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Building Code Reference

Got a CMC 405.1 correction?

Whole-house 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 405.1 requires every California dwelling unit to have a whole-house mechanical ventilation system that continuously delivers outdoor air to living spaces. The required airflow is calculated per ASHRAE 62.2: take 7.5 CFM per occupant (occupants = bedrooms + 1) and add 0.03 CFM per square foot of conditioned floor area. The system can be a dedicated supply fan, a bath exhaust fan running continuously, or a balanced HRV/ERV unit. Single-family and ADU permits are the most common jobs flagged for missing this calculation.

How to fix it

  1. Run the ASHRAE 62.2 airflow calculation

    Use the formula: CFM = (bedrooms + 1) x 7.5 + 0.03 x conditioned floor area (sq ft). Example: a 1,400 sq ft 2-bedroom ADU needs (2 + 1) x 7.5 = 22.5 CFM plus 0.03 x 1,400 = 42 CFM, total 64.5 CFM minimum (round up to the next commercially available fan size, usually 70 or 80 CFM). Write the full calculation out on the mechanical plan sheet, not just the result.

  2. Specify the system type and operating mode

    Pick one of the three compliant approaches and call it out explicitly on the plans: (a) Supply fan: a dedicated outdoor air intake fan that delivers fresh air directly to living areas; (b) Exhaust fan: a bath or utility fan sized to the ASHRAE 62.2 CFM minimum, wired to run continuously or on a 24-hour programmable timer; (c) Balanced HRV/ERV: a heat or energy recovery ventilator with supply and exhaust in balance. Don't just write "mechanical ventilation per CMC 405.1" on the plans.

  3. Add fan make, model, and rated CFM to the mechanical schedule

    Include a mechanical equipment schedule on the plans listing: fan manufacturer, model number, rated CFM at 0.1 in. w.g. static pressure (or the rated operating point), sone rating if noise matters to your client, and Energy Star certification status. Plan checkers often cross-reference listed appliances against HVI-certified product ratings. If you're using a bath fan for whole-house duty, confirm the model is rated for continuous operation, not just intermittent use.

  4. Show the ductwork layout and exterior termination

    Draw the duct route from the fan to the exterior termination on the floor plan or mechanical sheet. Label duct diameter, material (smooth metal preferred over flex), and termination cap type. For exhaust fans, the termination must exit to the outside, not into the attic or a soffit. For supply systems, show where fresh air enters the building envelope and how it distributes to habitable rooms. Missing duct details are a frequent secondary correction on resubmittals.

  5. Give the ADU its own independent ventilation system

    If the project includes an ADU, give it a dedicated whole-house ventilation calculation, equipment spec, and duct layout entirely separate from the main dwelling. Even if the ADU shares a roof or wall with the main unit, it is treated as an independent dwelling for CMC purposes. A note saying the ADU is served by the main unit's HVAC will not satisfy 405.1 and will come back as a correction.

  6. Coordinate ventilation specs with the Title 24 CF1R report

    The whole-house ventilation system must be consistent with the Title 24 Part 6 energy compliance documentation. If your CF1R report specifies an HRV with certain airflows, the mechanical plan must match those numbers exactly. Mismatches between the CF1R and the mechanical plan draw a secondary correction even when 405.1 itself looks clean. Have the energy consultant and mechanical designer compare notes before the submittal goes in.

  7. Document timer schedule and duty cycle if using cyclic operation

    ASHRAE 62.2 allows cyclic operation as long as the average hourly airflow meets the CFM minimum. If you're using cyclic operation, show the cycle fraction on the plans: a 110 CFM fan at 50% duty cycle delivers 55 CFM average. Include the timer make/model, programmed schedule, and the average CFM result. No schedule and no math means no approval.

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Why you got flagged

Plan checkers flag CMC 405.1 when the mechanical plans don't show a whole-house ventilation calculation at all. A lot of contractors assume the bath fan covers it, but a single 50 CFM bath fan running on a timer isn't the same as a code-compliant whole-house system spec'd with CFM, make/model, and operating schedule. If the plans just say "bath fan per code" without a calculation, expect a correction.

