What bathroom exhaust fan does an ADU need in California?

Each ADU bathroom needs a fan ducted to the outdoors that delivers at least 50 cfm when switched on as needed, or at least 20 cfm if it runs continuously. The 2025 CALGreen code adds that bathroom fans must be ENERGY STAR compliant and controlled by a humidity control unless the fan is part of a whole-house ventilation system.

Energy Code requirements

• Airflow. Section 150.0(o)1G requires local mechanical exhaust in every bathroom: 50 cfm for demand-controlled fans (Table 150.0-E) or 20 cfm for continuous fans (Table 150.0-F).
• Controls. A demand-controlled fan needs a readily accessible on-off control, or an automatic control that does not stop the occupant from turning it on. A continuous fan needs an on-off control the occupant can reach and must be designed to run during all occupiable hours.
• Sound. The CEC's Compliance Manual summarizes the ASHRAE 62.2 limits: demand-controlled local exhaust fans at 3.0 sones or less, and continuously operating fans at 1.0 sone or less.
• Proof of airflow. The installer either measures installed airflow or installs a fan and duct that match Table 150.0-H, which allows 25 feet of duct or less with no more than three elbows. For example, a fan rated at 50 cfm at 0.25 inch of water needs at least a 4-inch duct, and an 80 cfm fan needs at least a 5-inch duct.
• Backdraft damper. Section 150.0(m)7 requires a backdraft or automatic damper on fans that exhaust to the outside.

CALGreen requirements

CALGreen Section 4.506.1 calls for ENERGY STAR compliant exhaust fans in every bathroom, ducted to terminate outside the building, and controlled by a humidity control unless the fan functions as part of a whole-house ventilation system. Fans with built-in humidity sensing are widely available.

One fan, two jobs

A quiet, continuously running bath fan is a common way to provide the ADU's whole-dwelling ventilation. In that case it must deliver whichever required rate is higher, meet the 1.0 sone limit and carry the required label on its switch. The ECC rater (formerly HERS rater) then field-verifies the whole-dwelling ventilation airflow.

Common problems

• Fans that dump into the attic instead of a roof or wall cap.
• Long or crushed flex duct runs that choke airflow below the fan's rating.
• Fans chosen by cfm alone, without checking the sone rating.

What it means for your mechanical plan

The plan should show each bath fan's location, model, rated cfm and sone rating, the control type, the duct size and length, and the exterior termination. If a bath fan doubles as the whole-dwelling ventilation fan, the plan should say so and show the calculated rate. See ADU ventilation requirements and what mechanical plans include.

Sources: energy.ca.gov, energy.ca.gov, paradiseca.gov.