For all experienced folks out there. I have a squeaky bedroom floor right next a wall that encloses HVAC running up from the first floor. As a result, the subfloor is just a small section that probably stops at the ducts - about a foot wide strip that runs parallel to the wall. I'm now pretty sure that the squeak is from the wall sill plate nails to that short section of subfloor moving up and down when you walk along that area in the bedroom.

I have the carpet pulled back (will be replaced) and removed the tack strips along the wall and can see a gap between the subfloor and sill plate. It looks like the builders along one section applied construction adhesive along the gap.

Can I get away with gluing shims into the gap to stop the flex, or is the only way to really fix this to open up the wall and screw the sill plate down to the subfloor?

The complicating factor is that we painted the room already, so I'm kind of reluctant to break open the wall. I originally thought that I could solve this by screwing down the subfloor to the joists.

If I understand you correctly you might be able to remove the baseboard (carefully) and use some 3” screws to screw through the drywall diagonally and into the wall plate and subfloor.

I’m assuming that you can’t reach it from below.

Like smlsig suggested -

Screws through drywall behind the baseboard. I would use something like 3 1/2” star drive screws. Go all the way through the drywall, pulling up tight against the sill and going at an angle into the subfloor. Maybe one every foot. Once they are in walk around on the floor to see if the squeak has stopped. Then caulk the joint.

I think the ez shims would also work, but they might work out over time. If you try those you might anchor them in place with caulk (maybe) or you could toenail them in with screws. Once driven into place, cut off excess flush with the wall and them catch the last exposed edge of the wedge with the screw.

You guys are correct - no easy access from first floor. I'd be breaking into the ceiling and messing with crown molding. Pass.

My baseboards/walls are freshly painted so I was trying to avoid messing with them.

I do have access under the baseboards since they are up ~1/2" from subfloor for carpet to tuck under. This is why I'm trying to see if the shim method could work since I avoid messing up fresh paint/wall.

I'll try hammering in shims and if I must I'll come back later and open up the wall. I've got stairs I have to retread and two bathrooms to remodel before that.

Maybe try using an expanding adhesive with the shims like gorilla glue, or a shot of great stuff to help fill and hold things tight.