Hey folks, I'm dealing with a frustrating issue related to the humidity levels near the bathroom exhaust fans in my upstairs bathrooms. I've got some temperature and humidity sensors that I installed about 2 inches from the ceiling, and the readings are all over the place! When the fans are on, everything is normal, but as soon as I turn them off, the humidity spikes dramatically back to alarming levels. I've tested this with just one fan at a time to isolate the variables. I've checked everything – from insulation above the fans to ensuring there are no leaks in the roof. I even had my HVAC system inspected, and it's functioning well. If anyone has experience with this kind of problem or knows what might be causing these crazy fluctuations, please let me know!
5 Answers
You might want to check if the flaps in the exhaust fans have failed. I’ve had a similar issue where exterior humid air was getting sucked back in due to a malfunctioning flap. It’s odd for all bathrooms to have that problem, but it’s worth checking out.
From your graphs, it looks like humidity drops when the AC is running. Since you're on the second floor, have you compared the humidity at ceiling height versus 4 feet up? Humidity tends to be higher near the ceiling since warm air rises, which could explain the weird spikes.
It seems like when the AC is blowing, it keeps the humidity down as it cools the air near the ceiling. Once the AC stops, the warm air settles and raises the humidity level again. You might want to keep an eye on how your AC interacts with the humidity levels.
It might be that humid attic air is sneaking into your bathrooms. Ensure the fan housing is well-sealed, and that flappers in the ducts are functioning correctly. Also, check that the ductwork is angled upward towards the roof so condensation doesn’t build up and flow back into the bathroom.
It sounds like humid air from your attic might be getting pulled down through the exhaust fan when it's off. This often happens if there are issues with the ventilation or if other appliances like dryers are running. Double-check that no other exhaust fans are running simultaneously during your tests.