I plan to vent a bathroom (1/2 bath; no shower or tub) exhaust fan (Broan 744) through a soffit (only available option). I plan to use insulated duct from the fan to the soffit--is that necessary? The fan has a damper in its housing, but do I need another damper just inside the exhaust vent at the soffit to prevent condensation and icing problems in the duct during the winter? I live in the St Louis area and it gets below freezing in the winter.