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Building Code Reference

Got a CMC 403.7 correction?

Bathroom exhaust ventilation

For builders in California, Greater Seattle, the Las Vegas area, the Denver area and Kansas City · CrossBeam Permits

What this code requires, how to fix it, and how cities and counties review it

What this code requires

CMC 403.7 is the California Mechanical Code section that sets bathroom exhaust rates; for dwellings, CMC 405.3 requires every room with a tub or shower to have mechanical exhaust ducted directly to the outdoors, window or not. A recirculating fan that filters and returns air back into the bathroom does not satisfy this requirement. Residential bathrooms need a minimum of 50 CFM intermittent or 25 CFM continuous under CMC 405.3.1. ADU bathrooms, remodels that add or move a bathroom, and new residential construction all trigger this section.

How to fix it

  1. Confirm the bathroom qualifies for mechanical exhaust

    In a dwelling, CMC 405.3 requires mechanical exhaust directly to the outdoors in every room with a bathtub, shower, or tub/shower combination, and the 2025 Energy Code requires local mechanical exhaust in each bathroom. An openable window does not replace the fan, so you cannot substitute a window note on the plans. Mark each bathroom exhaust fan on the floor plan so the plan-checker can see it is provided.

  2. Select a compliant exhaust fan with the right CFM rating

    For residential bathrooms, the California Mechanical Code requires a minimum of 50 CFM for intermittent operation or 25 CFM for continuous operation (CMC 405.3.1). Verify the fan model you specify meets those minimums at installed static pressure — fan performance drops in longer duct runs. Specify the fan by model number and CFM on the mechanical schedule. Do not list ‘exhaust fan per code’ without a model number; plan-checkers will mark it incomplete. Include the manufacturer’s cut sheet in the submittal package when the jurisdiction requires it.

  3. Show duct routing to an exterior termination on the plans

    Draw or annotate the exhaust duct path from the fan to the exterior termination on your mechanical or architectural plans. Show the duct size (typically 4-inch round), direction of run, and the type of exterior cap (wall cap, soffit vent, or roof jack). The termination must discharge to the outdoors, not into an attic, crawl space, soffit cavity, or shared plenum. Mark the exterior termination on the elevation sheet or a dedicated detail. If the duct runs through a conditioned wall cavity, note that in the annotation so the inspector can verify insulation continuity.

  4. Specify a backdraft damper at the exterior cap

    Bath exhaust terminations must include a backdraft damper to prevent outside air from flowing back through the duct when the fan is off. Most exterior wall caps and roof jacks include an integral damper; confirm by noting ‘w/ backdraft damper’ on the plans next to the cap symbol. If your duct run is long or primarily horizontal, add an in-line damper at the fan housing as well. The damper also keeps pests out of the duct — a detail that field inspectors appreciate seeing addressed on the plans.

  5. Verify duct material and maximum run length

    Use smooth-wall metal duct (galvanized steel or aluminum) for bath exhaust. Flexible duct is allowed in short sections connecting the fan to the rigid duct, but an all-flex run collects moisture, sags over time, and restricts airflow. Keep the total equivalent duct length within the fan manufacturer’s rating; a 50-CFM fan rated for 50 feet of equivalent length will move inadequate air if the actual run is 60 feet with two elbows. Each 90-degree elbow typically equals 10 feet of equivalent length. Document the equivalent length calculation on the plans if the run is longer than 25 feet.

  6. Specify the fan control type on the electrical plan

    The fan needs a control strategy that ensures it actually runs long enough to remove moisture. Title 24 Part 6 energy compliance documents may require a humidistat, occupancy sensor, or a timer-based control depending on the project type and compliance path. Note the control type on the electrical plan — switch only, timer switch, humidistat, or occupancy sensor — and confirm it matches your Title 24 calculations. Some jurisdictions require the fan to be wired to run any time the bathroom light is on; others allow a separate switch if a timer is included.

  7. Add a plan note prohibiting recirculating fans

    If your plans reference a fan model that could be confused with a recirculating unit, or if the fan location makes exterior ducting look optional, add a plan note stating: ‘Exhaust fan shall discharge to outdoors via rigid duct. Recirculating fans not permitted per CMC 405.3.’ This note protects you during field inspection if a subcontractor swaps the specified fan for a recirculating model during construction, and it removes ambiguity for the plan-checker reviewing the submittal.

  8. Write a clear corrections response and resubmit

    When resubmitting after a CMC 403.7 correction, include a response letter that addresses each flagged item line by line. Reference the specific sheet and detail number where you addressed the correction — for example: ‘M-1, Detail 3: exhaust fan schedule updated with model number and CFM; duct routing shown on M-2; exterior wall cap noted on A-4, East Elevation.’ Cities with 60-day review cycles move faster when the corrections response is clear and specific. A generic ‘revised per corrections’ reply usually generates a follow-up comment asking you to clarify what changed.

