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Community and Q&A

How can I stop mold from growing on my A-frame ceiling?

user-6914891 | Posted in General Questions on

I have a problem with warm moist air accumulating at the peak of the ceiling of my A-frame vacation home during summer afternoons. This condenses on the surface of that T&G pine ceiling causing mold to grow. This occurs on days that our windows are wide open as well as during our absence when they are closed. If I aim a fan at the ceiling throughout the afternoon, the problem doesn’t occur.

One contractor advises that somehow a blockage must have formed between the soffit vents and the ridge vent. He recommends removing the entire ceiling and making sure air can flow from soffits to vent before rebuilding it. But another advises that it is a moisture problem in the living space, not between the ceiling and roof. (The home was built 30 years ago. The problem is only recent.)


  1. GBA Editor
    Martin Holladay | | #1

    First, can you tell us your name?

    Concerning the condensation problem: Condensation occurs when warm, humid air contacts a cold surface. So the first question we need to ask is: Is this building air conditioned?

  2. user-6914891 | | #2

    My name is Rich Visconti. There is no AC in the home. I understand that condensation occurs when warm moist air comes in contact with a cold surface. Here what is happening is that warm moist air gathers at the peak of the ceiling and condenses on that ceiling. I guess the ceiling in the summer although just below the roof is cool enough to cause this because of how hot and moist the air is at that point. Or maybe some cooler air floats up and causes condensation of the trapped warm moist air. If I could just move that warm, moist air out--by fan, vent, whatever, I would have no problem. But what is the best way to do that? Redo the venting above the ceiling (I assume there is a vapor barrier but I will take some down to see) by taking down the entire T&G pine ceiling or providing venting of some sort on the room side of the ceiling. A dehumidifier may not work because this ceiling is open to the entire house.

  3. GBA Editor
    Martin Holladay | | #3

    I must admit that the circumstances you describe are puzzling. If this phenomenon occurs on summer afternoons, the situation is particularly odd.

    In theory, during a summer afternoon, the ridge area of your roof assembly should be hot, due to solar radiation. It's hard to understand why your ceiling boards are cold enough to encourage condensation, especially at that time of day.

  4. user-2310254 | | #4


    Just to throw out another idea...

    Can you see moisture on ceiling? Do the stained areas have a musty smell? I am just wondering if you are seeing dirt/grim caused by stack-effect driven exfiltration. A photo might be helpful.

  5. user-6914891 | | #5

    The only "stain" that accumulates is that left by mildew. The condensation on the ceiling is enough that there are occasional drips of water from the beam that runs across the inside peak of the a-frame ceiling. Again, as far as I can tell, it just seems that enough hot warm air accumulates in the top several inches of the peak--on either side of this 8" beam--that it gets super saturated and condenses on the wood surface. The mildew forms on the underside of the beam and in various portions of the T&G pine within a foot or two of that beam. The mold forms when months pass that we are not using this vacation home--like last spring. Blowing the hot air around with a fan prevents this, but I would like a more permanent solution. Any idea whether ripping out the ceiling and making sure the soffits have air space to the ridge vent is the way to go, or might this problem have nothing to do with the venting between soffits and ridge vent but be an internal moisture accumulation problem? Little moisture is generated in the house; it is mainly humidity from the surrounding woods. Is there a simple solution to vent out or blow out this accumulated hot moist air?
    One other fact. The mold only forms along the peak in the back half of the house. The front half of the A-frame is a cathedral ceiling for the living room. It has no mold problem. The back half is a second-story bedroom that is essentially a loft in a dormer with a large opening to the front area and it has the center beam at the peak of the room (extra support for the dormer) where the front half ceiling has no central beam.
    The bedroom has large windows that are wide open when we are there during the summer. The front half cathedral ceiling has no windows that open or other venting other than the soffits and ridge vent. Yet even with windows open in the back half, on a mildly warm day the center beam drips.
    Rich V

  6. user-2310254 | | #6

    I agree with Jon. It is sounding more and more like a roof leak.

  7. user-6914891 | | #7

    Thanks guys, but this happens in completely dry weather; this happened both when the roof was old and after it was redone a few years ago; it happens when moist air gathers at the peak and condenses (I can put up my hand and feel the hot moist air at the peak). The water is on the surface of the pine, condensation, not seeping down from above. I am sure this is not a leak. Is moist air trapped in a peak and condensing really unique? That's what I seem to have. But again, thanks. Rich V

  8. GBA Editor
    Martin Holladay | | #8

    Does this building have metal roofing? I'm thinking the cause is night sky radiation cooling. In the early morning hours, your roofing is very cold. Humid air causes condensation on the underside of the roofing, and the condensation drips down to your ridge beam and ceiling boards.

