Bathroom ceiling condensation

[csdutton33]

Hi all, first-time poster here, so apologies in advance for any missteps!

Here's the issue:
I'm experiencing large water spots on my bathroom ceiling during winter months, likely due to condensation.

The details:
- The unit is on the top (3rd) floor of a triple-decker, right below the roof (flat rubber, warm roof with blown-in insulation)
- The fan is an adjustable Panasonic model, but I do not know the specific CFM it is set to (could be anywhere from 50-110)
- The fan connects to a flexible 4" duct that runs about horizontally before connecting to a vertical roof vent (with a candy cane shape)
- Unfortatelt I cannot access the attic space between the ceiling and roof (it's ~18" of space, with no access point)

My gut is that the condensation is likely due to one (or multiple) of the following causes:
- Underpowered fan
- Lack of insulation around the fan or duct
- Some issue with the roof vent

Has anyone experienced, and ideally solved, an issue like this? Just trying to figure out the best next steps to diagnose and fix this.

Thank you in advance!

 

[foxhole]

Fan may not be up to it or you open a window during use ? which also increases condensation .Keeping room warm during use also reduces condensation as it keeps moisture airborne longer so fan can remove.