Whole-House Ventilation Requirements
A home needs mechanical whole-house ventilation, sized using ASHRAE 62.2 as California adopts it — not the standard as published. California substitutes its own equation, sets a default envelope leakage of 2 ACH50, and excludes nine parts of the standard outright.
Why a house needs mechanical ventilation at all
A modern California home is built tight, and tight is the point — an envelope that does not leak is an envelope that does not waste conditioned air. But a house that cannot leak also cannot passively exchange air, so the fresh air has to be supplied deliberately.
That is the trade the code makes explicit: tighten the envelope, then ventilate on purpose at a known rate rather than accidentally at an unknown one.
ASHRAE 62.2, as California adopts it
The total required ventilation rate is calculated per ASHRAE 62.2 section 4.1.1, using California's own Equation 150.0-B. This is the detail that catches practitioners who know the standard: California does not simply adopt 62.2, it adopts it with substitutions.
Nine parts of the standard are expressly not required for compliance here — sections 4.1.1, 4.1.2, 4.1.4, 4.3, 4.6, 5, 6.1.1 and 6.1.3, plus Normative Appendix A. Working from a clean copy of 62.2 will therefore produce requirements California does not impose, and miss the equation it does.
The 2 ACH50 default
Q50 is calculated from the conditioned volume of the dwelling unit, using a default dwelling unit envelope leakage of 2 air changes per hour at 50 pascals.
Two things follow. The calculation is volume-driven, so ceiling height matters and not just floor area — a house with tall rooms needs more ventilation than its plan area suggests. And because 2 ACH50 is a default, it is an assumption standing in for a measurement, which is what makes it usable at design stage before the house exists to be tested.
Where it interacts with everything else
Ventilation is not an isolated requirement. It moves air the space conditioning system then has to condition, so it sits inside the load calculation. Where a ventilation fan draws through ductwork, duct sealing requirements apply to it. And ventilation was one of the areas the 2025 code strengthened across building types, so requirements carried forward from an earlier cycle should be rechecked.
What to settle in design
Conditioned volume rather than floor area, the ventilation strategy — exhaust, supply or balanced — and where the fan and any ductwork run. A ventilation system added after the mechanical design is settled tends to end up in whatever space is left, which is rarely where it works best or quietest.