  9. user-6914891 | | #9

    No, the roof has asphalt shingles, the condensation only begins in the afternoon after the warm air gathers at the peak, and it certainly looks like condensation on the ceiling, not water seeping through the T&G planks from above. Since it is clear that hot humid air does get trapped in this section of the peak of my home on warm afternoons, how do homes normally get rid of/ deal with such accumulated hot humid air if one were to assume it is room humidity and not dripping from above?

  10. GBA Editor
    Martin Holladay | | #10

    If your house had a tight air barrier, and adequate insulation above the ceiling, I'm guessing that this wouldn't happen. (It's still unclear why your tongue-and-groove ceiling boards are so cold -- if indeed they are cold. You might want to gather some data with a tall step ladder and an infrared thermometer.)

    The best investment would be measures to address the actual problem -- but we haven't really pinpointed the mechanism yet. In the meantime, aiming a fan at the ridge seems as good a solution as any.

  11. user-6914891 | | #11

    I don't believe the ceiling is cold. I believe it is sort of normal ambient temperature or a little higher if there is any heat coming through the roof. But the air that gathers at the peak is very warm and humid. It is this difference that seems to be causing the condensation. But when I am next in my vacation home, I will try to observe more closely what is going on and possible take down some of the ceiling to see what is happening above it. Thanks for your feedback. But you are right, I need to pinpoint the problem before investing in a solution. As a band aid I may install a ceiling fan and run it by a timer each afternoon.

  12. GBA Editor
    Martin Holladay | | #12

    Another useful data-gathering exercise: Buy one or more hygrometers, and tell us the indoor relative humidity.

  13. Jon_R | | #13

    My guess is a water leak somewhere, perhaps well below the peak and perhaps hidden behind the walls. This damp area evaporates and high humidity (nearly saturated) air rises to the peak. A moisture meter at various spots should help identify it.

    I expect that a very small amount of fan blowing on the area would disperse the moisture.

  14. Amundson | | #14


    I am curious to see if you have come to any good solution of this issue? We have an A frame cabin in the woods which sounds as if it is constructed very similar to yours. Cathedral ceiling in the front section loft bedroom in the back. We have the exact same condensation and mold issue. We have very thoroughly checked for any kind of leak and are completely confident that our issue is condensation not leakage.

    Any advice you have on solutions that have worked for you that you would be willing to share would be greatly appreciated.

    Thank you!

  15. GBA Editor
    Martin Holladay | | #15

    Information from this Q&A thread was incorporated into a recent GBA article titled "Summertime Condensation Near the Peak of a Cathedral Ceiling."

    You may want to read the information in that article. (Click the title of the article to read it.)

  16. user-2642926 | | #16

    Having read Martin's new article and re-reading this post today, it strikes me that "something has changed." The OP states the cabin is 30 years old, that it's a vented cathedral ceiling, and all that leads me to think that Jon R's theory sounds plausible.

    Is the water supply metered? Can a neighbor take a reading for you now and again in a week or two to see if there's water flow? Conversely if you turn off the water supply, does this problem dissipate?

    In case they're helpful, the SensorPush smart sensor are a really nice implementation of temperature/humidity sensors. The units themselves are perhaps 1.25" square and maybe 1/2" thick, and they can be integrated with a hub if you have WiFi at the cabin, so you can monitor temp and humidity remotely. If you prefer not to go with that complication, the sensors are battery powered and apparently the batteries last in excess of a year, so you can get the stored data when you return to the cabin.

    I think the software supports up to 8 units, and you can name them for ease of use, plus they're pretty easily hung by the little hole or you can use velcro tape to mount it on specific locations.

  17. manbut | | #17

    Hi Rich & Amundson,

    Did y’all ever fix this? I have an a frame built in 79 with a 1 yr old metal roof. TERRIBLE wetness at top of peak for about 7’ down. Drip stains everywhere. We have taken off the peak of the roof and it is totally dry up there so no roof leak. I also have no water or condensation lines up there or anywhere even close. Looking for what to do next to fix the issue.....

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