Q: My home has condensate on the windows. I want to put in a new bathroom fan, but I need to know the correct way to vent a bathroom exhaust fan and the correct size of motor the fan should have.
The airflow through a bathroom fan is measured in cubic feet per minute (CFM).
A bathroom needs a minimum of 50 CFM intermittently or 20 CFM continuously. Fans are labeled with a CFM rating, but the duct work you attach to the fan will affect its flow rate.
If you run ducting in long, 30-foot runs and/or lots of 90 degree turns, you will need to double the fan capacity to 100 CFM. Ducts for a bath fan exhaust can be run with plastic, such as ABS or PVC, or metal, such as 29gauge warm air snap seam.
Ducting made of smooth materials will let more air flow than the flexible slinky-style duct materials, which are only appropriate for very short lengths, approximately 5 feet.
Another use is for the connection between the fan and the ducting, in order to reduce vibration.
If the ductwork passes through the attic, or any unconditioned space, it will need to be sealed and insulated to prevent heat loss.
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.
If, after installing and using your fan, you continue to see condensation on your windows, you may want to consider a more substantial ventilation system for your home. Today, most building codes require some form of mechanical ventilation.
This can range from an appropriately sized exhaust fan operating with fresh air inlets installed in the living spaces to a Heat Recovery Ventilation System (HRV).
Regardless of which type of system is used, it must be sized and installed to meet the needs of the home it will be serving.