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Q&A Spotlight

How to Fix an Old Farmhouse Chimney

Persistent leaks and spalling bricks have the homeowners wondering whether the chimney should be repaired or replaced

Repair or replace? The chimney on this 1900 farmhouse has spalling bricks and a persistent leak, making the homeowner wonder whether it would make more sense to tear it down and start over.
Image Credit: Page Hyler

Between the spalling bricks and a persistent leak that has damaged a mudroom ceiling, the chimney on Page Hyler’s 1900 farmhouse is proving to be a problem that just can’t get fixed.

“We have had damage around the ceiling and wall of the chimney that I now know is efflorescence,” Hyler writes in a post at Green Building Advisor’s Q&A forum. Caulking the flashing at the roofline helped, at least temporarily, but Hyler eventually called in an out-of-town company to inspect and seal the bricks. Problems persist.

“I contacted someone else who refinishes old houses in the area and he thinks that we should patch up the bricks and repaint [them] with an oil-based paint because the chimney looks to be in decent shape otherwise,” Hyler writes. “He thinks the damage in the mudroom is the flashing around the chimney.

“I was wondering if there is efflorescence under tha painted brick would the oil paint suggestion solve problem or do I need to tear down the chimney and rebuild it.”

As if the situation wasn’t problematic enough, Hyler is planning to sell the house. Should that change the repair strategy?

Most likely causes: bad flashing or flue-gas condensation

Faulty flashing is one possible culprit, says GBA senior editor Martin Holladay, but it also could be moisture resulting from the condensation of flue gases.

Leaks at the intersection of chimney and roof aren’t at all uncommon, and while there are several possible corrections, Holladay thinks the most reliable way is with a technique called through-chimney flashing. “Proper installation of through-chimney flashing requires all of the bricks above the roofline to be disassembled,” Holladay writes. “After the flashing work is done, the chimney top can be rebuilt with the old bricks or with new bricks.”

Condensation occurs in flues that are too big or cold, Holladay adds, referring Hyler to an an article he originally wrote for The Journal of Light Construction. He suggests searching out a skilled mason with a good understanding of chimneys.

Dana Dorsett adds that a chimney can “rot out from the inside” because of flue gas condensation.

“It’s not about the quality of the brickwork,” Dorsett writes, “it’s the size of the liner relative to the BTU output of the boiler and the temperature and acidity of the exhaust. If the cross section of the flue is too large, it slows the velocity of the exhaust, and with lower temperature exhaust of higher-efficiency equipment, there is more moisture and acid being deposited on the masonry from the interior.

“The exterior degradation from rain/snow/ice moisture is often just the icing on the cake, not the underlying problem.”

Replacing the brick with a cheaper alternative

Given the problems, Hyler wonders whether the chimney, which vents a boiler and water heater, could be torn down and replaced with something less expensive. It would be no great loss to the house aesthetically, Hyler adds.

A stainless-steel chimney would probably cost less, and work better, than a masonry structure, Holladay says, so if the decision is to replace instead of repair that might be a good option. The stainless-steel chimney could be boxed in with framing and drywall so it wouldn’t be visible inside the house.

Hyler’s decision will be complicated by the fact the house will soon be sold. A simple repair, if one exists, might be more appealing financially than a major investment in a property Hyler will soon put on the market.

But in either case, potential buyers should be told of any existing problems that sellers are aware of.

“As far as I understand, in many states, home sellers are required by law to disclose any known defects,” Holladay says “Now that you have researched the topic and learned about your defective chimney — and now that your question has been posted in a public forum — you are probably required to disclose the defect to any interested buyer.

“If this were an ethics column instead of a construction advice column,” he adds, “here’s what I would answer: You are ethically obligated to fix the problem or to inform the buyer about any known defect in your home.”

Our expert’s opinion

Here’s what GBA technical director Peter Yost thought:

After looking at the photos and reading descriptions of this chimney, it sure looks to me to be a bulk water management issue, with flashing and condensation problems not necessarily lacking either.

But a couple of key points:

First, this chimney moves quickly from being surrounded by interior conditioned space to being outside the structure — so there is a big temperature drop during the heating season, and much less stack effect to support good draw. This increases the condensation potential pretty far down the stack.

This is a good reason to consider yanking the failing masonry chimney and, as Martin suggests, replacing it with a stainless-steel flue wrapped in cladding. But if the new clad stack is insulated, this will increase the height of the heated air column, increase the stack effect, and improve the chimney’s draw.

This is one of the reasons that many old homes have their main fireplace in the home’s center rather than at the gable end. The column of warm air in the fireplace is continuous and always draws well, whereas a gable-end fireplace with the chimney outside of the conditioned space often has draw problems.

Second, you can’t use paint to waterproof masonry, and you always want the water permeability of any coating to be very low (preventing wetting) while the vapor permeability is very high (to accommodate drying). For that, you need to use a siloxane as your chimney or masonry water seal. Siloxane water sealers are transparent, water-resistant, and vapor-permeable. They are, however, pretty expensive and pretty sensitive to UV degradation. You have to recoat per manufacturer’s recommendations to keep the water out over time.

It won’t be inexpensive to remove the old chimney and replace it with a clad and insulated chimney with metal flue, but it is the right thing to do.

2 Comments

  1. Justin Steinbach | | #1

    Great information
    I'm not a building professional, but I always try to follow articles on GBA just to learn more as home owner. This Q&A has some great information in it about old chimney's that I think will be helpful in the future as we look at homes to buy and things to look for and at regarding old chimney's before we further commit and have an inspector look over the house.

  2. Peter Walsh | | #2

    Awesome advice
    The chimney is one of those things that often gets forgotten, we know its there but because its job is for the most part hidden it can become neglected. I work for a company that manufactures fireplaces and we cannot stress the importance of having a working, safe chimney.

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