IMC 505 covers domestic cooking exhaust: hoods, downdrafts, and over-the-range microwaves in residential occupancies, plus the rules that carry over when domestic-style equipment is used in nonresidential or institutional settings.
Plain English: This section is shorter than 506 and 507, but it has more traps per paragraph than most engineers expect. Equipment must be listed. Makeup air kicks in at 400 cfm. Multistory shafts follow strict rules. When Group I-1 or I-2 is involved, additional requirements apply whether or not the equipment looks residential.
**505.1-505.2 Equipment and Listings**
Overhead hoods and non-integral downdraft fans: listed to UL 507.
Hoods and downdrafts with integral fans: also UL 507.
Ranges and cooktops with integral downdraft: ANSI Z21.1 or UL 858.
Over-the-range microwaves with integral exhaust: UL 923.
No unlisted field-built exhaust assemblies. If it is not on a label, it is not compliant.
**505.3 Duct Requirements**
Smooth rigid metal duct only. No flexible duct for the exhaust run.
No screen at the termination. Grease buildup will block the discharge.
Discharge must go outdoors. Recirculating hoods are allowed only where 505.2 expressly permits them (listed with charcoal filter).
**505.4 Makeup Air: The 400 cfm Threshold**
If a domestic kitchen hood system can exhaust more than 400 cfm, makeup air at approximately the same rate is required.
The MUA path must have a closure device (damper) and must be automatically interlocked with the hood.
On drawings: show the MUA path, damper, cfm, and interlock note. Missing this on a system over 400 cfm is a plan review redline.
**505.5 Common Multistory Domestic Kitchen Exhaust**
Multiple dwelling kitchen hoods connecting to a shared vertical riser must be in an IBC-rated shaft.
Dampers are prohibited in the exhaust duct (except as allowed by 505.3).
Use rigid steel duct inside the shaft: minimum 26-gauge, built per SMACNA.
Backdraft dampers are required at each individual unit connection.
**505.6-505.8 Non-Residential and Institutional Occupancies**
Other than Group R: If a nonresidential occupancy uses domestic-style cooking equipment that qualifies as residential-type, IMC 505 still applies. If the cooking load requires Type I or II classification, switch to IMC 506 and 507.
Group I-1 (assisted living, group homes): domestic hood installations per IBC 420.9 require a minimum 500 cfm hood, room ventilation per IMC 403.3.1, and outdoor discharge. Ductless hood allowed only if listed with charcoal filter.
Group I-2 (hospitals, nursing facilities): same requirements as I-1.
Code Path: IMC 505.1 (equipment listing) -> 505.2 (hood and downdraft requirements) -> 505.3 (duct requirements) -> 505.4 (makeup air threshold) -> 505.5 (multistory shaft) -> 505.6/505.7/505.8 (occupancy-specific).
Check: Before You Submit
Confirm every exhaust appliance has a listing standard on the equipment schedule (UL 507, UL 923, ANSI Z21.1, or UL 858 as applicable).
Flag systems over 400 cfm. Verify MUA path, damper, and interlock note are on the drawings.
If the project has a multistory residential shaft: confirm it is rated, has backdraft dampers at each unit, and uses rigid steel duct throughout.
Confirm ductless hoods are used only in permitted conditions and are listed with charcoal filters.
Review Risk:
Over-the-range microwaves with integral exhaust are missed on schedules more often than any other domestic exhaust device.
Multistory exhaust shafts with flexible duct at the unit connection fail inspection. Every unit connection must be rigid metal.
Makeup air above 400 cfm shown only as an arrow on the plan, without damper, cfm callout, or interlock note, generates a comment.
Field Tip: Run the 400 cfm check on every domestic kitchen hood in a multifamily or mixed-use project. Large decorative hoods specified by architects often exceed 400 cfm. If they do, the MUA duct, damper, and interlock all need to be in the set before submission.
Masterbuild QA Lens
Domestic cooking exhaust rarely gets a full code review at the drawing stage because it looks routine. The three items that generate the most corrections: (1) MUA not shown for a system over 400 cfm, (2) ductless hood used without a listed charcoal filter, (3) multistory shaft with wrong duct material or missing backdraft dampers at units.
Drawing / Submittal Check
Every domestic exhaust appliance on the equipment schedule should show: listing standard, exhaust cfm, duct material, duct termination location, and MUA note where cfm exceeds 400.
Common Review Risk
Multistory residential exhaust shafts designed like single-unit exhaust paths are the most common Section 505 failure on multifamily projects: missing IBC shaft classification, wrong duct material, and no backdraft dampers at the unit connections.