Q&A Chimney sweats - Condensation?

QandA Posted By QandA, Oct 5, 2005 at 2:20 PM

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  1. QandA

    New Member 2.
    Staff Member

    Nov 27, 2012

    This is more of a chimney question than a fireplace question, but perhaps you can help.

    Our chimney, which is visible in the attic, gets wet after rain. Sometimes it is just wet to the touch. Sometimes you can actually see very small drops of water.

    I've ruled out condensation, since parts of the chimney are completely dry, while other parts are very wet.

    I've had it inspected by a licensed chimney sweep and a home inspector. Neither could find anything wrong. I've had the the connection between the roof and the chimney inspected (is that the flashing?) and that is OK. Before we bought the house, the previous owner had sealed the outside of the chimney with a black thick rubbery sealant of some kind.

    The other problem with the chimney which may or may not be related is that on that the chimney is literally flaking off. There is lots of white material on the chimney (maybe minerals that are leaching out of the brick?) and also small pieces of the chimney sort of flake off and fall to the ground. Every month or so I vacuum up a pile of chimney dust that has flaked off.

    Do you have any suggestions on what is happening and what I should do about it.


    First, don't rule out condensation. It can happen just at certain spots. There are many possibilities here, and I assume you've exhausted many, but lets go thru them in case you missed any:

    1. Rain soaking into porous masonry - If you chimney does not have a rain car, get one....Also, check the condition of the "crown" (the cement on the top of the chimney) making sure that it is sound. Lastly, the outside of the chimney and mortar on the exterior of the house can be sealed with a masonry sealing compound - available at your local hardware or paint store.

    2. Is the chimney lined ? An unlined chimney (no terra-cotta or metal liner) can have lots of problems, including the ones mentioned.

    3. The part about sealing the chimney with the black goop is interesting.Masonry does need to breathe, and this could be holding in moisture that need to escape...Not much you can do about that one.

    4. Depending on the fuel type being burned, LOTS of moisture could be coming from inside the chimney..,.it might show up more on humid.rainy days. Gas, oil and wood all produce scads of moisture when burnt...Again, the lack of a liner would worsen this situation.

    The byproducts of combustion then can leach into the brick and cause deterioration of the chimney such as you describe.

    The age of the chimney, and therefore the quality of the original brick and mortar could have an effect also
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