Eliminating Exhaust Fan Drips

Q: Every time someone takes a shower, water drips out of the exhaust fan grill. This has been an ongoing headache over the last three years, mostly during cold weather. Black mold forms on the walls and ceiling wherever water runs repeatedly. What can I do to fix this? The flexible metal duct leading from the fan runs horizontally 6 or 7 feet through the attic before it reaches the outdoors.

A: Your problem is quite common and is caused by the non-insulated duct that travels through your attic. When warm, moist shower air cools in the duct during winter, water condenses inside, running back through the fan. The solution is to replace the existing duct with some insulated duct, the kind with a layer of fiberglass insulation around the outside, all sheathed under a continuous polyethylene vapour barrier sleeve. It’s often not enough to wrap the duct in insulation only. The seamless vapour barrier boosts thermal performance of the insulation and prevents external condensation forming on the outside of the pipe during warm, humid summertime weather.