The valve on my roof vent for our bathroom fans freeze shut during the cold Vermont winters. I have tried insulating the duct but this doesn't work. Do you have any suggestions to prevent the valve from freeziang shut?
Maybe you could install a light with say a 100watt bulb in side the duct close to the valve. You could use a tremostat to turn it on the heat from the bulb would keep the valve warm enough to prevent freezing.
That's why a roof exhaust is rarely used in this region. They are usually covered with snow during the winter and with the warm moist air from the vent creates problems like this and others as well. See if it's possible to relocate the ducting so it exits the gable end of the attic or through a wall. It will work much better on flat vertical surface.