2 ways to look at it.
if you have a window then you don't need a fan. but you don't want to leave the window open in winter.
the fan venting into the attic will put condensation on the bottom of the roof sheathing and cause it to eventually drip down into the insulation. have you seen some drip lines in the insulation? but if attic is vented well then you may not notice drips.
get a good fan with a good motor and a 4 inch duct line,. insulate the duct line with sleeve insulation.. use ridged ducting if you have to go a distance sideways to go out the roof. other wise go straight up and put in a roof vent that has a flap on it to keep the cold drafts out. and insulate on top of the fan also.
ice dams...check to see that there is adequate ventilation coming up from the soffits in that area.insulation tight against the roof will cause the ice dam as there is no cold air going by, keeping the bottom of the roof cold.
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