Background and The Problem
I am trying to solve the problem of mold growing on my bedroom ceiling. The house is a California Bulgalow built in about 1940. The house is single-brick with plaster interior walls. It is single story and with a roof cavity serveral meters high at the peak. There is insulation above the ceiling but not in the walls.
The floor of the house is about 1 foot above ground level. It's built in a low-lying area. There is no basement. There are vents every 2 or 3 meters around the perimeter. I checked and none of these are blocked.
There is an ensuite adjacent to the problem room. It has a exhaust fan that vents into the roof cavity. Two whirly-vents have been installed.
There is a reverse cycle air-conditioner, but this does not run often as it's programmed to heat at 16C (61F) and cool at 28C (82F). Summer was unusually wet.
Attempts So Far
So far I have attempted to remove the mold by scrubbing with beach, and when it came back, diluted vinegar. This worked temporarily, but did not solve the underlying problem. When I inspected the house in prior to purchase, there was a small of bleach which I put down to the vendor cleaning the bathroom. I now think they probably had this same problem.
I went into the roof cavity looking for leaks. The insulation batts make it easy to find any source of water. I could not find any roof leaks. The moisture from the bathroom exhaust fan just condenses on the flashing under the roof.
I bought a hygrometer to measure the relative humidity in the room. For a couple of weeks it was 50-60%. With recent rain, it went up to 60-75%. I could take moisture measurements in the sub-floor or attic if needed.
What should I do next?