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.
Investigations
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?
Best Answer
First, I'd redirect the bathroom vent out of the home. Best place is on the gable side of the house, but straight out the roof would also work. Depending on how much condensation you're seeing, and where it travels after condensing, this could be the cause.
Then I'd do some detective work. Start removing the insulation on the other side of the mold. Look for signs of water damage and follow them back to their source. There could be lots of causes, including a leak in the roof, plumbing problem, a clogged hvac drain line, or condensation on uninsulated hvac ducts. Water can travel a long distance before it finally settles on a low spot, so search far and wide.