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Chimney questions

nhbean | Posted in General Questions on

We are in the process of renovating a two-story 1910’s foursquare in climate zone 4.  We’re planning a re-roof later this summer, and I am working out what we’d like to do with our chimneys in preparation and could use some advice.  Currently the attic is insulated at the roof deck with 7” closed-cell spray foam.

Chimney #1 is a single-wythe unlined brick chimney serving an old coal-burning fireplace on the first floor.  Except where it penetrates the roof, it is entirely inside the thermal and air envelope of the house.  I would like to seal and insulate it semi-permanently (with the option of reopening it in the future).

I was thinking of wrapping wire or rope around stone wool batts pushed down from the chimney top, with the wire/rope tied to a rod running across the top of the chimney, and then sealing the chimney top with a metal cover and caulk.  Would this approach be feasible?

Chimney #2 is also an unlined single-wythe brick chimney.  At some point in the past, its top was removed to a point below the roof line and was used as a chase for a B-vent for a gas furnace and hot water heater located in the basement, both now abandoned and removed.  I would like to line this chimney and use it for a wood stove or masonry heater at some future point.

My question for this chimney is, would it be better to rebuild the removed masonry portion through the roof, or build a wood-framed chase on top of the existing chimney and apply thin brick veneer to it above the roofline?

Chimney #3 does not actually exist at this point.  As part of the renovation I am adding a radon mitigation system to our basement and crawl space.  The only space I could retrofit the 4” PVC pipe into passes through the interior space of the basement and first floor, but then exits the roof of a one-story bump-out along an exterior wall.  We are adding 2” of polyiso and a peel-and-stick air barrier to the exterior walls, and I am considering framing a chase to enclose the radon pipe and keep it within the 2” of polyiso.   If I did this, I’d probably want to continue the framed chase through the roof and apply a thin brick veneer above the roofline to be consistent with the other chimneys (below the roofline I would use the same lap siding as the rest of the house).

Are there any concerns with this approach I should be aware of?

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  1. Expert Member
    Michael Maines | | #1


    Chimney 1: your approach is better than doing nothing, but it very likely would not be airtight, and there is a lot of stack effect in a 2-story chimney, so it would continue to be an effective exhaust ventilation system--especially in cold weather. I would either add a foil balloon designed for chimney flues, or if you think you may never use the chimney, take it down below the roofline and cap it with an air-sealed lid. If you leave it sticking through the roof it is a good radiator, carrying your interior heat to the cold outdoors.

    Chimney 2: there is a slight thermal benefit to building a wooden frame and wrapping it with thin brick, covering an insulated metal chimney. But the thermal benefit is not huge, and it will probably cost less to rebuild with brick.

    Chimney 3: if you want the thin-brick version to look authentic, there are a lot of details that are easy to get wrong--especially the flashing at the roofline, and the chimney cap. Real brick chimneys are nearly always sized for full or half bricks; if you end up with 1/4 bricks at the end of a row, architectural purists will notice (but nobody else will). If you are saying that the chase would project from the wall and be wrapped with siding up to a point, with fake brick above, I don't think that would look right to most people. But it's your house.

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