The second common scenario is a CFM number that doesn't match the formula. The ASHRAE 62.2 calculation depends on both bedroom count and conditioned floor area. A 2,000 sq ft 3-bedroom home needs 90 CFM minimum: (3 + 1) × 7.5 = 30 CFM for occupants, plus 0.03 × 2,000 = 60 CFM for floor area. Submitting a 50 CFM spec for that home fails the math, and plan checkers will run the numbers before stamping anything.

A third trigger is ADU submittals where the ventilation schedule references the main house system. An ADU is a separate dwelling unit and needs its own independent whole-house ventilation. Tying it to the main unit's HVAC or writing "existing ventilation adequate" won't satisfy CMC 405.1 for the ADU. This catches a lot of garage-conversion projects where the mechanical drawings were copied from the main permit file.

Finally, some corrections cite 405.1 because the fan spec sheet is missing. Even when the CFM calculation is shown correctly, plan checkers want a listed appliance by make and model. "Broan 80 CFM bath fan" or an HRV cut sheet needs to appear in the mechanical schedule. A calculation without a matching product spec is still an incomplete submittal.

The official code

California Mechanical Code (CMC), Title 24 Part 4, current adopted edition — California Building Standards Commission

Read the source →

Common questions

Does a bathroom exhaust fan count as whole-house ventilation for CMC 405.1?
Yes, if it's sized correctly and spec'd to run continuously or on a code-compliant timer schedule. The bath fan must deliver at least the ASHRAE 62.2 CFM minimum for the entire dwelling, which is almost always more than the 50 CFM minimum the fan needs for the bathroom. A typical 3-bedroom 2,000 sq ft house needs at least 90 CFM. Most standard 50 CFM bath fans won't cover that. Spec a model rated for continuous operation and show the ASHRAE 62.2 calculation on the plans.
What CFM do I need for a 1,200 sq ft 2-bedroom ADU?
For a 1,200 sq ft 2-bedroom ADU: (2 + 1) x 7.5 = 22.5 CFM for occupants, plus 0.03 x 1,200 = 36 CFM for floor area, total 58.5 CFM minimum. Round up to the next available fan size, typically 60 or 70 CFM. Show the complete calculation on the mechanical plans, not just the rounded final number. Plan checkers want to see the math, not just the result.
Can I use an HRV or ERV to satisfy CMC 405.1?
Yes. A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) is one of the three compliant approaches under CMC 405.1, and often the preferred one in tight buildings or cold climates. The balanced ventilation it provides avoids the pressure imbalances that come with exhaust-only or supply-only systems. Make sure the HRV/ERV specs, duct layout, and airflow rates all appear on the mechanical plan and match what's in your Title 24 CF1R report.
Does the whole-house ventilation system need to be separate from the forced-air furnace?
Not necessarily. You can tie the fresh-air intake into the return-air plenum of a forced-air system as long as the air handler fan runs frequently enough to deliver the required average CFM. But the plans must show how the system delivers the ASHRAE 62.2 airflow minimum, including the fan runtime schedule. A note that says "fresh air damper on HVAC return" without an airflow calculation or timer spec won't pass plan check.
My ADU and the main house share a furnace. Does the ADU still need its own ventilation system?
Yes. Under CMC 405.1, an ADU is a separate dwelling unit and must have its own whole-house ventilation system independent of the main house. Sharing an HVAC system with the main dwelling doesn't satisfy the code. This is one of the most common corrections on garage-to-ADU conversions. Run a separate ASHRAE 62.2 calculation for the ADU, spec a separate fan or ERV, and show it on the plans.
What happens if my ventilation system runs on a timer instead of continuously?
ASHRAE 62.2 allows cyclic (intermittent) operation as long as the average airflow over the cycle meets the CFM minimum. The formula: fan rated CFM multiplied by duty cycle fraction equals average delivered CFM. Example: a 110 CFM fan running at 50% duty cycle delivers 55 CFM average. If the plan shows a timer, include the timer schedule, the duty cycle fraction, and the average CFM result. Plan checkers need to confirm the math works before they can approve.

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