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I have a fireplace exhaust going through an exterior wall. It is a double-wall one for a freestanding propane fireplace, with the supply air coming in the outer chamber. The wall has rockwool insulation, and is covered in 6mil poly vapor barrier.

Sealing the vapor barrier to the pipe would provide continuous vapor protection, but my gut reaction is plastic + fireplace exhaust is not a great idea. That said, the exterior of the stovepipe doesn't typically get very hot since it's pulling in outside air. What's the correct way to do this?

Stovepipe side view

Stovepipe front view

Edit: It's a freestanding propane stove, so part of this pipe comes through the wall and is visible. The only view from the side I can find has it covered in plastic (and before I removed the brick wall), but this shows roughly how it sits in the room.

wider side view of stove in room, before brick wall was removed


Edit:

I adapted the accepted answer:

I realized I had the square steel trim piece from the old install, so I used that just to avoid taping plastic directly to the pipe. It has a lip that fits around the pipe with about a 1/8" gap, which is sealed with foil tape. The vapor barrier is cut back to overlap the steel square by about 3/4" and sealed with tuck tape.

finished: stovepipe with square trim piece showing tuck and foil tape detail

I'm putting cement board and then some kind of stone over this, and got a new round (and smaller and nicer-looking) 7" trim collar to use to finish it off.

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  • For greatest fire safety(probably not needed), thin steel sheet with a hole in it. Caulk the outside edges to the vapour barrier and use high heat caulking between the sheet and pipe. If that pipe is zero clearance, caulking vapour barrier to the pipe might be enough.
    – crip659
    Sep 27, 2023 at 13:31
  • @crip659 it's freestanding (which I think means not zero-clearance), I added a picture to show.
    – gregmac
    Sep 27, 2023 at 14:06
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    Zero clearance is dependent on the pipe, not where the stove sits. It's pretty normal for concentric sealed combustion with the intake air in the outer ring to be zero clearance, but you need to check the documents...
    – Ecnerwal
    Sep 27, 2023 at 14:20
  • The stove and pipes will each have their own clearance values. The stove might need a foot clearance while the pipe might only need an inch. The actual values will be on instructions and usually a label on the stove and hopefully on the pipe.
    – crip659
    Sep 27, 2023 at 14:29
  • Ah, I see. This was all here before, so I'll have to see if I can find pipe clearances or specs anywhere. The clearances for the stove itself are on the back, no issues there.
    – gregmac
    Sep 27, 2023 at 19:38

2 Answers 2

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If (as I'd expect) the concentric stovepipe is rated "zero clearance to combustibles" you can seal right to it. The exhaust is in the inner pipe, and the outer pipe is cooled by the air coming in through it, so that's normal.

Use aluminum foil duct tape (actual metal foil with adhesive, not silver-colored cloth.) If it makes you happier, cut the plastic back an inch and use 2-inch wide tape to seal the gap.

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  • Thanks, did a slight variation by using the old square trim plate. Added a picture into the question.
    – gregmac
    Oct 6, 2023 at 2:22
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If you want a nonflammable sealant, there's always hydraulic cement. Since that expands to make a seal it might slightly crush the pipe, but (I think?) not enough to be noticable.

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