Adding warm moist air into a cold attic is not a good idea as the change in temperature causes the air to condense when it reaches its dew point. This condensation can accumulate on surfaces and cause rot or enough moisture to form running water that will run onto the ceiling or down the walls. Of course once the ceiling or wall materials are damp you may face a mold problem. It is best to vent all fans out through the gable ends or through the roof. Be sure to use insulated vent lines. These photos show what can happen to the sheathing if warm moist air hits the sheathing. This roof was very spongy and almost at the point that would require re-roofing the area.i13 i15