IRC 2024 Range Hood Venting: Duct Material, Size, and Exterior Termination

Quick Answer

Under IRC 2024 Section M1503, range hoods must vent to the exterior using smooth metal duct only — flexible duct and plastic duct are prohibited. The minimum duct size is 3¼ inches by 10 inches rectangular or 6 inches round. The duct must terminate with a backdraft damper to a location outside the building, not into an attic, crawl space, or wall cavity.

Under IRC 2024, for high-output hoods exceeding 400 CFM, makeup air must be provided to compensate for the air being exhausted. Recirculating (ductless) hoods that filter and return air to the kitchen are not acceptable for new construction under energy codes in most jurisdictions because they do not remove moisture — exterior venting is required.

What IRC 2024 Actually Requires

Section M1503.1 requires that range hoods, over-counter microwaves with exhaust fans, and other cooking appliance exhaust equipment discharge to the outdoors. The duct must be of smooth rigid or smooth flexible metallic construction — ribbed flexible metal duct and non-metallic duct are both prohibited. The smooth interior surface requirement exists because grease accumulates on duct walls, and ribbed or rough surfaces trap significantly more grease, increasing fire risk.

Minimum duct dimensions are specified in Section M1503.4 and the accompanying tables. A 3¼ inch by 10 inch rectangular duct is the minimum, which has a cross-sectional area of 32.5 square inches. The equivalent round duct is 6 inches in diameter (28.3 square inches), which is slightly smaller but is accepted as a standard minimum because round duct has lower resistance to airflow. Most range hood manufacturers specify a minimum 6-inch or 7-inch round duct for their units; always follow the manufacturer specification if it is more stringent than the code minimum.

Maximum duct length is determined by the CFM rating of the hood and the number of elbows. Section M1503 and its referenced tables specify equivalent length limits based on the hood’s airflow capacity. A standard 400 CFM hood with a 6-inch duct has a maximum equivalent duct length of approximately 40 feet, with each 90-degree elbow reducing the allowable straight-run length. High-CFM hoods (600 CFM and above) require larger duct diameters and have different length limits. Exceeding the maximum equivalent duct length starves the hood of airflow, causes the motor to work harder, and results in grease accumulation in the duct because the velocity drops below what is needed to carry grease particles to the exterior.

The exterior termination must discharge through a wall cap or roof cap with a backdraft damper that opens under airflow and closes when the fan is off. Screen covers over the termination are prohibited by Section M1503.1 because grease accumulates on screens, progressively blocking airflow and creating a fire hazard. The termination must discharge to the outdoors at least 3 feet from any opening into the building (window, door, or another vent).

Section M1503.4 requires makeup air when the exhaust hood exceeds 400 CFM. Makeup air is replacement air supplied to the kitchen to compensate for the volume of air being exhausted. Without makeup air, a high-CFM hood creates negative pressure in the house that can backdraft combustion appliances (furnaces, water heaters, fireplaces), pull contaminated air from attached garages, and make doors difficult to open. Makeup air systems range from simple passive transfer grilles from adjacent conditioned spaces to motorized supply air systems interlocked with the range hood.

Why This Rule Exists

Cooking produces grease-laden air, moisture, combustion byproducts (from gas ranges), carbon monoxide, and particulate matter. Venting this air to the exterior removes these contaminants from the living space. Moisture from cooking is a leading contributor to indoor humidity problems, mold growth on interior surfaces, and window condensation. Grease deposited in ductwork is a significant fire hazard — a grease fire that starts at the cooking surface can extend into the ductwork and propagate through the building structure if duct construction and routing are not compliant.

The smooth metal duct requirement directly addresses fire safety. Grease-laden air deposits grease on duct walls as it cools. On a smooth surface, grease can be wiped away during periodic cleaning. In ribbed flex duct, grease accumulates in the valleys of the ribbing and cannot be effectively cleaned, building up over time into a fuel load that can sustain a serious duct fire. Metal duct also resists ignition and limits fire spread in a way that flexible plastic duct cannot.

The prohibition on terminating into attics, crawl spaces, or wall cavities addresses both moisture and fire. Venting grease-laden, moisture-saturated cooking air into a building cavity deposits grease on structural members and insulation while simultaneously introducing the moisture that promotes rot and mold. A grease fire in a duct that terminates inside a building cavity can ignite framing, insulation, and other combustibles with no barrier to fire spread.

What the Inspector Checks at Rough and Final

At rough-in, the inspector verifies that the duct route is fully planned and that penetrations through fire-rated assemblies include approved fire-rated dampers where required. They check the duct material — no flex duct, no plastic duct — and verify that the duct diameter meets the minimum requirement. They confirm that the duct will terminate to the exterior, not into a building cavity. Where the duct passes through unconditioned spaces such as attics, they verify that insulation wrapping will be applied to prevent condensation on cold duct surfaces.

