I have a confusing problem that I cannot figure out. Sorry about the long post. Any help would be appreciated.
I have high velocity A/C. Unit works fine in the summer, no issues with cooling. I live in NH.
1-2 years ago I noticed some water staining on the ceiling next to an A/C port second floor below an unfinished attic. A/C port is in a bathroom that gets regular shower use, though we do use a fan/vent when we shower. Another port, just outside the bathroom door has some water damage next to it too. Recently I have seen some minimal water damage next to another port on the other side of the house. The roof does not leak.
I have a whole house fan in the attic, and I have three whirly bird vents on the roof. Initially, I thought the water damage in the bathroom AC port was due to water getting into the attic via the whirly bird vent and dropping on the outer surface of the A/C vent insulation (which is right below the whirly bird), and rolling down to the port and staining the ceiling. So I wrapped the surface of the insulated tubing in the attic with a towel to stop water from doing this.
However, recently there has been more staining at the bathroom port, but we have had no rain at all to prove my theory. Plus, the whirly bird vents are not above the other two A/C ports which have water stains.
So now I am thinking that this might be some kind of condensation thing. The attic is very cold now. I have never before blocked the air intake grill in the second floor hall ceiling. The AC blower is in the attic. I would think that if it was a condensation thing, wouldn't the condensation be INSIDE the AC tubing and not apparently on the outside where it is reaching the ceiling? I did notice some mildew inside the tubing.
Thanks in advance, photos below.