Do I Need a Ventilation Fan if I Can Open My Windows? Title 24
Windows cannot be your ventilation. Section 150.0(o)1A says window operation is not a permissible method of providing the whole-dwelling airflow. Yet many projects need no whole-dwelling fan at all: additions of 1,000 square feet or less, junior ADUs, and alterations in homes never required to have one.
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Why the answer is no
The Energy Code adopts ASHRAE 62.2 with amendments, and the first amendment closes the window argument: window operation is not a permissible way to provide the airflow set by Section 150.0(o)1C. The compliance manual repeats it and adds that continuous operation of a central-fan-integrated ventilation system does not count either. The Energy Commission also trims the national standard, dropping Sections 4.1.1, 4.1.2, 4.1.4, 4.3, 4.6, 5, 6.1.1, 6.1.3 and Normative Appendix A, which shows the Commission chose which parts of the national standard apply.
The cases where no fan is required
- Small calculated need. If the required mechanical rate (Qfan) works out under 10 CFM, no fan is required. Start with Qtot = 0.03 times floor area plus 7.5 times the number of bedrooms plus one; a 1,200 square foot, two-bedroom home starts at 58.5 CFM before the infiltration credit, which was only 2 CFM in one of the manual's examples.
- An addition of 1,000 square feet or less. The airflow requirement does not apply (Section 150.2(a)1Ci). Above 1,000, the rate is based on the whole dwelling, existing floor area included.
- A junior ADU that is an addition. Also excused.
- A remodel in an older house. If an existing fan is altered or replaced and no earlier permit required whole-dwelling ventilation, the home need not add it (Section 150.2(b)1M).
A new dwelling unit built as an addition is the exception: it must have the system.
What still applies without a fan
Windows or no windows, each kitchen and bathroom needs local mechanical exhaust (Section 150.0(o)1G), and additions must meet it too. Replacing a bathroom fan means the new one meets the same rules. An entirely new ventilation system, meaning a new fan and a new duct system, must meet all of Section 150.0(o) (Section 150.2(b)1L).
Next steps
If your project does need a fan, the sizing page and the ventilation fan page give the formula, and bath fans as the system shows a common shortcut. Check 1,000 square foot additions, the ADU ventilation rule and our residential reports.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
No. Window operation is not a permitted method, so the calculation does not give credit for openable windows. The credit that exists is for the natural air leakage of the building.
Not by itself. Alterations only reach that rule when a fan is altered or replaced in a home that an earlier permit required to have one. Ask the building department about your permit history.
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