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Help Solve Mold Issue Likely Related to Bathroom Fans and Ductwork

Mike Mold
4 years ago

Please help me solve this mystery:

Returned from a 10 day vacation to find mold on the ceiling mainly around the bathroom exhaust fans and tops of the wall. The attached photo, "mold1", is from the enclosed toilet area of the third floor master bathroom. The adjacent shower area has the exact same ceiling exhaust fan and a very small amount of mold was also present in that location, but only immediately around the fan. There are two other bathrooms on the 3rd floor of the house and a small amount of mold was similarly present around the ceiling exhaust fan in one of those bathrooms.

The AC was set pretty low - about 70 degrees. Outside temps in Atlanta at this time of the year hover around 90. Unknown variables: A dog sitter was present at the house during the time we were away. I do not believe the master bath was used during this period. Nor am I sure if the master bathroom toilet door was closed, but I think this was likely the case. I'm also not certain if the fans were left on during the entire period.

One mold remediation company suggested that the likely cause was the venting of exhaust fans into the attic instead of to the outside. This theory seemed like a reasonable assumption. I should note that there is no access to the very shallow attic space except by removing drywall (flat roof house). The same company claimed that there was very likely significant mold on the other side of the ceiling.

Another mold remediation company thought that the issue was related to the air in the bathroom being too cold (multiple AC vents in the bathroom) and it is a few degrees cooler than the adjacent master bedroom. This company did not think there was any mold on the attic side of the ceiling. Their principal suggested fix was to block off several of the AC vents to control the bathroom temperature.

Fast forward: We located exhaust vents for all the fans - see vent photo. The vents are positioned under the eaves, about 40ft above ground level. We got a handyman to conduct a smoke test for the main bathroom exhaust fan and while smoke did exit the vent, it did so very slowly and took much longer than it should have for all the smoke to exit the bathroom. The handyman was of the opinion that the vent exit screen/grate may be partially blocked which is restricting the air flow. While there is some accumulation of dust on the vent screen (photo looks worse than it is), I can clearly see into the duct pipe of the nearest vent. So it does not seem like this would be the major cause.

The handyman also cut out some drywall in the master bath toilet - zero mold on the attic side. However, what was revealed is flex duct piping (see duct photos) and the fan was aligned in such a way that two 90 degree turns were incorporated - one exiting the fan and one at the eaves exit. I don't think any rigid elbows were used, but I should be able to confirm tomorrow. BTW, the length of the duct run is about 14 feet. Also the existing fans seem to be rated at 50 or 70CFM.

So, it is my contention that the circulation issue in the master toilet area is more likely related to the fan alignment and the use of flex piping.


1) What is likely the main cause of the mold?

2) Replacing the entire run of flex pipe with rigid pipe may be too difficult and expensive as no direct attic access. Would a sufficient fix be to eliminate the 90 degree turn immediately by the fan and incorporating a rigid or semi rigid 90 degree at the eaves vent exit? If we do the latter, we will obviously get the eaves vents cleaned at the same time and can also change the exit vent to the type shown in the "replacement eaves vent" photo. Or should I leave with existing eaves vent type?

3) Seems like a more powerful fan will also help?

4) Is the existing flexible piping insulated? Should it be?

5) Any other suggestions? We already purchased a dehumidifier for the master bath and will likely keep it as a permanent fixture. Once all mold is cleaned/treated, we will also paint the ceiling with a mold resistant paint.

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