Obviously I’ve got some sort of problem with venting and excess humidity. I’ve partially opened one of the gable end windows and made sure that there’s no insulation blocking the soffit vents. How do I know if this is enough ventilation?

As for the excess humidity, I suspect it’s coming from the second floor bathroom. When the previous owner remodeled this bath, he put ½" drywall over the old plaster ceiling (which I would think would act as an added vapor barrier), but the added vent/heater fan assembly has some gaps around it. What is the best way to make a better seal - can I use expandable foam (e.g. Great Stuff)? Also, should I place some sort of plastic vapor barrier around fan assembly box before laying the fiberglass batting back on top? - (Although it’s a combination heater and vent fan, the metal assembly box doesn’t seem to get hot). Finally, should I replace the plastic flexible 4" vent hose with an insulated vent hose?

Any other things I should do to reduce the attic humidity?

I’m open for any/all ideas.

Thanks!