3.3 Whole-House Ventilation and Bathroom Exhaust
Key Takeaways
Table M1505.5 requires bathroom/toilet-room local exhaust capacity of 50 cfm intermittent or 20 cfm continuous.
Table M1505.5 requires kitchen local exhaust capacity of 100 cfm intermittent or 25 cfm continuous.
Whole-house ventilation is sized using the 2024 IRC M1505.4.3 method and its floor-area/bedroom inputs; intermittent operation uses the applicable runtime factor.
M1504.3 places ordinary exhaust openings at least 3 feet from property lines; exceptions affect separation from gravity openings and mechanical intakes.
Exhaust openings must generally be 10 feet from a mechanical air intake unless the exhaust is at least 3 feet above it or the listed combination-fitting exception applies.
Local exhaust and whole-house ventilation in the 2024 IRC
The 2024 IRC places mechanical ventilation in M1505. Local exhaust removes moisture and odors where they arise; a whole-house system supplies or exhausts outdoor air for the dwelling as a whole. M1505.4.1 permits supply fans, exhaust fans, or both, with associated ducts and controls. A local fan can also serve as part of a whole-house system when the design meets the whole-house provisions.
Local exhaust: Table M1505.5
| Space | Intermittent capacity | Continuous capacity |
|---|---|---|
| Bathroom or toilet room | 50 cfm | 20 cfm |
| Kitchen | 100 cfm | 25 cfm |
The listed bathroom/toilet-room fan airflow must meet the table at the static pressure specified by M1505.5; a free-air fan rating alone is not proof of installed performance. M1505.2 prohibits recirculating bathroom/toilet-room exhaust into a residence or another dwelling unit. Exhaust from kitchens, bathrooms, and toilet rooms must not discharge into attics, crawl spaces, or other interior areas. The listed ductless-range-hood exception is addressed in M1503.3.
Whole-house system: M1505.4
Inspect the chosen ventilation approach, outdoor-air pathway, airflow, fan listing, and controls together. The base whole-house rate is determined by Table M1505.4.3(1) or an applicable equation/exception in M1505.4.3, using floor area and bedrooms. If the system operates intermittently, M1505.4.3 requires qualifying controls and an increased operating airflow based on Table M1505.4.3(2). The permitted intermittent scheme operates for at least 25 percent of each four-hour segment; do not treat a timer as compliant without checking both runtime and delivered airflow. M1505.4.2 addresses system controls.
Exhaust-opening locations: M1504.3
Ordinary air exhaust openings must be at least 3 feet from property lines. They must be at least 3 feet from gravity air intakes and operable doors/windows, except where the exhaust is at least 1 foot above that opening. Mechanical air intakes require 10 feet of separation, except where the exhaust is at least 3 feet above the intake or an eligible factory-built intake/exhaust combination fitting is installed to the fan manufacturer's instructions. Check the actual termination geometry; the exceptions are not interchangeable. Exhaust systems discharge outdoors under M1501.1, not into an attic, soffit, ridge vent, or crawl space.
Which continuous local exhaust capacity does Table M1505.5 give for a bathroom or toilet room?
50 cfm
25 cfm
20 cfm
15 cfm
A whole-house system is designed to run intermittently. What must the inspector check besides the base continuous airflow?
That it uses a bathroom fan only
That controls, runtime, and increased operating airflow meet M1505.4.3
That each room has an operable window
That the supply duct is disconnected from the air handler
When may an ordinary exhaust opening be closer than 10 feet to a mechanical air intake under M1504.3?
Whenever it is 1 foot above the intake
When it is at least 3 feet above the intake, or an eligible factory-built combination fitting applies
Whenever the property line is 3 feet away
When the exhaust fan runs continuously
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