Fan-tastic Bath Venting Tips
Determining the minimum CFM of your bathroom is easy, simply multiply the room’s width × length × height × 0.13.
Many fans will list the maximum duct run that is allowed. Remember to include the duct run length and any elbows in your calculations. Adding an elbow connector or an external vent hood with a backdraft preventer will add approximately 10 to 15 feet of duct length. You may need a higher CFM rated fan if your ductwork is long or complicated.
You should also consider the ACH, the air exchange per hour, that is: an air flow rate sufficient to remove the air volume in a given room at a specified number of times each hour. Your bathroom exhaust fan should be sized properly to completely change out the air in the bathroom at the rate of 11-15 changes in one hour.
Locate your bathroom fan to ensure maximum moist air extraction.
Steam rises, so the shower ceiling is always a good spot for a shower fan. If that location is not feasible then place the fan just outside the shower.
In cathedral ceilings sometimes a sofit built over the shower can accommodate the fan or a high-mounted wall fan can be installed.
Install the Fan Properly
Bath fan ducting can negatively affect fan performance. Un-insulated, undersized, or droopy flex ducting and excessive length or excessive use of elbows all can negatively affect the fan’s rated airflow.
In order to keep the fan effective, try to lay out your installation with minimal turns.
The best duct pipe to use is solid smooth aluminum. Solid duct pipe results in better airflow because it has less air drag and is easier to keep in a straight run to avoid drooping.
When installing solid pipe, create a slight pitch toward the exterior of the building in case water condenses in the pipe. You want to avoid moisture draining back toward the fan.
Take steps to properly support your pipe run either from below, with blocking or from above, with plumbing strap or large, heavy-duty cable ties (with screw holes in them). I use metal plumber’s tape—it’s a galvanized strapping with holes every 1/2 inch for screws.
Install the pipe with all seams facing up and seal these seams and all joints with aluminum tape (not duct tape). Keeping your pipe seams at the top will prevent moisture leakage in the house. Small sheet metal screws are acceptable with bath vent pipe, but the aluminum tape works fine without them.