IRC 2024 Kitchen Exhaust Duct: Smooth Metal, Maximum Length, and Exterior Termination

Quick Answer

IRC 2024 Section M1503.1 requires kitchen exhaust ducts to be constructed of smooth, rigid metal — not flexible duct — with a minimum cross-section of 3.25 inches by 10 inches for rectangular duct or 6 inches in diameter for round duct. Every kitchen exhaust system must terminate to the exterior of the building through a grease-resistant cap. Recirculating hoods that filter and return air to the kitchen do not satisfy the exhaust requirement for new construction under current energy code standards in most jurisdictions.

A common field shortcut — using a short section of flexible aluminum duct between the hood and the rigid run — is a code violation regardless of the length of that flexible section. The code requires the entire duct system, from the hood collar to the exterior termination cap, to be smooth rigid metal. This rule applies even where the hood must be slightly repositioned for alignment, where a vibration-isolation connection might seem desirable, or where the cabinet framing makes a rigid direct connection difficult to achieve without modification.

What IRC 2024 Actually Requires

Section M1503.1 establishes that exhaust ducts for domestic kitchen cooking equipment must meet three core standards: material, minimum size, and termination location.

Material: Exhaust ducts shall be constructed of smooth interior surface metal. The code explicitly permits steel or copper. Flexible plastic or foil duct is prohibited for kitchen exhaust — both because flexible materials accumulate grease in the corrugations and because they present a higher fire risk. Smooth-wall metal allows grease condensate to drain back toward the hood rather than pooling in wrinkles.

Minimum duct size: Section M1503.4 ties minimum duct size to the airflow rating of the range hood. At a minimum, rectangular duct must be no smaller than 3.25 inches by 10 inches (32.5 square inches) and round duct must be no smaller than 6 inches in diameter. Higher-CFM hoods typically require larger duct diameter to maintain acceptable velocity and reduce noise.

Equivalent length limits: IRC 2024 Table M1503.4 sets maximum equivalent duct length based on hood CFM rating and duct diameter. As an example, a 6-inch round duct serving a 400-CFM hood is limited in total equivalent length. Each 90-degree elbow counts as several feet of equivalent length. Reducing elbows to 45-degree sweeps, minimizing turns, and routing ducts in a straight vertical path are the most effective strategies for staying within limits.

Exterior termination: All kitchen exhaust must terminate outside the building envelope. The code prohibits termination into an attic, crawl space, soffit, or any concealed interior space. The termination fitting must be a grease-resistant damper cap that opens under airflow and closes by gravity when the fan is off. This prevents pest entry and back-drafting.

Why This Rule Exists

Kitchen exhaust removes two hazardous byproducts of cooking: moisture and grease-laden air. Grease is combustible. If grease accumulates in a duct made of flexible corrugated material, or if the duct terminates into an attic, a grease fire in the duct can ignite surrounding framing with no warning. The smooth metal requirement ensures grease cannot accumulate in corrugations. The exterior termination requirement ensures that in the event of a duct fire, it stays isolated from building cavities.

Moisture is the second concern. Cooking generates significant humidity. Allowing that moisture to exhaust into an attic or soffit leads to mold, rot, and structural damage — often not visible for years. Exterior termination eliminates this risk entirely.

IRC 2021 required the same smooth metal duct and exterior termination, but IRC 2024 updated Table M1503.4 to include additional CFM thresholds that trigger makeup air, tightening the rules around high-capacity professional-grade ranges that are increasingly common in residential kitchens. If you are working from older reference materials, verify that the equivalent length table you are using reflects the 2024 edition.

What the Inspector Checks at Rough and Final

At rough-in inspection (before drywall), the inspector verifies that the duct path is all rigid metal with no flexible sections, that joints are not held with sheet-metal screws that penetrate into the duct interior (screws catch grease and debris), and that the duct is supported at appropriate intervals. Duct joints must be sealed with listed duct sealant or metal tape — not standard duct tape, which fails at elevated temperatures.

At final inspection, the inspector checks that the exterior cap is installed, opens freely, closes under gravity, has no screen on the termination (screens trap grease and are prohibited for kitchen exhaust caps), and that the cap is rated for grease-laden exhaust. The inspector may also verify makeup air provisions if the hood is rated above 400 CFM.

One detail that trips up many contractors at final inspection is the duct support interval. IRC M1503 and the equipment listing typically require rigid round duct to be supported at intervals not exceeding 6 feet, and rectangular duct at intervals not exceeding 4 feet. In open basement or crawlspace runs, inadequately supported duct can sag over time, creating low spots where grease condensate pools rather than draining toward the exterior. The inspector checks support intervals specifically because sagging duct is both a grease-fire risk and an equivalent-length compliance issue — a sagging duct run with a low point effectively adds friction loss to the system.

