First thought would be to look for spray coming out the threaded connnection where the showerhead screws onto the shower arm pipe. BTW, is it mainly the ceiling surface or is the area saturated and mushy to the touch?

Ahower or bathtub?

While a corectly installed shower should not leak even with bad caulking, getting one installed correctly can be rather hit or miss.

If it is a tub check the caulking of the wall tile to the tub rim.

Next is water getting behind the trim plates of the controls, and the last place is often the tub overflow.

Hi, I presume you are concerned about codensation.You have no bathroom above you. If you do have pipes above you for the tub they would leak 24/7.
Condesation doesn't always form where one thinks it should.
Do you have fan or a window? Does the mirror get condensation?
A test you can do is the next time you use the shower turn on the fan or open the window and leave the bathroom door open (not the shower door). Let us know if the condensation is elimated.
Good Luck Woodbutcher

Check the seal for the tub drain. It may have loosened or the seal may be bad. Also the trap connected to tub may be leaking. Unfortunately the only way you will really be able to tell is by pulling out the ceiling below shower, run the shower and observe where water is coming from. Frequently there is a bedroom on backside of the bathroom and a closet may be adjacent to the shower in that room. So you may be able to inspect the wall in that closet for any water damage that would be coming from the plumbing to shower head.
RJ

Thanks. I already shower with the door open and the fan on (and the shower curtain closed). The mirrors don't get any condensation on them. It's possible there are other spots that are getting condensation on them that I can't see, but these two areas get wet enough that I can see the water. One is about 14"x6", the further one is about 6"x6", both irregularly shaped.

rjh2o, I didn't see your post, sorry. This is a one-story house, not an upstairs bathroom leaking below it. Unless I'm misunderstanding you, the tub drain can't leak 8 feet up. :)

I did look down in the basement and I can see the copper pipes running along there, so Woodbutcher is right - not that I doubted you! - that there aren't any pipes above the bathroom. I also checked for water around the shower head and between the tub/shower fixture (not sure what to call it) and the wall, and it was all dry after my shower (but not particularly clean). The caulk between the tub/shower and wall looked intact, but of course you can't tell for sure just by looking at it.

The thing I find so strange is that this bathroom is at least three times the size of my old one, and my old one didn't have a fan, and the mirror would fog up about halfway when I showered there, but I never saw specific areas get wetter than other areas. I also never saw the paint peel, which is what is starting to happen in the middle of these after only two months - so far only 1" spots. Maybe my real issue is the paint? I think it's just builder grade. Should I try a quality paint made for bathrooms? I can ask in the paint forum if that's more appropriate.

Based on the description and the comment that it is localized to two specific popints, my conclusion is a leaking exhaust duct for your bathroom fan. Try showering with the fan off and see if the leaks return.

I agree with the couch potato. I have seen this before:

An uninsulated flexible aluminum duct is run through an uninsulated space. I have seen this with dryer vents and bathroom vents. The water vapor condenses inside the aluminum duct. The liquid water accumulates and flows down to the lowest point(s). Sometimes the water drips out the air intake grill. Sometimes there is a pinhole in the duct, or it could be at the joint. The water will leak out of the duct wherever it can.

The solution is to use an insulated exhaust duct to prevent the water vapor from cooling to the condensation point in the first place. All that steam needs to get outside as steam.

You could test for the problem by running the shower with only cold water for 10 minutes with the fan running. The next day try it with the hot water. If you can't replicate the wet spots without running the hot water, it is definitely a steam condensation issue inside the ducts.

The water spots form even when the fan is off - in fact, the fan seems to make no difference whatsoever. However, I had someone come out to take a look at it, and he looked at the exhaust duct, and there were places where the all-powerful duct tape had failed, venting steam into the attic. He can fix that and also install a more powerful fan, and we'll see if that helps.
Thanks all.

"places where the all-powerful duct tape had failed,"

Ducts are actually about the last place to use duct tape.

It NEVER stays on very long.

The heat of an attic cooks the glue so badly it fails in very short order.

You can go a little longer with aluminum foil tape if it is rubbed down thoroughly (a plastic spreader for auto body compound works well).

Prehaps the wet spots corresopnd with places the insulation is missing above. Sometimes builders leave out insulation right near an eave or soffit vent because they are worried about the insulation blocking the vent. It is a better practice to make a shield to hold the insulation in place and still allow airflow.