The most common option for bathroom exhaust fan venting is the ceiling to wall venting option. The second most common is the roof vent installation. The third most common is probably the vertical wall installation.
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.
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.
Types of Bathroom Extractor Fans available
The most common types of bathroom extractor are either Axial, Centrifugal or inline. Axial extractors connect directly to external walls whereas centrifugal are primarily designed for internal walls with longer runs of ducting.
A ductless bathroom fan can perform the same job as a ducted bathroom fan but requires less space. Ductless bathroom fans come in a variety of style options and can be installed in any bathroom.
Ductless bathroom vent fans can be legit, especially if you have a window for natural ventilation. While they might not be as powerful as vented options, they're still effective in reducing humidity and preventing mold.
For example, flexi duct can be used for a bathroom fan's system, but is prohibited in dryer ventilation systems, and PVC and PVC coated aluminum can be used for bathroom fan ducts, but is very dangerous to use for dryers.
The duct should vent to the outside. If you don't have an attic duct, see the installation steps later in these instructions.
Recirculating (ductless) exhaust fans are not allowed if mechanical ventilation of a space is required, but such fans are allowed in naturally ventilated spaces.
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.
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.
Ceiling-mount bathroom exhaust fans are the most common models. They connect to an exhaust duct that runs through the attic or between floors. Wall-mount bathroom exhaust fans are designed for rooms where venting through the ceiling isn't practical.
Issue 3: Your fan is ducted improperly
Venting to the attic or crawlspace leads to dangerous and destructive moisture issues with potential mold and mildew and other damages. Make sure to use the shortest run of duct possible, with as few elbows as possible. You want the straightest shot to the outside you can get.
What to Do If There is No Exhaust Fan in the Bathroom. It's not uncommon especially for older homes to lack the necessary ventilation that modern-day bathrooms need. There are some simple solutions, like installing a window-mounted fan or a ceiling vent.
The flexible insulated duct is recommended for this application to minimize condensation in bathrooms (or other warm and moist areas), and can also be helpful in reducing fan noise.
Recirculating fans, used when venting outside isn't an option, do offer some odour control by trapping dust and particles in a filter. However, they don't address moisture effectively. The trapped moisture needs regular filter changes to maintain functionality.
Ductless exhaust fans have a charcoal filter system, allowing them to remove moisture from the air as well as mitigate odors. They can be attached to the wall or ceiling and are a quiet and energy efficient ventilation solution.
For an easy solution that doesn't require an exhaust fan, leave the bathroom door open and run a dehumidifier. Immediately dry any wet spots that occur. For a more permanent solution, add a ceiling, wall, or floor duct vent fan. These don't usually cost more than $2,000 USD to install.
By their nature, bathrooms create much steam and water vapour. Without proper ventilation, this can turn to condensation and damp, not just in the bathroom, but also in nearby rooms. There are several ventilation solutions that can help, but one of the simplest and most effective is to fit an extractor fan.
You usually find ductless fans in bathrooms without tubs or showers. For them to be up to building code, you must also have a window. While the fan does a good job of keeping air circulating, leading to a more comfortable environment, the window will aid in helping to remove moisture from the air.
The moist air will fuel the growth of mold and mildew on the roof framing and the sheathing that covers the timbers. Wood rot is an absolute certainty over time if you dump bath exhaust into an attic. This moist air must exhaust to the exterior of your home. This is non-negotiable.