Moisture and mildew

If I say that I'm having a humidity and mildew problem in the master bathroom, one is likely to assume it's the kind of long-term ongoing thing that often crops up in the autumn when temperatures drop and windows are closed. I think I know a little more than that about what's caused this one but I don't know what that means about how to fix it.

The house is six years old, and we have had a mildew problem on at least two previous occasions; each time, it came on fairly quickly and was visible first as spots in the ceiling of the shower. Each time we called out the contractor who built the house.

The first time, we were told the problem was that the duct from the ceiling fan to the vent on the outside of the house was not a rigid duct, so it had had a bit of a sag in it; the cooler weather meant that steam condensed and gathered into the sag, which caused more sagging, which caused more condensation to accumulate, etc. This caused the vent to become less effective, and eventually to form ice. They cleared out the water and ice, re-levelled the duct, and repaired the water damage on the shower ceiling. The problem went away for at least a year.

The second time, or perhaps it was a third but in any case the most recent time (a couple of years ago at least), they said the same thing had happened, but decided they didn't want to keep coming out and fixing it. So they replaced the duct and routed it, instead of a long path over the master bedroom to a vent in the house's gable end, a much shorter distance to a vent in the soffits just outside the bathroom. Again, the problem went away and has stayed away for a few years.

This year it came on even more abruptly and intensely than ever. In the space of only a few weeks we've got mildew on the ceiling not just in the shower but all over the bathroom. It seems unlikely that, this many years on and with them having come out to fix it several times, we can get away with insisting on another repair from the contractors at no cost, and I don't want another "for now" fix, I'd like to make the problem go away for good. Trouble is, I don't know where to start.

As far as I know the fan itself is fine. It's not dirty or clogged, it sounds normal when I run it, and it's a model which is known for being rock-solid and reliable. I've never been in the attic and don't think I'd be particularly suited to get into it -- it's really more of a crawlspace than a proper attic and it'd take a skinny person to get through the door into it anyway. So I can't check the vent duct or even confirm that they rerouted it. Nor would I know what to do about it if I could get to it.