How to ventilate the bathroom
With the very tight homes being built in Alaska today, moisture loads can reach damaging levels more readily without mechanical ventilation. Building codes require bathrooms to be mechanically ventilated to the outside to reduce the high humidity levels created by showering and bathing. The latest codes specify a ventilation rate of 50 cubic feet per minute (CFM) for intermittent exhaust – for example, a bath fan operated by a timer.
When choosing a fan, pick the quietest and most energy efficient fan you can afford. Fan noise is rated in sones, and code requires a sone rating of 3 or less for an intermittent fan. One sone is approximately the noise produced by a quiet running refrigerator.
Bath fans will be most effective if they are installed in the ceiling in order to capture rising heated air and the odors and moisture contained in that air. The fan should also be located close to the moisture source and away from the door in order to avoid short-circuiting the airflow if the bathroom door is open. The fan can be controlled by a shut-off timer, an on-off switch, or sensors.
The ducting run used to exhaust the air outside should be as short as possible with minimal elbows or other restrictions. Ducting made of smooth materials allows more airflow than the flexible slinky-style duct materials. Slinky-like flexible ducting is only appropriate for very short lengths, approximately five feet, and should be avoided whenever possible. Twenty-nine gauge smooth wall metal ducting is a common choice for bath fan systems.
If the ductwork passes through the attic, or any unconditioned space, it will need to be very well sealed and insulated to prevent heat loss and condensation problems. As a rule, try to run the ducting in heated space whenever possible. Ducting should slope slightly away from the fan toward the exterior, so if any condensation takes place, the moisture will run outside rather than back into the fan housing. At minimum, the fan exhaust should have an exterior hood and a flapper type damper to prevent cold winter air from entering the bathroom when the fan is turned off. Installing an inline damper in the ducting on the warm side of the exterior wall will provide improved air sealing.
There are several local companies that can assess your situation and install the right system, or sell the appropriate hardware if you want to do the job yourself.