IRC 2024 Kitchen Range Hood Exhaust: Minimum CFM, Duct Size, and Grease Filter Rules

Quick Answer

Under IRC 2024 Section M1503, a kitchen range hood must exhaust at least 100 CFM, use rigid smooth-wall metal duct (minimum 3.25 inches by 10 inches rectangular or 6 inches round), be equipped with a listed and removable grease filter, and serve only the range hood — no other appliances share the duct. The hood must be installed at least 24 inches above the cooking surface, with 30 inches the more common installed height. Length limits in IRC Table M1503.3 cap the duct run to prevent velocity loss and grease accumulation.

Under IRC 2024, flexible duct and plastic duct are prohibited. The termination must be to the exterior through a listed cap with a backdraft damper.

What IRC 2024 Actually Requires

Section M1503.1 defines the scope: every dwelling unit with a cooking appliance shall have a mechanical exhaust system, a listed factory-built kitchen hood, a downdraft exhaust system, or a listed ductless range hood with recirculating filtration. For ducted systems, Section M1503.2 sets the minimum flow rate at 100 CFM when the hood is operated intermittently. Continuously operating hoods must provide a minimum of 25 CFM. These are delivered-air requirements measured at the grille, not bench-rated values.

Section M1503.3 governs the duct itself. Material must be smooth-interior steel, stainless steel, or aluminum — no flexible duct, no corrugated duct, and no plastic. The minimum dimensions are 3.25 inches by 10 inches for rectangular cross-sections or 6 inches in diameter for round duct. The duct must be exclusive to the range hood: no other exhaust appliances or systems may be connected to the same duct. Joints must be made mechanically, running in the direction of airflow (downstream pipe overlaps upstream pipe), sealed with listed mastic or foil tape.

IRC Table M1503.3 limits total equivalent duct length based on duct size and elbow count. For a 6-inch round duct, the maximum equivalent length is typically 140 feet for a 100 CFM hood and shorter for higher CFM ratings. Every 90-degree elbow reduces the allowable straight run by approximately 10 feet, and every 45-degree elbow by 5 feet. Hoods rated above approximately 400 CFM trigger the makeup air requirement in Section M1503.4, discussed separately.

Section M1503.5 requires a listed grease filter that is accessible for removal and cleaning. The filter must be positioned no closer than 18 inches above an open-flame cooking surface and must be listed for grease-filter service. The filter captures airborne grease before it enters the duct, reducing grease accumulation inside the duct that would otherwise be a fire hazard. Filters that are not listed, not accessible, or not removed regularly for cleaning are among the most common range hood violations.

Why This Rule Exists

Kitchen fires are the leading cause of residential fires in the United States. Grease vapors produced by cooking are combustible and deposit on every surface they contact, including the interior of the exhaust duct. Without a listed grease filter, the duct accumulates a continuous grease coating that becomes ignition fuel. Without adequate CFM, the hood draws only a fraction of the cooking plume, and most of the grease migrates to wall surfaces, cabinet interiors, and duct exteriors. The combination of inadequate airflow and no filter creates conditions that allow a grease fire to travel from the cooktop surface into the duct and potentially into the building cavity.

The exclusive-duct requirement prevents cross-contamination between the range hood and other mechanical systems. Connecting a dryer duct or bath fan to the range hood duct is a code violation regardless of how it might seem to simplify installation, because each exhaust system has different temperature, grease, and moisture requirements that are incompatible in a shared duct.

The smooth-interior requirement exists because corrugated metal and flex duct create ridges where grease accumulates. A grease-filled flex duct between the hood and the wall is a fire hazard that fire investigators encounter regularly. Smooth rigid metal can be cleaned; corrugated flex cannot practically be degreased.

What the Inspector Checks at Rough and Final

At rough-in, the inspector traces the range hood duct from the cabinet rough opening to the exterior termination. They verify the duct material is smooth-wall rigid metal, confirm minimum dimensions meet the 3.25-by-10 or 6-inch round requirement, count and verify elbow equivalent lengths against Table M1503.3, and confirm the duct is exclusive to the range hood with no tees or splices to other systems. They check that the duct penetrates the exterior wall or roof and terminates outside the building envelope.

At final, the inspector looks for the listed termination cap with an operable backdraft damper, the listed grease filter in its housing, and proper clearance between the hood canopy bottom and the cooktop surface. They will typically run the hood to confirm the damper opens fully and that airflow is perceptible at the termination. They verify the grease filter is accessible without tools. If the hood requires makeup air per Section M1503.4, the makeup air system is also inspected at final.

