Hello,
I have a 650C chimney (11" diameter with 2" insulation so 7" flue, stamped GSW from Parry Sound, Ontario, so I think a Selkirk? label is worn off) in my basement, which is being refinished. Currently there's a natural gas fireplace with a 4" vent that runs through the flue and up, but in the future I may see how bad my insurance would be converting to a wood stove. The chimney runs horizontally through my foundation to the outside, and then vertically up the side of the house ending above the roofline. Where it runs through the wall into my basement, there is a piece of flashing/thimble that was bolted onto the concrete block foundation, as the rough in hole is bigger than the chimney itself (see attached). As we had sprayfoam done on the walls, a bit had crept under the flashing/thimble and I removed it to scrape off any sprayfoam/debris that worked its way underneath, to respect the 2" combustible clearance.
Now my problem is, I'm not sure what I should be using to seal it back up. I removed some grey silicone caulking (it felt fairly rigid, but still pliable) in order to remove the flashing. I know the chimney is insulated but heaven forbid I put an inadequate silicone caulk on it. Should I be using a high temperature silicone sealant (500F? 600F? higher?), or a DAP Fire Stop/3M Fire Barrier (CP 25WB+, FD 150+), or something else? I couldn't find a temperature rating on the DAP/3M sealants. The caulking around this flashing won't be visible as there's another piece that is supposed to sit over top of the chimney on the inside, but I would probably use it to touch up the outside section where the T i as I can see it's starting to separate.
I have a 650C chimney (11" diameter with 2" insulation so 7" flue, stamped GSW from Parry Sound, Ontario, so I think a Selkirk? label is worn off) in my basement, which is being refinished. Currently there's a natural gas fireplace with a 4" vent that runs through the flue and up, but in the future I may see how bad my insurance would be converting to a wood stove. The chimney runs horizontally through my foundation to the outside, and then vertically up the side of the house ending above the roofline. Where it runs through the wall into my basement, there is a piece of flashing/thimble that was bolted onto the concrete block foundation, as the rough in hole is bigger than the chimney itself (see attached). As we had sprayfoam done on the walls, a bit had crept under the flashing/thimble and I removed it to scrape off any sprayfoam/debris that worked its way underneath, to respect the 2" combustible clearance.
Now my problem is, I'm not sure what I should be using to seal it back up. I removed some grey silicone caulking (it felt fairly rigid, but still pliable) in order to remove the flashing. I know the chimney is insulated but heaven forbid I put an inadequate silicone caulk on it. Should I be using a high temperature silicone sealant (500F? 600F? higher?), or a DAP Fire Stop/3M Fire Barrier (CP 25WB+, FD 150+), or something else? I couldn't find a temperature rating on the DAP/3M sealants. The caulking around this flashing won't be visible as there's another piece that is supposed to sit over top of the chimney on the inside, but I would probably use it to touch up the outside section where the T i as I can see it's starting to separate.
![[Hearth.com] Chimney and "Thimble" Caulking](/talk/data/attachments/262/262415-e3abeb0f9d196f3b6d213617e0b4b4ff.jpg?hash=ALLeIg7Q6f)
