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Community and Q&A

what if a chimney is not used, but cannot be removed?

crankyankee | Posted in Energy Efficiency and Durability on

Hello! I am a (volunteer) trustee of a condo association. Our building is a historic school, originally built 1898. It was converted to condos in 1987. Climate zone 5A (Massachusetts).

Our building is all brick, 4 stories tall, and has a large chimney. The chimney is no longer in use. All fireplaces/flues and other connections between the interior and the chimney were closed off long ago, so there is no concern about air tightness.

But the top of the chimney is still open, so rain freely falls in. The brick at the base of the chimney is showing efflorescence and flaking. I think it is happening because water gets in there and has nowhere to go. So we are looking at closing the top of the chimney. My specific question is: should the closure at the top be air-tight, or not?

The not-air-tight option would be a common metal chimney cap – the kind with mesh around the side that would still allow airflow/drying, and a little metal roof over the top. 

The air-tight option might be a new concrete topping slab over the top of the whole thing, or a wood deck with membrane roofing, and drip edge.

Thoughts?

Thanks in advance!

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Replies

  1. Expert Member
    DCcontrarian | | #1

    Why can't it be removed?

  2. crankyankee | | #2

    We can't remove the chimney because of Historic. We are national register listed, as well as a strict local historic commission. The chimney is a major feature of the exterior appearance of the building. No way they would approve removal.

  3. Expert Member
    Michael Maines | | #3

    I have a large, center chimney in my 1830 Greek Revival house. Last week I finally got around to plugging five unused flues, keeping only the one for the furnace open. I wanted the plugs to be reversible so I cut scrap 2" polyiso to just a bit smaller than the flue opening and sealed the perimeters with canned spray foam. That will keep conditioned air from escaping.

    I was still concerned about rain saturating the foam so I cut high-quality OSB to cover all but the open flue and spray-foamed that in place as well. That's a lazy solution but it's easily reversible. If I wanted a more permanent solution I would have formed and poured a 2" mortar cap. (It currently has a thin wash coat of mortar, standard practice here but it's not very durable.)

  4. crankyankee | | #4

    Thanks Michael. So it sounds like you opted for the air-tight closure. How long ago did you do it? Has it worked well? Eg, any concerns with trapped moisture, mold, etc inside the closed flues?

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