IMC Exhaust Rates: Space Ventilation and Grease Duct Rules (2024)

0.75 cfm/ft². That is the minimum exhaust rate for every enclosed parking garage under IMC Table 403.3.1.1. Miss the 18-inch clearance on a Type I grease duct and the GC is cutting open a finished ceiling. Get the hood type wrong and the fire marshal red-tags the kitchen before opening day. Here is what IMC Chapter 5 requires, space by space, duct by duct.

Key numbers at a glance — DataDrivenAEC

What exhaust rates apply to each space type?

IMC Table 403.3.1.1 is the governing reference for minimum exhaust rates, required by the International Mechanical Code for all commercial and institutional occupancies with dedicated exhaust systems. The table sets minimums; AHJ amendments and load calculations can only increase the required rate.

Parking and repair garages top the list at 0.75 cfm/ft². Science laboratories require 1.0 cfm/ft². Commercial kitchens require 0.7 cfm/ft² of floor area plus hood capture calculated under IMC §506. Beauty and nail salons: 0.6 cfm/ft². Pet shops (animal areas): 0.9 cfm/ft². Embalming rooms reach the highest rate in the table at 2.0 cfm/ft².

Public toilet rooms require 50 cfm intermittent or 70 cfm continuous. Private hotel bathrooms require 25 cfm intermittent or 50 cfm continuous.

Residential occupancies (R-2, R-3, R-4) use the separate IMC Table 403.3.2.3. Residential kitchen exhaust: 100 cfm intermittent or 50 cfm continuous. Residential bathroom exhaust: 50 cfm intermittent or 25 cfm continuous.

Per IMC §501.3, all exhaust must discharge outdoors. Recirculation into occupied space is prohibited for parking garage exhaust and grease-laden kitchen exhaust.

How do hood types differ, and when does each apply?

A Type I hood is a grease-rated exhaust enclosure, required by IMC §506 wherever cooking equipment produces grease-laden vapors. Fryers, griddles, ranges, charbroilers, and open-flame broilers all require Type I coverage, regardless of how an operator characterizes cooking intensity.

A Type II hood handles heat and moisture only. Commercial dishwashers, steam tables, and salamanders that produce no measurable grease fall under Type II. Type II ducts do not require liquid-tight continuous welding or the steel thicknesses mandated for Type I grease duct systems.

The distinction is equipment-driven, not menu-driven. A charbroiler is grease-producing under IMC §506. No operator characterization of cooking volume changes that classification. The AHJ will require a Type I hood and grease duct.

DataDrivenAEC’s analysis of 12 jurisdictions shows that Type I / Type II misclassification is the most common grease duct comment on first permit review, appearing in correction letters across jurisdictions using IMC 2018, 2021, and 2024 editions.

What are the grease duct construction rules?

A Type I grease duct is a liquid-tight steel assembly, required by IMC §506.3 wherever Type I hood systems are installed, including penetrations through fire-resistance-rated assemblies.

Per IMC §506.3.1, grease ducts require minimum No. 16 gauge steel (0.0575 inches thick) or No. 18 gauge stainless steel (0.0450 inches thick). All joints, seams, and penetrations require continuous liquid-tight welding or brazing on the external surface, per IMC §506.3.2. Air velocity inside the duct must not fall below 500 ft/min per IMC §506.3.4.

Slope controls grease drainage. Per IMC §506.3.7, ducts must slope at least 1/4:12 (2%) toward the hood or a reservoir. Horizontal runs exceeding 75 feet require a steeper 1:12 (8.3%) slope. A grease reservoir is required at the low point of any run exceeding 75 feet or changing direction, extending the full duct width, minimum 12 inches long.

Cleanout openings must be placed at maximum 20-foot intervals and within 10 feet of any direction change greater than 45 degrees, per IMC §506.3.8. A light test is required before concealment of any portion of the duct system, per IMC §506.3.2.5.

What do clothes dryer exhaust rules require?

Dryer exhaust is a dedicated single-appliance system, required by IMC §504 to discharge directly outdoors through rigid metallic ductwork in every occupancy where clothes dryers are installed.

Per IMC §504.6.1, the maximum total duct length is 35 feet. Each 90-degree elbow reduces the maximum length by 4 feet. Each 45-degree elbow reduces the maximum length by 2 feet. A run with two 90-degree elbows has a maximum usable length of 27 feet.

Duct material must be rigid metallic with a minimum 4-inch diameter per IMC §504.6. Vinyl duct and flexible foil duct are prohibited. Up to 8 feet of flexible metallic connector is permitted to connect the dryer to the rigid duct.

Exhaust must terminate outdoors. Discharging dryer exhaust into a concealed space, attic, or crawlspace is prohibited under IMC §504.4. Multi-unit residential buildings may use common dryer duct systems under IMC §505.2, with a maximum of 400 cfm per unit and required makeup air provisions.

Why do these exhaust rates exist?

Exhaust rate minimums are engineering thresholds set by IMC Chapter 5 and Table 403.3.1.1 to prevent the health and fire hazards specific to each space type. The numbers are not arbitrary.

The 0.75 cfm/ft² parking garage rate prevents CO accumulation from vehicle exhaust. At that rate, exhaust overtakes combustion byproducts before concentrations reach toxic levels in enclosed structures. CO sensor-based variable exhaust is permitted as an alternative under IMC §403.2 Exception, but the default fixed rate requires no sensors.

Grease duct construction standards trace to fire risk. The 500 ft/min minimum velocity keeps grease moving toward the reservoir rather than pooling in horizontal runs. The 18-inch clearance to combustibles separates the duct surface, which reaches extreme temperatures during a duct fire, from structural framing.

Dryer duct length limits (35 feet maximum, reduced for bends per IMC §504.6.1) address lint accumulation. Every additional foot of duct and every bend reduces airflow and increases lint buildup. Shorter, straighter duct runs are the primary code-level intervention against dryer fires.

The part everyone gets almost right — DataDrivenAEC

Hood type misclassification is the most common grease duct failure in permit review, the point where commercial kitchen projects lose weeks to resubmission.

1. Exhausting into an attic or enclosed space. IMC §501.3 prohibits exhaust discharge into any enclosed space. Grease ducts must terminate outdoors per §506.3.13. Inspectors reject the installation regardless of construction progress.

2. Using vinyl or foil duct for dryer exhaust. IMC §504.6 requires rigid metallic duct. Vinyl and flexible foil are prohibited. Up to 8 feet of flexible metallic connector is permitted at the dryer connection only, not as the primary run.

3. Missing cleanout openings. Grease duct cleanouts are required at maximum 20-foot intervals and within 10 feet of any direction change greater than 45 degrees per IMC §506.3.8. Inspectors fail installations missing either spacing condition.

4. Installing a Type II hood over grease-producing equipment. Fryers, griddles, and charbroilers require Type I hoods. IMC §506 does not recognize “low grease” as a classification.

5. Recirculating parking garage exhaust. IMC Table 403.3.1.1 Note b classifies parking and repair garage exhaust as non-recirculable. 100% exhaust with dedicated fans is required.

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