IRC 2024 Range Hood CFM: Minimum Airflow and Makeup Air Threshold

What IRC 2024 § M1503.3 requires

IRC 2024 Section M1503.3 does not mandate a specific minimum CFM for residential range hoods, but ASHRAE 62.2 (referenced by the IRC energy provisions) and many state and local codes specify a minimum of 100 CFM for kitchen exhaust fans over ranges. More critically, IRC 2024 Section M1503.4 requires makeup air when the exhaust hood is rated above 400 CFM. The hood must be HVI-certified, and island hoods generally require 20 to 30 percent higher CFM than equivalent wall-mounted hoods due to cross-drafts.

Under IRC 2024, sound rating (sones) matters for livability but is not a code requirement in most jurisdictions.

Section M1503 governs domestic kitchen exhaust equipment. The CFM requirements interact with both the IRC mechanical provisions and the energy code provisions referenced therein.

Minimum CFM: IRC 2024 does not contain a table specifying minimum CFM for residential range hoods in the base code. However, ASHRAE Standard 62.2, which is referenced by the IRC’s mechanical ventilation provisions and adopted by many jurisdictions through their energy codes, establishes 100 CFM as the minimum intermittent kitchen exhaust rate. Local amendments in many states (California, Washington, and others) have codified this 100 CFM minimum directly. Confirm with the local AHJ whether a numeric minimum applies in your jurisdiction.

HVI certification: The hood must be HVI (Home Ventilating Institute) certified, meaning the CFM and sone ratings on the label reflect actual tested performance, not a nominal manufacturer claim. HVI certification ensures that the 400 CFM labeled on the hood actually delivers 400 CFM at the duct static pressure conditions specified in HVI’s test standard.

Makeup air threshold: Section M1503.4 is unambiguous: when the exhaust hood is rated above 400 CFM, makeup air must be provided. Makeup air replaces the air exhausted by the hood, preventing the building from depressurizing. Without makeup air, a 600-CFM hood creates significant negative pressure that pulls combustion gases from furnaces and water heaters back into the living space (backdrafting), slams doors, and makes the hood itself operate less effectively as it fights to pull air against the pressure differential.

Types of makeup air: Makeup air may be passive (an automatically operated damper that opens when the hood runs, allowing exterior air to enter through a duct) or active (a powered supply fan interlocked with the exhaust hood). Passive systems are less expensive but introduce unconditioned outdoor air directly into or near the kitchen. Active systems can temper the makeup air (heat or cool it) before introducing it to the space, but require more equipment and coordination with the HVAC system. A third approach is “transfer air” from the rest of the house, but this only works when the home has sufficient interior air volume and the HVAC system can compensate for the additional pressure imbalance.

Island hoods vs. wall-mounted hoods: Wall-mounted range hoods capture cooking effluent in a relatively contained zone between the hood and the wall. Island hoods hang over a range or cooktop in the center of the kitchen with no walls to contain cross-drafts. Cross-drafts from HVAC registers, open windows, and foot traffic carry cooking effluent away from the island hood before it can be captured. For this reason, hood manufacturers and building science practitioners consistently recommend 20 to 30 percent higher CFM for island hoods than for equivalent wall-mounted installations.

Why This Rule Exists

Kitchen exhaust removes two primary pollutants: airborne grease and combustion byproducts. High-BTU ranges (gas ranges with 15,000 to 25,000 BTU burners) produce significantly more combustion byproducts than lower-output ranges. A 400 CFM hood that adequately captures effluent from a standard residential range may be badly undersized for a 48-inch commercial-style range with 20,000 BTU burners. The makeup air requirement at 400 CFM exists because above this threshold, the hood creates enough building depressurization to trigger backdrafting of atmospherically vented appliances. Below 400 CFM, the depressurization in typical residential construction is manageable without dedicated makeup air.

What the Inspector Checks at Rough and Final

At rough-in, the inspector confirms that the duct is properly sized for the hood’s rated CFM (a 600-CFM hood on a 4-inch duct is undersized), that the duct is smooth rigid metal, and that if makeup air is required, the makeup air duct and damper are roughed in. The makeup air system is often overlooked during rough-in — installing makeup air after drywall is much more difficult and expensive than planning it from the start.

