Range Hoods and Kitchen Ventilation: Sizing, Ducting and Makeup Air

A range hood has one job: catch the heat, grease, moisture and combustion gases that rise off the cooktop and send them outside. Washington writes more of that job into its residential code than most states do. The rules below are from the 2021 Washington State Residential Code, in force since 15 March 2024 and current as of October 2026.

What the Washington code requires

Kitchens must have a local exhaust system (IRC M1505.4.4.1 as amended by WAC 51-51-1505). A new or replaced hood has to meet the minimums in Table M1505.4.4.3:

Capture efficiency is measured under ASTM E3087. Few hoods publish that figure yet, so in practice most are selected on the cfm number. The code also requires the hood to be HVI or AHAM rated so the airflow and sound values can be confirmed (M1505.4.4.3.1).

Ducted or recirculating

Plan on a ducted hood. Washington's amendment to M1503.3 says domestic cooking exhaust equipment "shall discharge to the outdoors through a duct." A recirculating (ductless) hood is allowed only where it is listed and labeled, installed to the manufacturer's instructions, and the kitchen is an enclosed kitchen that also has continuous local exhaust at the rate in the table.

That exception has a practical consequence. Continuous exhaust is not permitted as the method for an open kitchen, so a ductless hood is not a compliant choice in an open-plan kitchen. If you are opening a wall to join the kitchen to a living area, the hood needs a route outside.

Duct rules that shape the layout

The duct is usually what decides where the range can go. Under M1503.3, M1503.4 and M1504.3 it must:

• Have a smooth interior surface and be airtight, with a backdraft damper.
• Be independent of all other exhaust systems. It cannot share a duct with a bath fan or dryer.
• Be made of galvanized steel, stainless steel or copper in a single-family home. Flexible foil and plastic duct are not on the list.
• End outdoors, never in an attic, a crawl space or inside the building.
• Terminate at least 3 feet from property lines, at least 3 feet from operable windows, doors and gravity air intakes, and at least 10 feet from a mechanical air intake unless the exhaust is at least 3 feet above it.

The code's prescriptive duct sizing table covers small fans only, so the duct diameter for a range hood comes from the manufacturer's instructions. Follow them exactly. A duct that is too small, too long or has too many elbows will make a strong hood perform like a weak one, and louder.

The shortest route wins. A range on an exterior wall can vent straight out the back. A range on an interior wall needs a path up through the ceiling or along a joist bay, and in a two-story house the room above decides whether that path exists. Settle this while you are still choosing between kitchen layouts.

Sizing beyond the minimum

The code minimum is a floor. Three things matter more than chasing a bigger fan:

• Coverage. A hood at least as wide as the cooking surface, and deep enough to reach over the front burners, captures far more than a narrow one with a larger motor.
• Mounting height. Follow the range and hood manufacturers' instructions. Mounted too high, a hood lets the plume spread past its edges.
• How you cook. Frequent searing, frying or wok cooking justifies more capacity. Simmering and baking do not.

Clearance above the range is also a code matter. The residential code requires at least 30 inches from the cooking top to unprotected combustible material, with reduced clearances only as the listing of the hood allows (IRC M1901.1), and gas ranges have their own version of the rule in G2447.5. The guide to gas, electric and induction ranges covers how fuel choice changes the hood you need.

Makeup air: the 400 cfm threshold

A hood that pulls a lot of air out of a house has to get that air from somewhere. In a tight house it can pull it backward down the flue of a gas water heater or furnace. The code addresses this in IRC M1503.6, which Washington adopts without amendment:

• It applies where the home has a gas, liquid or solid fuel-burning appliance that is neither direct-vent nor mechanically drafted. A natural-draft gas water heater or a wood stove are typical examples.
• In that home, each exhaust system capable of more than 400 cfm must be provided with makeup air at a rate approximately equal to the exhaust rate.
• The makeup air must enter the same room as the hood or a room connected to it by permanent openings, through at least one damper that opens automatically when the hood runs (M1503.6.1, M1503.6.2).

Read the condition carefully. An all-electric home, or one whose only fuel-burning appliances are direct-vent or power-vented, is outside the trigger as written. If you are in the first group, the simplest answer is often to choose a hood rated at 400 cfm or less, which still clears the 250 cfm minimum for gas by a wide margin. A makeup air system adds a duct, a motorized damper and controls, and in winter it brings in cold air that has to go somewhere comfortable.

Noise

Washington limits intermittent kitchen exhaust to a maximum of 3 sones, measured at one or more airflow settings of at least 100 cfm (M1505.4.4.2). Continuous kitchen exhaust is limited to 1 sone. A hood you will not turn on because it is too loud is not ventilating anything, so compare sone ratings at the working speed, not only at the lowest one. A remote or inline blower moves the motor out of the kitchen and is worth asking about for larger hoods.

Island hoods and downdraft

A hood over an island cooktop has no wall or cabinets to contain the plume, so cross-drafts carry more of it away. Expect to specify a wider, deeper hood than you would on a wall, and expect the duct to run up through the ceiling and across to an exterior wall or up through the roof.

Downdraft systems pull air down and sideways, against the direction hot air wants to travel. The code treats them as "other intermittent kitchen exhaust" with a 300 cfm minimum. They still have to duct outdoors in metal. The one exception in the duct material rule allows schedule 40 PVC for a downdraft duct run under a concrete slab on grade, with conditions.

Power, permits and inspection

A cord-and-plug connected hood needs an individual branch circuit, and a hardwired hood may not be fed from the countertop small-appliance circuits (2023 NEC, sections 422.16 and 210.52). See the kitchen electrical code page for how that fits with the rest of the kitchen.

One scheduled change to watch: the State Building Code Council plans final adoption of the 2024 residential and mechanical codes on 16 October 2026, with an effective date of 3 May 2027. A remodel permitted before that date is reviewed under the 2021 rules on this page. Confirm the ventilation numbers again if your permit will be issued later.

Custom hoods

A plaster, wood or metal hood surround is a shell around a rated blower insert. The insert is what has to meet the airflow, sound and listing rules, and the shell has to respect the clearances in the insert's instructions. In JDI Construction's published planning ranges, a custom plaster or metal hood adds $3,000-$9,000 before sales tax. That is a planning range, not a quote. Our team sorts out the duct route and any makeup air question during design, before cabinets are ordered. Learn more about kitchen remodeling in Vancouver, or continue to the overview of Washington building codes for kitchens.