Bathroom fan installation requires outside ventilation. If the bathroom exhaust fan or venting fan isn't accessible through an attic, you'll need to vent through a sidewall of your house. If you're venting a bathroom fan through an external sidewall, then you'll need a wall mounted bathroom exhaust fan.
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.
Surprisingly, some building codes do not require bathroom fans. All municipalities have different requirements, but some do not draw a hard line on requiring exhaust fans. Bathroom ventilation is needed in those areas, but it can be from a window or fan—you choose.
Without an exhaust fan, you can expect to have mold growing in your shower, peeling paint, and even damaged drywall. Prolonged moisture buildup is a disaster for the bathroom; picture damaged walls, floors, ceilings, fixtures, and even doors. That's something no homeowner wants to face!
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.
The duct should vent to the outside. If you don't have an attic duct, see the installation steps later in these instructions. Turn on the power and check that the fan works. Some fans are extremely quiet, so listen carefully.
California. Under section 1203.4. 2.1, California's building code requires that any bathroom that contains a bathtub, shower, spa or similar, will require ventilation by an exhaust fan.
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.
Exhaust fans are required in bathrooms that don't have a window or if the window is smaller than 3 square feet. The window must also open.
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.
Never terminate bathroom exhaust ducts in an attic, where added humidity can lead to mold issues and expensive repairs. Venting a bathroom fan into an attic adds unnecessary humidity to that space, which can lead to condensation problems.
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.
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.
Did you know that bathroom exhaust vents discharging into your attic can cause mold? Moreover, it can cause water stains on your ceilings that could lead you to believe you have a roof leak. Bathroom vents should always be vented to the exterior of your home and it is required by code.
Bathroom exhaust fan installation cost
Bathroom exhaust fan installation costs $150 to $550 when replacing an existing fan. The cost to install a new bathroom fan is $250 to $950 to vent outside through the wall or roof. The bathroom extractor fan itself costs $20 to $400, depending on the type and features.
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.
Behind or within the Wall: In most cases, the vent pipe for a toilet should run vertically from the toilet's waste pipe (known as the closet bend) and extend upward to connect with the main vent stack or a branch vent line that ultimately exits through the roof of your home.
Signs that your bathroom needs more ventilation are:
Stuffiness when you enter the room. Mildew on walls or tiling. Stains forming on walls or ceiling. Smells that linger for a long time.
The Home Ventilating Institute (HVI) recommends that exhaust fans for bathrooms up to 100 square feet provide 1 CFM per square foot. For bathrooms over 100 square feet, HVI recommends basing ventilation on the number of fixtures—50 CFM each for the toilet, shower, and tub, or 100 CFM for a jetted tub.
Should the bathroom fan be vented outside or into the attic? According to the building code, it must vent to the outside. Often, older homes just vented them into the attic, but this can lead to a buildup of humidity, and moisture can lead to mold.
Apart from pushing cooled air into your room, air vents also draw warm air from your room back into the air ducts. This warm air is then transported back to the air conditioner, which is cooled again, maintaining an efficient air conditioning cycle.
Exhaust ducts should always terminate outdoors (refer to applicable codes for vent termination clearance requirements). Running exhaust ductwork up to a vented attic or just in front of an attic vent (gable, ridge, or soffit) is not sufficient. The exhaust should be fully ducted to a point exterior to the home.
Venting to the Outside is Typically Considered Mandatory
This can get mold started in the insulation, rot roof timbers, and otherwise cause serious problems. Because of this, many areas have building codes that require outside venting for bathroom exhaust fans.