Why is my wall wet?

I have a one room seasonal cottage in Rochester NY. The walls and ceiling are cedar. The ceiling is attached directly to the roof; there is insulation and a vapor barrier, but no air space. It is about 15' high at the peak. I have been in there a few times, but it hasn't been used since last fall.
Today I got a ladder out to change the smoke detector battery, and noticed it got very warm and humid as I went up. The smoke detector actually had water on it, and the top couple feet of the wall (up near the peak) was wet.
It is right next to the chimney, so the first thing I thought of was that the flashing was leaking, but I would think that the water would have come further down the wall and I don't see why the smoke detector would have water on it.
Instead I figured that there were a few warm humid days followed by some cold weather. The warm humid air got trapped near the ceiling and condensed when the roof cooled off.
Does this make any sense, or should I get a roofer out? Any other ideas? There was a major roof leak before I bought the place, but it was reroofed and seems to have been fine since. Thanks.
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Toller wrote:

I would want to check out any exhaust fans or chimneys or exhaust gas vents in the area to make sure you don't have a leak or that someone did not terminate a bath or kitchen vent to an enclosed area.
--
Joseph Meehan

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Toller wrote:

exhaust dumps into that room. Then see if you can add some passive venting to the room. Passive so that it can still vent when unoccupied. My suspicion is that its just condensation from somewhere, somehow. For such a limited use place I suspect you do not want to run a dehimidifier or AC all the time.
I too have a cottage. No AC. Its designed so soffit vents into the structure itself. There is no attic, no insulation, no ceiling. From inside second floor all you see is the underside of the roof sheathing and rafters. Yea, we may get a few buggies from time to time but its a very rustic cottage with good ventilation!
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