The fan’s damper can also get stuck in the open position, allowing hot air into the cool duct and creating condensation. Check the small damper at the fan. It should open when the fan is on and close when the fan turns off. This damper responds to fan pressure and gravity. Most vent connectors through the roof or sidewall should also have a damper to keep cold air out, and it should open and close with fan operation.
For many years, contractors installed bath fan vent ducting incorrectly, creating a bend or low loop to catch condensation. This just allows water to accumulate and may cause a large leak when the water lets go.
Also assure the bath vent terminates to the exterior. Any bath vent that terminates in the attic or near an attic vents can cause moisture issues in the attic.
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