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Community and Q&A

What can I do to address a condensation and mildew issue on a cathedral ceiling?

asl58ene | Posted in GBA Pro Help on

Our house is 35 years old and has a cathedral ceiling in the living area located in the center of the house. We have never had an issue with the ceiling until about 5 years ago. Condensation develops during the summer months on the ceiling and ultimately results in mildew that spreads across the entire ceiling. We live in LA and the heat in the summer is in the 90s during those months. Approximately 8 years ago we replaced the roof on the house, using the architectural shingles and removing the power vents and replacing them with a ridge vent system. That is only difference that I am aware of that may have contributed to this problem. Five years ago we redid the ceiling and the painter treated the mildew and used a special paint that was mildew resistant. For the last 3 years we have been o.k. But this year the problem has developed again. Before I redo the ceiling, I want to solve the problem that is causing it. Can you help?

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  1. GBA Editor
    Martin Holladay | | #1

    Condensation and mold occur on cold surfaces. You need two conditions for the situation you describe:
    1. Something is making your ceiling cold.
    2. Humid air is coming in contact with your cold ceiling.

    First of all, you need to describe the air barrier in your ceiling and the insulation layers in your ceiling. I suspect that your cathedral ceiling ventilation may be contributing to the problem.

    Second, is your house air conditioned during the summer?

    Third, are there any HVAC ducts (air conditioning ducts) or air conditioning registers in your ceiling?

    Fourth, do you have any idea whether your house is build with attention to airtightness? Or is it instead an old, leaky house?

  2. charlie_sullivan | | #2

    Interesting puzzle--the more of Martin's questions you can answer, the more clues we will have. But I realize you might not know the answers to all of them. One more question in case you know the answer is what the temperature and humidity are inside during the weeks that this is a problem. I would recommend getting a humidity meter, but I don't know of a cheap and accurate one to recommend.

    The two reasons I can think of are:

    1) If there's vent space above the insulation, and no air or vapor barrier between that vent space and the insulation, then humid outside air going through the vent space can diffuse down through the insulation and hit the top of the drywall ceiling. If the ceiling is cool because the space is air conditioned, the moisture condenses, and then in soaks through to where you see it.

    2) If the A/C ducts are in the ceiling, they could make the ceiling colder than the space below, and condense moisture from the room on the ceiling.

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