what if a chimney is not used, but cannot be removed?
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
Why can't it be removed?
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.
If it is easier, you could make it watertight at the top, and airtight at the bottom.
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.)
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?
**EDIT** I see now that you said you just did it last week. So my question above is irrelevant. Sorry.
Cranky, I have capped a few others over the years, but as far as I can recall, all were below the roofline. The last two I did I put polyiso over the entire chimney and secured it in place with canned foam and a few bricks on top.
In any case, it's best to also seal the lower end of the chimney, because warm, moist air will still rise into the chimney and condense wherever it reaches a cold-enough surface. I suppose mold is a risk but with the flue sealed at both top and bottom, little air should get in.
I actually did that at my house last year, for our largest open fireplace. I cut polyiso to fit where the damper had been and cut a piece of polysio to fit there. I had planned to spray-foam that into place as well but the fit was tight enough, and the surrounding area was clean enough, that I could just tape it in place.
I have a 3X8 chimney that is active. Leaking was primarily not from the flues, but from the cap.
I would cap the entire top surface, I used bent sheet metal.
Hi Gus, yes, I think that if the question is “do I seal only the top of the flue, or the entire top of the chimney?”, then the clear answer is “the whole top”. Edge to edge, with good drip edge around the perimeter, etc.
Here is my real concern: the interior of the flue is not conditioned space. All connections from the building interior to the flue were sealed many years ago. So if I also seal the top, I am creating an enclosed, unconditioned, and unvented space. If any water does find its way in, it has nowhere to go. No weeps to drain out the bottom, no way to evaporate out.
I know the saying with water is… “don’t keep it out, let it out”, and that’s the only reason why I hesitate to do a fully airtight closure across the top…
It's unlikely the whole thing is vapor-tight, masonry is pretty permeable. If the interior is just covered with drywall and paint, those are both permeable too. Masonry supports capillary action, so moisture can wick up into the above-roof portion and evaporate there. Presumably the building is heated, so in winter you're going to have a strong vapor drive toward the exterior.
So long as it's just a little water I don't see a problem.
If you are worried about trapped moisture make a sort of vapor diffusion port. Use a permeable rigid board insulation and cap it with a high perm air barrier taped to the chimney, then give your chimney vent cap a half inch gap around edge and between the chimney.
That plus interior seal with vapor impermeable seems like you won’t have issues
IMHO water is an issue
Moisture is not