The most popular venting option for a bathroom exhaust fan is a pipe extending vertically through the roof. This choice only works on home designs that allow a straight vertical pipe with no bends. It's the most efficient means of pulling moisture out with the least chance of moisture re-entering the home.
Rigid is best but for bath exhaust flex metal is OK. Vent to outside, ideal is straight up through the roof (every bend introduces resistance and reduces efficiency) but if you have to go out via the soffit, it's better than no vent at all.
If you have enough room in the void, then always go for round ducting as this gives the best overall airflow performance through the system. If you just don't have the height, then go for flat ducting next.
Rigid Metal Duct
Rigid duct is considered as the best since it is more durable and less resistive than flexible duct. Professionals consider it the best choice for a bathroom fan in terms of performance, namely venting air and moisture from the bathroom.
Disadvantages of Flex Duct
Flexible ducts are more prone to air leakage around the seams, which can lead to lower HVAC system performance. Additionally, the pliable nature of flex duct tends to make them prone to tangling or crushing when they pass through tight spaces or fixtures.
Ventilation rates based upon eight air changes per hour are generally suggested. For most bathrooms this works out to one CFM per square foot of bathroom area. For example, a 7' x 10' bathroom would require a 70 CFM fan. A 50 CFM rating is recommended as a minimum for bathrooms 50 sq.
The duct should vent to the outside. If you don't have an attic duct, see the installation steps later in these instructions.
Aiming for the space in between the shower and bathroom door gives the vent fan the best chance to remove the humid, unwanted air as well as pull in fresh air from the rest of the house.
The best solution is to vent the bath fan directly through the roof. In this article and the video above, we'll explore the step-by-step process of installing a roof-mounted bath fan vent, as demonstrated by This Old House general contractor, Tom Silva.
As a general rule of thumb, it's always a good idea to oversize your bathroom exhaust fan. This way you can be sure that your exhaust fan will be powerful enough to do its job without being so powerful that it's overkill.
The extraction rate you need for your bathroom will depend on the room's size. However, building regulations state that the minimum fan extraction rate your bathroom should have is 50 m^3/hr (or 15 l/s) if the room doesn't have an openable window, so do ensure you at least meet this level.
Semi-Rigid Aluminum Duct is ideal for bathroom fan exhausting.
The bigger the bathroom, the higher the CFM rating you'll need. A fan should have a CFM rating high enough to replace the air in your bathroom at a rate of eight times per hour. To figure out the right fan size, first determine the area of your bathroom.
To fulfill the local exhaust airflow requirements of ASHRAE 62.2 (2010) and the International Residential Code (IRC 2021, Section M1507), bathroom fans should have a mechanical exhaust capacity of ≥ 50 cfm for intermittent operation or ≥ 20 cfm of ventilation when operated continuously.
The most popular venting option for a bathroom exhaust fan is a pipe extending vertically through the roof. This choice only works on home designs that allow a straight vertical pipe with no bends. It's the most efficient means of pulling moisture out with the least chance of moisture re-entering the home.
The installation of a recirculating exhaust fan is one of the easiest ways to vent a bathroom without an exterior exit. Recirculating fans filter the air via a set of charcoal or HEPA filters before releasing it back into the bathroom, in contrast to ordinary exhaust fans, which exhaust air outside.
Note that the bathroom vent fan must always exhaust to the outdoors; never allow the duct to simply blow into an attic, crawlspace or other enclosed area.
To adjust the CFM, you can change the speed of the blower or alter the ductwork size. Increasing the blower speed won't have much effect if a room's duct is too narrow. A 12' x 12' foot room is 144 square feet. You generally need at least 1 CFM per square foot, meaning that room's duct should have a 144 CFM capacity.
A fan that is too big can cause a bathroom to be cold and noisy. This is why calculating the correct CFM is important.
A ventilation fan brings fresh air into a space, improving circulation, while an exhaust fan removes stale, humid, or polluted air, typically from kitchens or bathrooms. The key difference lies in airflow direction: ventilation fans pull outdoor air in, while exhaust fans push indoor air out.
We recommend at least an 80 CFM for most bathrooms but a 100/110 for a larger master bathroom. This will help move the moisture out of the bathroom and reduce the risk of mold.