Requirements & code

Commercial Kitchen Ventilation: Hoods, CFM & Code

Compiled by Resultex Media Group·Updated August 2026·How we research & cite

Reviewed against primary sources — the International Mechanical Code (IMC §507) and NFPA 96, manufacturer-listed hood data, and your local mechanical code. This is educational information, not professional engineering, legal, or code advice; verify with a licensed mechanical engineer or contractor and your local authority having jurisdiction (AHJ).

Commercial kitchen ventilation basics: Type I vs Type II hoods, capture velocity, exhaust CFM sizing, and make-up air balance under IMC 507/508 and NFPA 96.

Educational, not legal advice. Codes vary by jurisdiction — always confirm with your local health department and building authority (AHJ).

FoodServiceNerd EditorialResearched from the FDA Food Code, manufacturer specs & industry sourcesUpdated Aug 2026

Sources & standards

The technical guidance on this page reflects these published codes and standards, which your local authority (AHJ) adopts, amends, and enforces. It is educational, not engineering or legal advice — confirm the current requirements for your jurisdiction.

International Mechanical Code (IMC) — commercial kitchen exhaust & make-up air
NFPA 96 — Ventilation Control & Fire Protection of Commercial Cooking Operations — Type I hoods, grease ducts, duct velocity, clearances, cleaning frequency
UL 300 — Fire Testing of Fire Extinguishing Systems — wet-chemical fire suppression for cooking equipment

Commercial kitchen ventilation removes grease, smoke, heat, and moisture from cooking. Type I hoods handle grease-laden vapor and require duct fire protection; Type II handle heat and steam only. The IMC and NFPA 96 govern hood type, exhaust CFM, clearances, and the make-up air that balances the exhaust.

What kitchen ventilation has to accomplish

Commercial cooking throws off a mix of grease-laden vapor, smoke, heat, moisture, and odors. A kitchen ventilation system captures those effluents at the source, carries them safely out of the building, and replaces the removed air. Done right, it keeps the kitchen safe and workable and keeps grease out of the ductwork; done poorly, it lets grease coat the duct, pushes smoke into the dining room, and overheats the line. Two code families govern the work: the International Mechanical Code (IMC), chiefly Sections 507 and 508, and NFPA 96, which covers fire protection of the exhaust system.

Type I vs Type II hoods

The first design decision is hood type, and it depends entirely on whether the appliance produces grease.

	Type I hood	Type II hood
Handles	Grease-laden vapor and smoke	Heat, steam, moisture, odors (no grease)
Typical over	Fryers, ranges, griddles, charbroilers, woks	Dishwashers, steamers, ovens, kettles
Needs grease filters	Yes (baffle filters)	No
Needs fire suppression and grease duct	Yes	No

A Type I hood is the classic grease hood: it uses baffle filters to knock grease out of the airstream, discharges through a dedicated grease duct, and is protected by a fire-suppression system. A Type II hood is a lighter-duty condensate or heat hood used only where there is no grease, such as over a dishwasher or a steamer. Putting a Type II hood over a fryer is a serious code violation. Whenever grease is present, you need a Type I system and everything that comes with it.

Capture velocity and CFM sizing

A hood works by pulling the rising column of cooking effluent up into the canopy before it escapes into the room. Two related ideas drive the sizing:

Capture and containment: the hood must be large enough and pull hard enough to capture the plume and hold it under the canopy. Designers reference the airflow needed to keep effluent from rolling out of the hood; this is often discussed as a capture velocity at the hood face.
Exhaust CFM: the actual volume of air the fan removes, in cubic feet per minute. CFM is driven by hood type, hood length and style (wall-mounted, island, backshelf), how far the hood overhangs the appliances, and the duty of the appliances underneath, since a charbroiler produces far more effluent than a warming oven.

There is no single universal CFM number for a kitchen; it is calculated for the specific hood and appliance lineup, and the IMC and ASHRAE 154 provide the sizing methods. As a rule, heavier-duty cooking and longer hoods demand higher exhaust CFM, and undersizing the exhaust is the most common reason a hood fails to capture smoke.

