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Problem with moisture in attic and second-floor ceiling of unheated house

JohnnyO46 | Posted in General Questions on

Remodeling a 40 yr. old cottage in NE NY to make it year round. No heat yet but after 3 days of 60+ temps last week, a 2-3 sq. ft. section of ceiling adjacent to the masonry, 2 flue chimney is wet and there is frost on the chimney in the attic. There are two gable vents, each 1.5 sq’. located opposite each other across the 28′ width of the attic. They’re as close to the ridge of the 5/12 pitch roof as possible. The perpendicular attic length is 23′ for a total of 644 sq.’

One vent faces a lake w/ several trees on the shore 50′ away and the opposite vent is 100′ from trees though the areas parallel to that space are heavily wooded.

The roof is not leaking but there is frost on some rafters. I just insulated (FG R-38 in the new section but R-21 nearest the chimney) and rocked the ceiling in August and everything has been fine until now.
If ventilation is the issue, thinking of a gable vent fan but not sure which one to use if I go that route. According to architectural info I’ve read, I need 1 sq.’ of vent for every 300 sq.’ of attic. Would appreciate guidance on this one.
Thanks, JTO

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  1. user-2310254 | | #1

    Have you done any air sealing on your cottage? Is there a gap around the chimney where it passes through the attic?

  2. JohnnyO46 | | #2

    Steve, thanks for responding. There is a 1/4" space between the chimney and the ceiling immediately adjacent to the section of the ceiling that's affected. I intend to seal it when the weather breaks. I didn't think it was critical until my heat plant was installed/functional. If the temp in the attic becomes greater than that below the ceiling, I don't understand why the condensate would form on the attic side of the ceiling.

  3. user-2310254 | | #3

    More knowledgeable members will chime in, but I suspect that air infiltration and exfiltration due to stack effect are causing at least some of the issues you noted.

  4. GBA Editor
    Martin Holladay | | #4

    In an unheated house, all of the components of the house (including the chimney) get quite cold during extended spells of cold weather. Because a masonry chimney has a lot of thermal mass, it will stay cold for many hours, even after the weather changes.

    When an extended cold spell is followed by warm weather, the warm, humid air contacts the cold masonry chimney. That's when the entire chimney is quickly coated by a layer of condensation or frost. In some cases, this condensation can soak a ceiling or adjacent materials.

    Once the house is heated, the chimney won't get as cold as it's getting this winter, and you won't see this phenomenon.

    The tasks you face are the same tasks faced by anyone fixing up an old house: perform air sealing work and improve the insulation R-values. For more information on these tasks, see these articles:

    Air Sealing an Attic

    Borrowing a Cellulose Blower From a Big Box Store

    -- Martin Holladay

  5. JohnnyO46 | | #5

    Thanks Martin. I clearly discounted the effect of spaces between the drywall and the chimney. I'll get that sealed up quickly.

  6. user-2310254 | | #6

    See this article for best practices:

    You also need to air seal other areas of the house. The sidebar contains several related links.

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