The answer to this question is that moisture is condensing somewhere in the exhaust run, check the following items.
- Operate the fan for a period of time after a shower (bathroom fan) or cooking (range hood) to ensure all moist air is exhausted.
- In the case of a bathroom fan ensure the space between the fan housing and finished ceiling is completely sealed with caulking. When the fan operates it can draw air from the unconditioned space above the ceiling, this air can cause condensation to form behind the grill.
- Use insulated duct when venting through unconditioned spaces (attics, crawl spaces etc.).
- If using flexible duct, ensure it is pulled as tight and smooth as possible. The ridges in flexible ducting can act like baffles forcing moisture out of the air.
- When completing the installation to the outside ensure the exhaust duct is sloped in a manner that any moisture will run to the exterior of the home.
- Ensure the exhaust fan is vented through a proper wall, roof or soffit exhaust vent cap.