What Contractors Need to Know

Size the duct for the hood’s actual rated CFM, not just the code minimum. An island hood rated at 600 CFM cannot be served by 6-inch round duct, which is sized for 100 to 200 CFM. Underduct a high-CFM hood and the static pressure buildup will cause the hood to deliver far less than its rating, generate excessive noise, and stress the motor. Use the hood manufacturer’s duct sizing chart alongside Table M1503.3.

Plan the routing before framing. A range hood on an exterior wall is simple — a short horizontal run through the wall with a cap. A range on a kitchen island requires a duct rising through the ceiling, traveling horizontally across the attic or through an interior soffit, and eventually reaching an exterior wall or the roof. Island installations drive up equivalent length quickly and often require 7-inch or 8-inch round duct to stay within velocity limits.

Install the termination cap before closing in the wall or roof to ensure proper flashing. A termination cap installed after siding is up often produces a sealant-only waterproof detail that fails within 5 years. Use listed exterior hoods with integral flashing flanges designed for the wall material (stucco, fiber cement, wood siding, etc.).

Keep the hood at 24 to 30 inches above the cooking surface. Below 24 inches is a code violation because the hood becomes a fire ignition hazard if it is too close to a flame. Above 30 inches, capture efficiency drops and the installed CFM at the cooking surface falls below the 100 CFM minimum for many hood designs. Most manufacturers specify 30 inches as the optimal installed height for performance and clearance.

What Homeowners Get Wrong

The most common homeowner mistake is choosing a decorative hood for appearance rather than for performance. A tall chimney-style hood that looks impressive may have a blower rated at 400 CFM but deliver 150 CFM at the installed conditions because the duct is undersized, too long, or includes too many elbows. Checking the delivered CFM at the actual installation conditions, not just the rated bench value, is the only accurate way to know whether the hood meets the 100 CFM minimum.

Many homeowners also do not realize that a ductless recirculating hood is a legal option under IRC 2024 but provides dramatically different performance than a ducted hood. A recirculating hood filters grease and odors through activated carbon and grease filters and returns air to the kitchen. It never achieves the moisture removal of a true exhaust system and must have listed filters replaced on schedule. In humid climates or for high-BTU cooktops, a recirculating hood is rarely adequate in practice even when it is code-compliant.

Grease filter neglect is universal. The listed grease filter in a range hood should be cleaned every one to three months depending on cooking frequency. Homeowners who deep-fry regularly can block a grease filter in a few weeks. A blocked filter chokes airflow to a fraction of the hood’s rated CFM and defeats the purpose of the system.

State and Local Amendments

California Title 24 Part 6 applies energy efficiency requirements to range hoods that exceed certain CFM thresholds, mandating makeup air with tempering for hoods above 400 CFM in some climate zones. Some California jurisdictions also require ENERGY STAR certified range hoods in new residential construction.

Washington state energy code requires that range hood ducts penetrating the envelope be dampered and sealed to limit infiltration when the hood is not operating. Several jurisdictions in the Pacific Northwest require makeup air for hoods over 300 CFM due to tight-construction standards that make depressurization a combustion-safety concern. Always check the local mechanical code for CFM thresholds that trigger makeup air requirements in your jurisdiction.

When to Hire a Professional

Installing or relocating a range hood duct that penetrates an exterior wall or roof, passes through an attic or soffit, or requires makeup air is a project where professional installation protects the structure and ensures compliance. A licensed HVAC contractor or kitchen exhaust specialist has the tools to properly flash a roof penetration, select appropriate duct sizing for the actual installed conditions, and commission the hood at its rated CFM. If the hood is gas-adjacent or the kitchen includes a gas range, a licensed plumber or HVAC contractor should verify that the exhaust system does not create depressurization conditions that could backdraft the gas appliance. For hoods requiring makeup air above 400 CFM, a mechanical engineer or experienced HVAC designer should size the makeup air system.

Common Violations Found at Inspection

• Flexible duct or corrugated metal used for the range hood duct instead of smooth-wall rigid metal.
• Duct smaller than 3.25 by 10 inches rectangular or 6 inches round for the required CFM.
• Range hood duct connected to the same duct as a bath fan, dryer, or other exhaust appliance.
• Grease filter missing, not listed, or permanently installed without a mechanism for removal and cleaning.
• Hood installed less than 24 inches above the cooking surface.
• Equivalent duct length exceeds Table M1503.3 limits, causing delivered CFM to fall below the 100 CFM minimum.
• Termination cap without a backdraft damper or with a damper that is stuck closed.
• Duct joints assembled with the upstream pipe overlapping the downstream pipe, creating an interior lip that collects grease.