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?

3 Answers 3


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.


I had a similar situation in my 1950 SoCal home. Bricks are porous and to a degree so is plaster. You have hard, heavy sponges full of moisture, or at least I did. I waited until summer to have the bricks dry out, then waterproofed the plaster on the inside and stuccoed the outside. This was at the recommendation of an abatement company. We shall see the results over the long haul, but it certainly sounds similar to your case.

  • And did it work?
    – WW.
    Commented Feb 24, 2020 at 3:24
  • 1
    Yes. There has been little to no moisture issues in these rooms.
    – pennstump
    Commented Feb 25, 2020 at 4:34

There’s the possibility that the mold that you’re seeing is old- that it’s resurfacing after not being completely cleared in past attempts to remove it. And if this is the case, dealing with humidity/ moisture issues is obviously a good thing, and should slow growth down, but probably won’t solve the problem.

Bleach is a great mold-killing solution on non porous surfaces (glass, plastic, metal, etc.) but on porous surfaces, it can actually make mold worse. Mold creates deep roots in porous material (https://www.healthline.com/health/does-bleach-kill-mold#does-it-work) and bleach doesn’t get down to these roots. The chlorine in bleach can’t penetrate below the surface, so all that does get below the surface is the water in bleach. And store bought bleach is about 94-95% water. (https://clevelandwaterandfire.com/blog/mold-remediation-6-common-misconceptions/) This means that the bleach isn’t only failing to kill mold at its roots, but it is adding moisture to it.

There’s a good chance that vinegar could work better, since it is able to get to the roots of the mold. But it shouldn’t be diluted, and keep in mind that it is only able to kill 82% of mold species. (https://extension.msstate.edu/sites/default/files/newsletter/dawg-tracks-safety-talk/2016/dts_16_201606.pdf)

Depending on how deeply the mold has penetrated the ceiling, and how much it has spread, going in and removing affected material may be the best option. As others have said here, having professional help would be a good idea- they can at least asses the spread and help you decide next steps. And if any amount of demolition is involved, professionals will be able to take care of that safely. You don’t want spores becoming airborne and spreading to other parts of your house. (https://www.epa.gov/mold/mold-course-chapter-6#:~:text=Full%20containment%20requires%20double%20layers,out%20of%20the%20remediation%20area.)

I know this post is old, and hopefully this problem has been dealt with for a long time! But hopefully this info can be helpful to someone in a similar situation.

  • The answer is a bit hard to read. Could you please improve it by breaking it into paragraphs. Commented Mar 7 at 11:31
  • Yep, thanks for the feedback! Commented Mar 7 at 17:10

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