At final inspection, the inspector checks the exterior termination cap for proper backdraft damper function and confirms no screen is installed over the termination. They verify the total equivalent duct length does not exceed the limit for the hood’s CFM rating. They check that grease duct clearances to combustibles are maintained — generally 3 inches to combustible materials along the duct length except at the hood itself. For hoods over 400 CFM, they verify that a makeup air system is installed and functional.

What Contractors Need to Know

Plan the duct route before framing is complete whenever possible. Running a smooth metal duct through a finished kitchen is significantly more difficult than roughing in the duct path during framing. The most efficient route minimizes elbows — each 90-degree elbow is equivalent to approximately 10 feet of straight duct run. A route with two 90-degree elbows and 20 feet of straight run has a total equivalent length of 40 feet, which reaches the limit for a standard 400 CFM, 6-inch duct.

When running duct through the exterior wall or roof, use a dedicated range hood termination cap, not a standard dryer vent cap. Range hood caps are designed with larger damper openings to handle the higher airflow and with grease-collection features on some models. Ensure the cap is installed with a proper flashing if it penetrates the roof. Exterior wall terminations should have the duct slope slightly downward toward the exterior to allow any condensation to drain out rather than back into the kitchen.

For high-CFM professional-style hoods (600 CFM and above), the duct diameter must increase. A 600 CFM hood typically requires a minimum 7-inch round duct; many require 8-inch or 10-inch round. Using an undersized duct on a high-output hood overloads the blower motor, creates excessive noise, and may cause the automatic grease fire suppression systems in commercial-style hoods to be ineffective. Always match the duct diameter to the hood manufacturer’s specification, not just the code minimum.

What Homeowners Get Wrong

The most pervasive homeowner misconception is that a recirculating (ductless) range hood is equivalent to a ducted range hood. Recirculating hoods filter grease through a grease filter and pass the air through an activated carbon filter before returning it to the kitchen. They remove some grease particles but do nothing for moisture and provide little benefit for combustion byproducts from gas ranges. For new construction, recirculating hoods are not acceptable under the energy code’s indoor air quality requirements in most jurisdictions. For existing kitchens where ducting is genuinely not possible, a recirculating hood is better than nothing, but it is not a code-compliant substitute for exterior venting in new construction.

Homeowners who replace a range hood themselves sometimes use the flexible aluminum foil duct that comes in the box with the hood. This is prohibited under IRC 2024 M1503. The ribbed foil duct traps grease, is difficult to clean, and is a fire hazard. Smooth rigid metal duct or smooth flexible metallic duct (not ribbed) is required. Smooth flexible metallic duct is only appropriate for the final short connection between a rigid duct run and the hood itself, not for long duct runs.

Many homeowners do not know that the exterior vent cap should never have a screen. An inspector who finds a screened range hood termination will require it to be replaced. If you have an existing screened termination, replace the cap with a damper-only model — it is an inexpensive fix that also improves your hood’s airflow performance.

State and Local Amendments

California’s Title 24 Part 6 (Energy Code) explicitly requires mechanical ventilation in kitchens that vents to the exterior. Recirculating range hoods do not satisfy California’s kitchen ventilation requirement in new construction, and hoods must meet specific minimum airflow performance ratings. California also regulates maximum sound levels for range hood fans in certain residential energy compliance pathways.

Many jurisdictions have adopted local fire code amendments that impose stricter grease duct clearance requirements in residential kitchens, particularly for high-output commercial-style ranges. If you are installing a high-BTU professional range in a residential kitchen in a jurisdiction that has adopted the International Fire Code alongside the IRC, verify that the duct system meets both residential and fire code requirements. Some jurisdictions treat professional-style ranges in residential kitchens as commercial cooking equipment for duct purposes, requiring listed Type I grease duct assemblies.

When to Hire a Professional

Range hood duct installation that involves penetrating the roof, running duct through finished walls, or routing duct through fire-rated assemblies requires a permit and, depending on jurisdiction, a licensed mechanical or HVAC contractor. Makeup air systems for hoods exceeding 400 CFM must be designed by a mechanical engineer or licensed HVAC contractor in many jurisdictions because they must balance the supply and exhaust airflows and integrate with the existing HVAC system to maintain combustion safety. Simple duct replacement on an existing hood with an accessible existing duct route may be within homeowner scope, but always verify with your local building department before starting work.

Common Violations Found at Inspection

• Flexible ribbed aluminum foil duct used for range hood — only smooth metal duct is permitted, not ribbed flex duct.
• Range hood terminating into attic space instead of directly to the exterior through a proper termination cap.
• Screen installed over the exterior termination cap, which accumulates grease and is explicitly prohibited by M1503.1.
• Duct diameter smaller than the 6-inch round or 3¼ by 10 inch rectangular minimum, or smaller than what the manufacturer requires for the hood’s CFM rating.
• Total equivalent duct length (including elbows) exceeding the maximum for the installed hood’s CFM rating.
• No makeup air provided for a hood rated over 400 CFM, creating depressurization and backdrafting risk.
• Recirculating (ductless) hood installed in new construction where exterior venting is required.
• Duct joints not secured and sealed — grease-laden air leaking into wall cavities or attic at joints.