What is ASHRAE 62.2 and how does it apply to ADU ventilation?

ASHRAE 62.2 is the national standard for residential ventilation and indoor air quality, published by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. California has incorporated it into the Title 24 energy code, which makes a continuously operating, whole-building mechanical ventilation system mandatory in every new ADU — regardless of size.

What ASHRAE 62.2 actually requires

The standard recognizes that modern, tightly built homes no longer "breathe" through gaps and leaks the way old construction did. Without a deliberate ventilation system, indoor pollutants — moisture, CO2, VOCs from finishes and furnishings, cooking byproducts — accumulate. ASHRAE 62.2 addresses this with two layers:

• Whole-building (dilution) ventilation: a continuous, mechanically driven supply of outdoor air sized to the dwelling's floor area and bedroom count. This is the headline requirement and the one most often missed on ADU plans.
• Local (spot) exhaust: exhaust fans that remove moisture and pollutants at the source — kitchen range hoods and bathroom fans. These are in addition to whole-building ventilation, not a substitute for it.

Typical ADU ventilation rates

The required continuous rate rises with floor area and bedroom count. As rough benchmarks for common ADUs:

The engineer calculates the exact target for your unit; these figures illustrate the order of magnitude.

How ADUs meet the requirement

Several approaches satisfy whole-building ventilation, and the mechanical plan must specify which one is used:

• Continuous exhaust: a quiet, energy-efficient bath fan rated for continuous duty, wired to run continuously (often with a timer or controls). Simple and economical.
• Balanced ventilation with an ERV or HRV: an energy- or heat-recovery ventilator exchanges stale indoor air for fresh outdoor air while recovering most of the heat (and, for ERVs, moisture). The most comfortable and efficient option, increasingly common in higher-performance ADUs.
• Supply ventilation: ducting a measured amount of outdoor air to the HVAC air handler so incoming air is filtered and tempered.

Verification and documentation

ASHRAE 62.2 compliance is not just a paper requirement — a HERS rater field-measures the ventilation airflow to confirm the installed system delivers the required CFM. To pass plan check and inspection cleanly, the mechanical plan should show the ventilation strategy, the equipment make/model/CFM, the controls, and the calculated rate confirming compliance.

ASHRAE 62.2 checklist for ADU plans

1. Calculate the required whole-building rate from floor area and bedroom count.
2. Select a strategy (continuous exhaust, ERV/HRV, or supply) and specify equipment and CFM.
3. Confirm the system is wired for continuous operation with appropriate controls.
4. Add local exhaust: a ducted-to-exterior kitchen hood and bathroom fan(s) — separate from whole-building ventilation.
5. Note HERS verification of ventilation airflow on the plans.

Ventilation rates and verification requirements vary by code cycle and jurisdiction — confirm current details. Our mechanical plans include the ASHRAE 62.2 design, calculated rate, and equipment schedule, and pair cleanly with a bundled Title 24 report. Get started when ready.