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Why you got flagged

Here’s why plan-checkers flag CMC 403.7 corrections so often.

The most common trigger is a bathroom remodel or addition where the plans show a ceiling fan but don’t specify that it ducts to the exterior. "Fan per code" or "exhaust fan" noted on the mechanical schedule isn’t enough — the plan-checker needs to see a duct routing to an exterior termination. If your plans don’t show where that duct exits the building, expect a correction.

A second frequent scenario involves recirculating fans. Some fan models with charcoal filters are marketed as bathroom ventilation, but CMC 403.7 is explicit: air must be discharged to the outdoors. Interior recirculation doesn’t count, regardless of the filter quality. If the fan spec sheet shown on your plans is for a filterless recirculating model, the plan-checker will catch it.

Third, interior bathrooms in multi-unit buildings often get flagged when the mechanical drawings show exhaust routing through a shared plenum or into the attic. Both are non-compliant. Each bathroom exhaust must terminate at the exterior with a backdraft damper — typically through the soffit, exterior wall, or roof cap. Routing into an attic or common corridor fails CMC 403.7.

Finally, small half-baths get overlooked because contractors assume only full baths with showers need exhaust. CMC 403.7 applies to any bathroom that lacks an openable window, regardless of whether it has a tub or shower.

The official code

California Mechanical Code, Part 4 — Section 403.7 Bathroom Exhaust Ventilation

Read the source →

Common questions

Does CMC 403.7 apply to ADU bathrooms?
Yes. ADU bathrooms follow the same rules as primary dwelling bathrooms under CMC 403.7. An ADU bathroom with a tub or shower needs a ducted mechanical exhaust fan rated at 50 CFM intermittent or 25 CFM continuous, window or not. This is a common correction on ADU permits because designers sometimes assume ADU interior baths can use a recirculating fan to save duct run cost. They cannot. ADU projects already face 3 to 6 months of total delay when corrections pile up — catching this in your plans before submission saves the round.
Can I use a recirculating fan to comply with CMC 403.7?
No. CMC 403.7 explicitly requires exhaust air to be discharged to the outdoors. A recirculating fan — even one with a charcoal or HEPA filter — does not move air out of the building and does not satisfy the code. The only compliant option is a ducted fan that terminates at an exterior cap. If running a duct to the exterior is impractical due to the bathroom location, consult with the building department before submitting — but do not assume an alternative will be accepted without prior approval. Submitting plans with a recirculating fan and hoping the plan-checker misses it is a reliable path to a correction letter.
What CFM is required for a bathroom exhaust fan in California?
The California Mechanical Code requires a minimum of 50 CFM for intermittent operation or 25 CFM for continuous operation for residential bathrooms (CMC 405.3.1). Those numbers apply at the installed conditions, not just the nameplate rating. A fan rated at 50 CFM in a lab test may deliver 35 CFM in the field if the duct run is long or includes multiple elbows. Check the fan manufacturer’s airflow curve at the expected static pressure for your duct run, and size up if needed. For large bathrooms — generally over 100 square feet — some local interpretations require higher airflow; confirm with your building department.
Can bathroom exhaust duct terminate into the attic?
No. Terminating a bath exhaust duct into the attic is a direct CMC 403.7 violation and will fail inspection. Moist air from a bathroom dumped into an attic causes mold, wood rot, and degraded insulation — the exact problems the exhaust requirement is designed to prevent. The duct must exit through the exterior wall, through a properly flashed soffit vent with an exterior cap, or through the roof with a weather-resistant roof jack. Mark the termination point clearly on your plans. If the attic termination was installed in the field before the plans were corrected, the inspector will require it to be rerouted.
Does CMC 403.7 apply to half-baths with no shower or tub?
Yes, if the half-bath has no openable window. CMC 403.7 covers any bathroom that lacks natural ventilation through an openable window, regardless of whether it has a tub or shower. A powder room — toilet and sink only — in an interior space without a window still needs a ducted exhaust fan. This catches contractors off guard on interior remodels where a half-bath gets relocated into a hallway or closet conversion. The code doesn’t care about the fixture count; it cares about whether the space can be naturally ventilated.
How long can a bathroom exhaust duct run be?
The maximum depends on the fan model and duct diameter. Most residential bath fans with 4-inch round smooth-wall duct are rated for 50 to 100 feet of equivalent duct length by the manufacturer. Each 90-degree elbow counts as approximately 10 feet of equivalent length; a 45-degree elbow is roughly 5 feet. If the run exceeds the fan’s rated equivalent length, actual airflow drops below 50 CFM and the installation fails the intent of CMC 403.7 even if it passes visual inspection. Upgrade to a higher-CFM fan, use 6-inch duct, or redesign the duct path to reduce elbows.

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