Balancing exhaust with make-up air

Exhaust is only half of ventilation. Every cubic foot removed has to be replaced with make-up air, and the IMC (Section 508) requires supplied air to be approximately equal to the exhaust. Kitchens are typically designed with make-up air around 80 to 90 percent of exhaust CFM so the space stays slightly negative relative to the dining room, which keeps odors and heat moving toward the hood. If make-up air is inadequate, the exhaust fan cannot move its rated volume, hood capture collapses, doors bind, and combustion appliances can backdraft. In cold climates the make-up air is tempered so raw winter air does not pour onto the line.

Ductwork, clearances, and fire protection

Type I exhaust must run in a dedicated grease duct built and installed to NFPA 96. Because that duct can carry burning grease, NFPA 96 sets minimum clearances to combustibles for hoods, grease-removal devices, fans, and ducts where no rated enclosure is used:

Adjacent material	Minimum clearance
Combustible construction	18 inches
Limited-combustible material	3 inches
Noncombustible material	0 inches

These clearances can be reduced only with a listed grease-duct enclosure or approved reduction method evaluated for that purpose. The duct must slope to drain, provide access panels for cleaning, and generally run continuously to the exhaust fan and roof. The hood over grease appliances also carries a UL 300 fire-suppression system, and the whole grease-bearing system must be cleaned on the NFPA 96 schedule. Ventilation, suppression, and cleaning are three parts of one safety system, and a change to any appliance can force a re-evaluation of all of them.

Make up air requirements

Kitchen exhaust hood cleaning

Ansul fire suppression systems

Hood filters

Permits & licensing

Sources

ICC — International Mechanical Code, Section 507 Commercial Kitchen Hoods and Section 508 Makeup Air. Governs hood type, exhaust systems, and make-up air for commercial kitchens.
NFPA — NFPA 96, Standard for Ventilation Control and Fire Protection of Commercial Cooking Operations. Sets grease-duct construction, clearances to combustibles (18 in./3 in./0 in.), access, and fire protection.
ICC (UpCodes) — Grease duct clearances. Consolidated code text on required grease-duct clearances and allowed reductions.

Frequently asked

What is the difference between a Type I and Type II hood?

A Type I hood handles grease-laden vapor and smoke from appliances like fryers, ranges, and charbroilers. It uses baffle filters, a dedicated grease duct, and fire suppression. A Type II hood handles only heat, steam, and moisture, such as over a dishwasher or steamer, and has no grease filters or suppression. If grease is present, you must use a Type I system.

How is exhaust CFM for a kitchen hood determined?

Exhaust CFM is calculated for the specific hood and the appliances under it, not from a single fixed number. It depends on hood type, length and style, how far the hood overhangs the appliances, and appliance duty, since a charbroiler produces far more effluent than a warming oven. The IMC and ASHRAE Standard 154 provide the sizing methods designers use.

What clearance does a grease duct need from combustibles?

Where no rated enclosure is used, NFPA 96 requires Type I hoods, grease-removal devices, fans, and ducts to keep at least 18 inches from combustible construction, 3 inches from limited-combustible material, and 0 inches from noncombustible material. These clearances may be reduced only with a listed grease-duct enclosure or approved method evaluated for that purpose.

Which codes govern commercial kitchen ventilation?

The International Mechanical Code, primarily Sections 507 (commercial kitchen hoods) and 508 (make-up air), sets hood, exhaust, and replacement-air requirements. NFPA 96 governs fire protection of the exhaust system, including grease ducts, clearances, cleaning, and suppression. ASHRAE Standard 154 is the engineering reference for sizing. Local amendments apply, so confirm details with your authority having jurisdiction.

Why does my hood need make-up air to work?

A hood can only exhaust as much air as the room can replace. Without adequate make-up air, the kitchen goes strongly negative, the fan cannot reach its rated CFM, and the hood stops capturing smoke and grease. The IMC requires make-up air roughly equal to exhaust; kitchens are usually designed around 80 to 90 percent to stay slightly negative for good capture.