At final inspection, the inspector verifies that the HVI-certified hood is installed, that the makeup air damper (if required) is motorized and interlocked with the exhaust hood, and that the exterior termination cap has no screen. The inspector may also ask for the hood’s HVI certification documentation if the label is not clearly visible.

What Contractors Need to Know

Hood CFM selection should follow a loading calculation, not just a rule of thumb. The general building science recommendation is 1 CFM per 100 BTU of total range output. A range with four 15,000-BTU burners and a 25,000-BTU oven produces 85,000 BTU total — which would suggest an 850-CFM hood. At 850 CFM, a robust makeup air system is not optional — it is essential for both code compliance and safe operation of other gas appliances in the home.

Variable-speed hoods offer a practical solution: a 600-CFM hood operated at low speed for everyday cooking produces far less depressurization than a 600-CFM hood at full speed. Some energy codes allow variable-speed hoods to bypass certain makeup air requirements when the minimum speed setting is below 400 CFM, but confirm this with the AHJ.

For island hood installations, confirm with the hood manufacturer what CFM rating they recommend for the specific cooking surface and island configuration before specifying the duct size. Under-sizing an island hood is a common source of homeowner complaints after installation.

What Homeowners Get Wrong

The most common error is purchasing a range hood primarily based on aesthetics or price without verifying the HVI-certified CFM rating. Many hoods carry large CFM numbers on their packaging that reflect motor capacity, not actual duct performance. The HVI-tested CFM (at a specific static pressure) is the only number that matters for code compliance and real-world performance.

A second mistake is selecting a powerful hood (above 400 CFM) without understanding the makeup air obligation. Many homeowners and even some contractors install a 600-CFM hood without providing makeup air, then discover that the kitchen door slams every time the hood is turned on, that the gas furnace draft hood spills combustion gas, and that the hood itself sounds labored. The fix after installation is expensive — it requires cutting a new penetration for a makeup air duct and installing an interlocked damper.

A third error is ignoring sone ratings. A powerful hood running at 8 sones is as loud as a dishwasher and uncomfortable to work near. Higher-quality hoods with EC motors can move 400 CFM at 2 to 3 sones — a dramatically more pleasant kitchen environment. While sone ratings are not typically a code requirement, they significantly affect the likelihood that the hood will actually be used.

State and Local Amendments

California’s Title 24 requires makeup air for hoods exceeding 400 CFM and specifies that makeup air must be tempered (heated or cooled to within a specified temperature range of indoor air) when the outdoor temperature is below 50°F or above 100°F. Washington State requires all kitchen exhaust hoods in new construction to be HVI-certified and establishes additional limits on maximum sone levels for continuously operating fans. Several jurisdictions in the Northeast have adopted ASHRAE 62.2-2022 provisions that require kitchen exhaust as part of the whole-building ventilation strategy, with specific requirements for intermittent vs. continuous operation.

When to Hire a Professional

Any installation where the hood exceeds 400 CFM requires professional coordination between the exhaust system and the makeup air system. The makeup air system must be engineered to provide approximately the same volume of air that the hood exhausts, properly balanced so it does not create its own pressure problems. This involves duct sizing, damper selection, motor interlocking with the exhaust hood, and — in cold climates — a tempering system to prevent cold drafts from the makeup air inlet.

For island installations, a professional hood designer or kitchen ventilation specialist can help select the right CFM, position the island hood at the optimal height above the cooktop (typically 28 to 36 inches depending on hood design), and specify the duct configuration that minimizes noise and maximizes capture efficiency.

Common Violations Found at Inspection

• Hood above 400 CFM installed without makeup air provisions
• Makeup air damper not interlocked with the exhaust hood (operates independently)
• Non-HVI-certified hood where certification is required by local code
• Duct undersized for the hood’s rated CFM (e.g., 4-inch duct on a 600-CFM hood)
• Island hood installed at the same CFM as an equivalent wall-mounted hood without accounting for cross-drafts
• Exterior cap fitted with a screen (prohibited for kitchen exhaust)
• Passive makeup air inlet located so that it introduces cold outdoor air directly onto food preparation surfaces
• Variable-speed hood wired so the speed control bypasses the motorized damper interlock
• Hood recirculating air (no exterior duct) in new construction without AHJ approval
• Makeup air system sized for only partial replacement of exhaust volume, leaving residual depressurization