I ran up to the attic - ok, trudged once I met a near-solid wall of hot air - to check for leaks but saw nothing. I broke out in a sweat within seconds of entering the attic, so I skedaddled out of there quickly.
Here's the room in question, though this picture was taken last summer when there appeared to be no mold issues:
Darwin was on his way home when I noticed the mold, and I pointed it out to him when he got there. He noticed that the heat from the attic stairs or just from heat rising, whatever, began exactly where the mold began around the top third of the room. I could reach my arm over my head and feel the strata of heat above. It doesn't help that there's no A/C vent in this particular segment of the hall (the middle section), so it's slightly hotter than the other places in the house.
The trouble is, short of cleaning off the mold over and over, I'm not sure what to do to stop this from happening. The hardware is missing to work the transom window, so I wonder if it would help the condensation if we bought some new hardware from Van Dyke's and kept the transom open (we keep the doors to the bedroom closed to prevent cat hair from accumulating all over our bed). Maybe that would help the cool air circulate better?
I've also considered painting the whole area with that mold- and moisture-resistant paint they sell for bathrooms. And there's an attic fan we tried out last year, but the motor started smoking. Would an attic fan - with a new motor - help?
Oh dear, what do we do?