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Abandoned brick chimney: is it hurting energy efficiency?

jpritzen1 | Posted in General Questions on

Brick single-story house, brick chimney protrudes from the exterior wall from ground level, basement.

Furnace & water heater used to exhaust – clay liner.

Water heater was then orphaned, smaller metal liner inside the clay liner.

Water heater was then removed. Metal liner capped from the bottom.

At this point the chimney is unused. It’s still open from the top, has a cap, but has a couple “tin-can” caps at the bottom where the furnace & water heater flues would connect.

Is the chimney in this state hurting energy efficiency? What’s the proper actions to take once you’ve abandoned your chimney, but do not want to remove it?

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  1. GBA Editor
    Martin Holladay | | #1

    There are two ways that this type of chimney hurts your home's energy performance.

    It encourages conditioned indoor air to escape up the flue (because the caps on the thimbles are probably leaky, and the stack effect pulls air into these cracks). And the brick chimney probably represents an uninsulated thermal bridge that allows heat to escape through conduction and convection.

    You can address the air leakage issue by building an airtight and weatherproof cap. This can be a plywood cap roofed with copper or EPDM, or it can be a concrete cap. Whatever type of cap you build, strive for airtightness. (For starters, you can stuff an old fiberglass batt down the flue and then seal the top of the flue with canned spray foam. Then build your cap.)

    Once you've installed your cap, go indoors and seal the cleanout door at the base of the chimney and every single thimble you can find. Again, stuff some fiberglass batts in there and go to town with spray foam. Or build a custom plug out of plywood and seal the edges of the plywood with caulk.

    Once that's done, you can add insulation on the interior side of the chimney. Start with 4 inches of rigid foam, and protect the rigid foam on the interior with gypsum drywall.

  2. jpritzen1 | | #2

    Thank you for that information, Martin.

    My next question is regarding the chimney's ability to dry out (if sealed up as you prescribe). Is this not a valid concern considering it's abandoned, assuming the cap & roof-chimney flashing details are correct?

    With the chimney in its current condition of having an open chimney cap and free air passage within the clay & metal liners as well as between the brick & clay liner, when bricks get wet, I assume they dry out pretty easily. Now if I seal those air spaces, will I not have to worry about wet bricks? Should the chimney be sprayed with some waterproof sealant?

    The brick house has never seen any waterproof sealants due to concerns about 'being able to dry out' especially since the insulation behind exterior walls is 1" paper-faced fiberglass batts, which I read is bad.

  3. GBA Editor
    Martin Holladay | | #3

    If the brick chimney is exposed to the exterior air, it will dry out easily.

    If the brick chimney is inside your home's thermal envelope (until it penetrates the roof), it will stay warm and dry.

    In neither case is there any worry.

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