floor is made up of tongue and groove exterior plywood subfloor, tongue and groove pine boards, another layer of tongue and groove exterior plywood, the top is a layer of fiberglass mesh with about 1/8" latex concrete (no cracks in 16 years).
The squeaks are in the floor in the middle of the 8' french door which is centered in the middle of the 20' outside wall between the living room and porch. I think the living room floor joists and the deck floor joist are sistered together over the I-Beam, I don't think the builder in 1978 would have used 22' floor joists!
There is no access from below to the area where squeaking occurs. Since the ceiling is popcorn (sprayed on joint cement) it would be difficult to remove part of it to gain access. There is about 400 square feet of ceiling, patching and resurfacing would be a nightmare.
My Stepson asked about boring holes in the 1/2" space between the laminate flooring and the door sill and squirting foam in the joist cavities. I would like your opinions.