What Contractors Need to Know

Plan the duct route before ordering materials. The shortest, straightest path to the exterior is always correct. Where the hood must exhaust through an exterior wall, keep the duct horizontal with a very slight downward pitch toward the exterior so any condensate drains out rather than back into the fan. Where the duct must run vertically through cabinetry or a chase, plan for access to the grease damper at the base of the run for cleaning.

Never use sheet-metal screws that penetrate the duct interior — use pop rivets or drive screws only into the lap of the joint. Seal every joint with UL-listed foil tape or mastic. If the duct passes through a fire-rated assembly, the penetration must be protected with an approved fire-rated sleeve or wrap.

Duct diameter matters for noise. A 400-CFM hood on a 6-inch duct runs loud. Upsizing to 7 or 8 inches reduces velocity, dramatically lowering noise, and also gives you more equivalent-length margin under Table M1503.4.

What Homeowners Get Wrong

The most common homeowner mistake is purchasing a recirculating (ductless) range hood and assuming it satisfies code. Recirculating hoods filter grease through a mesh filter and return air to the kitchen — they remove some grease but no moisture, and they do not meet the exhaust requirement of M1503.1 for new construction.

The second mistake is allowing a contractor to use flexible aluminum duct “just for the last couple of feet” to connect the hood to the rigid run. Flexible duct anywhere in a kitchen exhaust system is a code violation and a grease-fire risk. The connection from the hood to the duct must be rigid metal as well.

A third error is failing to clean the grease filter and exterior cap. Even a properly installed system becomes a fire hazard if the grease cap is allowed to become clogged. Caps should be cleaned or replaced annually in active cooking kitchens.

State and Local Amendments

California’s Title 24 energy code imposes additional restrictions on kitchen exhaust, including mandatory makeup air systems for hoods exceeding 400 CFM and tighter limits on uncontrolled infiltration. Some jurisdictions have adopted amendments that prohibit recirculating hoods entirely, even in remodels. Other jurisdictions have adopted lower CFM thresholds at which makeup air is triggered. Always verify with the local Authority Having Jurisdiction (AHJ) before finalizing hood selection and duct design.

High-wind coastal jurisdictions sometimes require heavier-gauge duct and more robust exterior termination caps to resist wind-driven rain entry. Check local flood and wind zone amendments when selecting exterior cap hardware.

In jurisdictions that have adopted stricter energy codes, the makeup air requirement may activate at thresholds as low as 300 CFM. Some energy codes also require that makeup air be conditioned (heated or cooled) when the flow rate is high enough that unconditioned air would significantly affect indoor comfort or energy performance. These local variations can significantly impact project costs, so confirm the applicable energy code with your AHJ at the design stage.

When to Hire a Professional

If the duct must run more than 10 to 12 feet to reach the exterior, if the route requires more than two elbows, or if the hood is rated above 600 CFM, engaging a mechanical engineer or experienced HVAC contractor to design the duct system is worthwhile. High-capacity hoods also trigger makeup air requirements that involve additional ductwork, dampers, and possibly a dedicated makeup air unit — systems that require professional design and installation.

If the planned duct route passes through a fire-rated floor-ceiling assembly or an exterior wall with structural framing, a contractor experienced in fire-stopping and structural framing is necessary to ensure the penetration is made correctly without compromising either the fire rating or the wall structure.

Common Violations Found at Inspection

• Flexible foil or plastic duct used for any portion of the kitchen exhaust run
• Duct terminating into the attic, soffit, or crawl space instead of the exterior
• Exterior cap fitted with a screen that traps grease and creates a fire hazard
• Sheet-metal screws penetrating the duct interior at joints
• Duct joints sealed with standard cloth duct tape instead of listed foil tape or mastic
• Total equivalent duct length exceeding the Table M1503.4 limit for the installed hood CFM
• Duct diameter smaller than the 6-inch round (or 3.25x10-inch rectangular) minimum
• Recirculating hood installed in new construction without AHJ approval
• Makeup air system absent when hood CFM exceeds 400
• Exterior cap damper seized shut by grease accumulation at final inspection
• Duct run inadequately supported, creating a low-point sag where grease condensate pools
• Fire-stop protection missing where duct penetrates a fire-rated floor-ceiling or wall assembly

Inspectors occasionally flag a less obvious issue: a duct that meets the minimum diameter requirement but was upsized mid-run using an eccentric reducer, creating a velocity drop that allows grease to fall out of suspension and coat the duct walls at that transition point. Maintain consistent diameter throughout the run, or upsize the entire run from the hood collar to the termination cap rather than partially upsizing a section.