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Home / Code Guides / Range Hood Venting

Range Hood Venting & Makeup Air — Duct Rules and the 400 CFM Line

Range hood rules come down to three things: the air has to go outside, the duct has to be smooth metal, and if you install a powerful hood, you may have to bring replacement air back in. That last one — makeup air above 400 cfm — is the requirement that surprises the most homeowners shopping for pro-style hoods.

RequirementModel Code StandardSection
Discharge to outdoorsRange hood exhaust must discharge to the outdoors — never into an attic, crawl space, soffit, or between joistsIRC M1503.3, M1501.1
Duct materialHood ducts must be galvanized steel, stainless steel, or copper with a smooth interior surface, airtight joints, and a backdraft damper — flexible duct and screws protruding into the airstream that catch grease are outIRC M1503.2
Recirculating hoodsListed and labeled ductless (recirculating) range hoods are permitted where installed per the manufacturer’s instructions and where the kitchen has other ventilation — they’re the exception to the outdoor-discharge ruleIRC M1503.3 Exc.
Makeup air thresholdA hood system capable of exhausting more than 400 cfm requires makeup air at approximately the exhaust rate — under the 2021 IRC this applies where atmospherically vented gas/liquid/solid-fuel appliances are inside the home’s air barrierIRC M1503.6
Makeup air dampersMakeup air openings need a gravity or electrically operated damper that opens automatically when the exhaust runs — the electric damper is typically interlocked with the hood switchIRC M1503.6.2
Termination locationThe exhaust termination must be at least 3 feet from property lines and from operable openings into the building, with a corrosion-resistant cap and damperIRC M1504.3

Worth knowing

  • The makeup air rule exists to prevent backdrafting: a 900 cfm hood in a tight house can pull combustion gases down the water heater flue. The 2021 IRC narrowed the requirement to homes that actually have atmospherically vented appliances — all-electric and direct-vent homes are off the hook, but check local amendments.
  • A hood “capable of” 600 cfm triggers makeup air even if you plan to run it on low — inspectors go by the rating, not your habits.
  • Where kitchen local exhaust is required for whole-house ventilation compliance, the minimum is 100 cfm intermittent or 25 cfm continuous (IRC M1505.4.4).
  • Over-the-range microwaves follow their listing: most can be vented out or recirculated, but the duct rules above apply the moment you duct one outside.
Sections cite the 2021 International Residential Code and 2023 National Electrical Code as model standards. This is BuildSpec’s own summary — not ICC or NFPA text; read the authoritative language at codes.iccsafe.org. Your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.
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