Last year we visited a home to replace a "bathroom fan" but when we arrived, we found that it was vented directly into the attic. If you've been watching my show, you know that should never be! That was in the middle of the winter with ice on a steep roof, so we promised to go back and do it right after the snow was gone.
Here are a few of the details of a good bathroom fan installation when you have no choice but to go through the roof. Remember, you never want to go through the soffits as the hot moist air will just turn around and re-enter the attic through the soffits themselves. If you have no vertical gable end to the attic, then you must go through the roof, the least desirable of the choices because some condensation will always occur in the ductwork and it may drip back into the bathroom. But if you have to, you have to.
You can now purchase roof top exhaust vents, the best of which are metal from my friends at Ventilation Maximum. What is the difference between a roof vent and a roof exhaust vent? A roof vent is simply a rain cover over a hole in the roof. A roof exhaust vent is a sealed duct that hooks onto an exhaust duct and assures that the air is discharged outside.
One of the things that I do recommend in a job like this, even if you are an avid do-it-yourselfer, is to hire a good roofer to install the exhaust vent itself. One, you don't fall off the roof. Two, he makes sure that there will be no leaks in the roof. Then you can install the fan and the ducting yourself. If he doesn't have, or doesn't know what a roof exhaust hood is, get another roofer. Avenue Road Roofing in Toronto put our's in.
Don't forget that any ducting in the attic must be insulated as heavily as possible. If you are going up-hill to an exhaust outlet, do not allow any low loops in the ducting as these will collect water and grow mould. Make sure that the duct is mechanically attached to and sealed to that exhaust hood. If it were to fall back into the attic, all that bathroom moisture will most likely rot out your roof.
Seal the fan box itself and seal it to the bathroom drywall. Then bury it all heavily under the attic insulation.
I always prefer to run ducts down through the warm part of the house and out through the basement, but often the attic is the only practical way to go. You do have to be much more careful to avoid moisture, frost and mould problems in this freezing cold part of the house.