ADU Whole-House Ventilation Requirements
A new detached ADU needs mechanical whole-dwelling ventilation at any size, and so does an attached ADU that is a new dwelling unit. A JADU addition is excused. The rate is 0.03 cfm per square foot plus 7.5 cfm times bedrooms plus one, and an ECC-Rater measures it on site.
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Which ADUs need a ventilation system
- New detached ADU: always. The Energy Commission ADU FAQ says no size exception exists.
- Addition that is a new dwelling unit: yes, sized on the ADU's own floor area under Section 150.2(a)1C.
- JADU addition: excused from the airflow, though the Mechanical Code may still ask for ventilation. The Commission points those questions to HCD.
- Conversion of conditioned space: only when a whole-dwelling fan is replaced (a new system meets Section 150.0(o) in full). If an earlier permit required ventilation, the replacement meets the airflow and is tested; if not, nothing is required.
Run the numbers for a small unit
The total rate is 0.03 times floor area plus 7.5 times (bedrooms plus 1), counting at least one bedroom. A 400 square foot studio comes to 27 cfm, a 600 square foot one-bedroom to 33 cfm and a 900 square foot two-bedroom to 49.5 cfm. The fan supplies that total less an infiltration credit, based on a default of 2 ACH50 unless a blower door test proves less. The manual adds that if the result is under 10 cfm no fan is required.
What the ECC-Rater tests
The rater measures delivered airflow at the fan with a flow hood or flow grid. An exhaust or supply system passes at or above the required rate. A balanced system passes when supply and exhaust are within 20 percent of each other and average at least the requirement, so readings of 80 and 90 cfm against 86 required fail at 85. Every operating mode is tested. The installer completes CF2R-MCH-27-H and the rater CF3R-MCH-27-H.
Details that sink a pass
The fan must be rated at 1.0 sone or less, and its on-off switch needs the Section 150.0(o)1J label. A bathroom fan can serve as the whole-dwelling fan if it meets those ratings and the measured airflow. Leave a margin, because a label rating is not what a ducted fan delivers.
See whole-house ventilation, the IAQ fan page, addition versus new construction, bathroom fans and range hoods. Order from the ADU page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
A new detached ADU does not. The ADU FAQ says all newly constructed dwelling units must meet Section 150.0(o) however small, and a studio still counts as one bedroom.
No. Section 150.0(o)1A says window operation is not a permissible way to provide the dwelling unit ventilation airflow.
Explore ADU and JADU
- ADU Window SHGC Requirements
- ADU Window U-Factor Requirements
- Attached ADU Title 24 Checklist
- California ADU Rules and Title 24
- Can a New ADU Use a Gas Furnace?
View all ADU and JADU guides → · Permits and Compliance · Title 24 guide