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Community and Q&A

Regarding Efflorescence on Chimney in Attic

GBA Editor | Posted in General Questions on

Before insulating our attic I would like to know if I should be concerned about a heavy white efflorescence on the bricks of the chimney in our attic. There is no efflorescence on the brick above or below the attic. We live in a 1950’s two story home located in an area with low humidity, warm summers & very cold winters. Thanks in advance for your help.

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  1. GBA Editor
    Martin Holladay | | #1

    The efflorescence is evidence that your chimney is regularly wet. When the moisture evaporates, minerals that had been dissolved in the water are left behind as a visible sign of the evaporation.

    There are two possible sources of the moisture:
    1. Either the chimney is getting soaked by rain, or
    2. There is condensation of moisture in the flue gas.

    Problem #1 occurs more frequently when the chimney is poorly flashed. Do you understand the principles of chimney flashing? If so, have you inspected the flashing?

    Problem #2 occurs more often in unlined chimneys (chimneys without flue tile), and in "orphaned chimney" situations. (An orphaned chimney refers to a chimney that formerly accommodated two appliances -- most often a furnace and a water heater -- that now accommodates only one appliance -- in many cases, because a new direct-vent furnace is installed). When that occurs, your chimney is now oversized. The chimney in the attic is cold, so that's where the flue gas moisture condenses.

    The solution to problem #2 is to line the chimney with a properly sized stainless-steel flue.

  2. Brent Lerwill, Brentwood Building & Home Inspections | | #2

    I concur with everything that Martin has said. What I would add is that if there is no cap installed at the top of the chimney, and you are in an area that gets a lot of rain, water running down inside the flue can contribute to the condition you have described. I build and inspect on the Oregon coast where we get 60 to 100 inches of rain a year. I am continually amazed that I see so many chimneys with no cap. When I do, I will see this condition, or worse. On an older chimney the mortar and masonry is often crumbling and in some cases needs to be completely rebuilt.
    The other comment I would make about inspecting the flashings is: Don't do it yourself, hire a "real" pro who knows what to look for. It's not enough that you can see flashing around the base of the chimney: it has to be properly keyed into the masonry and the roofing material.

  3. Colin Kindrachuk | | #3

    Thank you for your prompt responses Martin & Brent.
    Regarding problem #2 - The chimney appears to be lined with clay tile and is not "orphaned"
    Regarding problem #1 - The flashing looks in good condition & a review of our home inspection report from three years ago indicates it was in good condition then.
    I just looked at the framing around the chimney in the attic & observed evidence of water leakage in the past. It is bone dry now even though we are having record amounts of rain. I therefore think that the efflorescence must be the result of a flashing problem (as you suggested) that has been fixed before we bought the house. As the chimney seems to otherwise be in good condition I think this problem is in the past now. Thanks a bunch for helping me figure this out.

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