Keith in Ste. Agathe, Manitoba writes: "I installed a new fan in the bathroom ceiling. I ran the exhaust pipe into the attic, then on a gentle down slope to the gable end of my house. On the outside wall I installed a 3 flap door for the exhaust to come out. In the winter there is so much moisture that the flaps freezes shut. Is thare anyway to prevent this?"
Well Keith, you are doing a lot of things right, but in a real tough climate.
First, to get as much heat to that flapper as possible, insulate the duct heavily in the attic, even insulate the fan box heavily.
Congratulations for sloping it downhill and going out the gable end, you have been listening about keeping problems out of the attic.
Now the problem with that cheap little flapper is that there is simply too much exposure to those cold Manitoba winds. I would suggest replacing it with a hooded damper and then extend the hood down about 12 inches or more as a wind shield. That will cut out the wind chil factor, and permit the heat from the duct to keep it functioning.
If you can't locate a wind baffled exhaust, you can have a sheet metal hood made that simply goes over the typical exhaust hoods and drops down the wall a bit as outlined in the last graphic.
One off-the-shelf exhaust hood that works well at preventint wind entry and avoiding freezing is the series of "Exterior Wall Vent Covers" made by Ventilation-Maximum: Ventilation-Maximum.com