Kitchen ventilation code requirements come primarily from Chapter 15 of the International Residential Code (IRC), which most states adopt as their baseline and local jurisdictions amend. The core rules: a range hood must move at least 100 CFM intermittently or 25 CFM continuously, ductwork must be metal and terminate outdoors, hoods over 400 CFM in a home with a non-direct-vented combustion appliance need an automatic makeup air system, and clearances above the cooktop follow the hood manufacturer’s listing. Local amendments can tighten any of these, so verify with your building department before you buy equipment.
Minimum Exhaust Airflow
The IRC sets two airflow floors. An intermittent system (a range hood or downdraft you switch on while cooking) has to move at least 100 CFM. A continuously operating system has to sustain at least 25 CFM.1UpCodes. Chapter 15 Exhaust Systems: GSA Residential Code 2024 These are minimums, not design targets. A 100 CFM fan covers a standard electric cooktop in a modestly sized kitchen; it will not keep up with a high-output gas range.
Inspectors confirm airflow capacity by reading the equipment’s rated CFM off the label or specification sheet, usually at rough-in or final inspection. If the rated capacity falls below the minimum for the type of operation, the system fails. Confirm the rating before the ductwork gets enclosed in finished walls.
Sizing a Hood for a Gas Range
The 100 CFM floor is a code minimum, not a sizing rule. Gas ranges with high-BTU burners need substantially more airflow than that. The widely used sizing calculation divides the range’s total BTU output by 100, so a six-burner professional-style range rated at 90,000 BTU calls for a 900 CFM hood. For an all-gas range, add the BTU ratings of every surface burner, griddle, and grill to the highest-rated oven burner. If the range has two oven burners but only one can fire at a time, use only the higher number. Dual-fuel ranges use a similar formula, converting the electric oven’s watts to BTU at roughly 3.5 BTU per watt.2Trade-Wind. Determining CFM Requirements for your Range
A modest oversize is better than hitting the number exactly, because duct length and elbows reduce effective airflow at the hood opening. Undersizing the hood against the range’s heat output is the most common mistake in kitchen ventilation projects.
Duct Material, Length, and Termination
Kitchen exhaust ducts have to be galvanized steel, stainless steel, or copper. Plastic and flexible aluminum are not allowed. The one narrow exception: a downdraft system can use schedule 40 PVC when the duct runs under a slab on grade, the trench is backfilled with sand or gravel, and the PVC extends no more than one inch above the floor indoors or above grade outdoors.3UpCodes. Section M1503 Domestic Cooking Exhaust Equipment Everywhere else, metal is the only compliant option.
The reason is grease and heat. Flexible ducting traps grease in its ridges and fails at temperatures metal shrugs off. Joints have to be sealed airtight and the interior surface has to be smooth to limit grease accumulation.4UpCodes. Exhaust Discharge The flex duct sold at hardware stores for dryer vents looks like it should work and is the most common code violation inspectors see on DIY kitchen projects.
Allowable duct length depends on fan CFM and duct diameter, laid out in IRC Table M1504.2. There’s no single maximum. Higher CFM paired with a larger diameter gets a longer run. Each 90-degree elbow costs 15 feet from your allowable length, so two elbows deduct 30 feet from whatever the table permits.5UpCodes. Section M1504 Exhaust Ducts and Exhaust Openings If your run exceeds the table, you can still pass by following the manufacturer’s published design criteria or by having installed airflow verified with a flow hood or measuring device.
The duct has to terminate outside the building. Discharging exhaust into an attic, crawl space, or soffit violates code. The termination needs a backdraft damper, and the duct must be independent of any other exhaust system.1UpCodes. Chapter 15 Exhaust Systems: GSA Residential Code 2024 Position the exterior outlet away from windows, doors, and fresh-air intakes so exhaust doesn’t get pulled back inside. Where the duct passes through an unconditioned attic, insulate it to prevent condensation; some jurisdictions set a minimum of R-4 depending on climate zone.
