The original post: /r/diy by /u/blahdiddyblahblah on 2025-03-11 19:46:06.

The 70+ yr old chimney straddles an interior wall upstairs. I.e., the front half is exposed in a bedroom as part of the “wall”, the back half is exposed inside an attic space where our furnace lives. There is a slight irregular gap between where the bricks meet the drywall.

Since it’s a gas furnace, I want to seal up the gaps around the chimney, where it meets the walls, that could allow CO leakage into the bedroom.

The gap is 1/4-1/2 inch wide. What’s a safe material to use, since the chimney could be warm when in use? I doubt it has a liner since it is so old.