Makeup Air Over 400 CFM
Any exhaust hood capable of moving more than 400 CFM, installed in a home that also contains a gas, propane, oil, or solid-fuel appliance that isn’t direct-vented or mechanically drafted, requires a dedicated makeup air system. The makeup air supply has to be roughly equal to the exhaust rate.3UpCodes. Section M1503 Domestic Cooking Exhaust Equipment
The rule exists because a powerful hood pulls the entire house into negative pressure if nothing replaces the exhausted air. That pressure drop can reverse the natural draft in a gas water heater or furnace flue, pulling combustion gases, including carbon monoxide, into the living space instead of up the chimney.6National Library of Medicine. Causes and Consequences of Backdrafting of Vented Gas Appliances
The makeup air damper has to be automated. It must open when the exhaust fan activates and close when the fan shuts off, with no manual step. Dampers must be either gravity-operated or electrically controlled to synchronize with the exhaust system.7Pennsylvania Housing Research Center. Residential Makeup Air Systems and Requirements Budget for makeup air from the start if you’re planning a high-output hood. Retrofitting one later usually means new exterior wall penetrations, more ductwork, and sometimes a tempering unit so cold outdoor air doesn’t blast into the kitchen in winter.
Clearances Above the Cooktop
Vertical clearance between the cooktop and the bottom of the hood follows the hood manufacturer’s listing and local code. The IRC requires at least 30 inches between the cooking surface and any unprotected combustible surface above it, like a wood cabinet.8UpCodes. Hood Size and Location A metal hood isn’t combustible, so the hood itself can usually sit closer, but the manufacturer sets the specific minimum. Most listings call for 24 to 30 inches depending on model and fuel type, with gas typically requiring the greater distance.
Over-the-range microwave-hood combinations follow a different rule. They typically mount with the top at 66 inches from the floor, putting the bottom roughly 13 inches above the cooktop, and they must not be installed above any gas range or cooktop with a combined burner rating over 60,000 BTU.9GE Appliances. General List of Minimum Air Clearance Requirements A professional-style range at 90,000 BTU can’t use one. You need a dedicated hood.
Combustible cabinets flanking or above the hood also have to respect clearance rules. These can sometimes be reduced if a non-combustible heat shield is installed between the hood and the cabinet, with the exact reduction set by the hood’s listing. Where the manufacturer’s spec and local code differ, the stricter requirement wins.
Ductless and Natural Ventilation
The IRC permits ductless (recirculating) range hoods, but only when the hood is listed and labeled for ductless installation, installed per the manufacturer’s instructions, and the kitchen has mechanical or natural ventilation provided by other means.10UpCodes. Chapter 15 Exhaust Systems: 2021 IRC A ductless hood doesn’t eliminate the ventilation requirement; it just moves it elsewhere, to an operable window or a whole-house system. Ductless hoods remove some grease and odor through charcoal or mesh filters but don’t remove heat, moisture, or combustion byproducts, which is why a ducted hood is meaningfully safer for gas cooking. Some local jurisdictions don’t accept ductless hoods at all.
Natural ventilation through operable windows and doors can also satisfy general ventilation requirements if the total openable area is at least 4 percent of the floor area being ventilated.11UpCodes. Natural Ventilation This is a room-level air exchange standard, not a kitchen-specific capture rule, and a window across the room doesn’t do what a hood directly above the burners does. Some jurisdictions override the natural ventilation allowance for kitchens entirely, particularly in new construction under energy codes. Confirm with the building department before planning around it.
Permits and Inspection
Installing or replacing a ducted range hood generally requires a mechanical permit because the work involves an exterior wall penetration, duct connections, and sometimes electrical. Permit fees vary widely by jurisdiction, commonly from $50 to several hundred dollars. The permit triggers at least one inspection, sometimes two: a rough-in before walls close up, and a final once the system runs.
Skipping the permit compounds the risk. Unpermitted work surfaces during a home sale when the buyer’s inspector flags it, forcing retroactive permitting and sometimes rebuilding. Insurance carriers can deny fire or water damage claims connected to unpermitted work, and some insurers will cancel or refuse to renew a policy if they discover unpermitted modifications during a claims investigation.
At inspection, expect the inspector to verify that the fan’s rated CFM matches the range below, that the duct is the right material and terminates outdoors with a backdraft damper, that clearances to combustible surfaces are met, and for systems over 400 CFM, that the makeup air damper activates automatically with the fan. Have the manufacturer’s installation manual and the equipment’s specification sheet on-site.