Plain English:
401 (General): Chapter 4 governs occupied spaces. Ventilate when occupied (401.3). Choose NAT-402 or MECH-403. Keep OA intakes separated/protected (401.4-401.5). Capture point sources with exhaust to outdoors (401.6).
402 (Nat): Openings controllable by occupants (402.1). Openable area >= 4% of floor area (402.2). Vent via adjoining spaces only with >= 8% transfer (min 25 sf) (402.3). Below-grade needs 1.5x horizontal clear space (402.4).
403 (Mech): Provide supply and return/exhaust (403.1). Determine OA via 403.3; follow recirculation limits (403.2.1) and transfer-air rules (403.2.2).
403.3.1: Table 403.3.1.1 -> Vbz = Rp·Pz + Ra·Az -> Ez (Tbl 403.3.1.1.1.2) -> Voz = Vbz/Ez -> system OA via Eq 4-3/4-4 or Zp/Ev/D (Eq 4-5/4-6/4-8). Provide table exhaust (403.3.1.2). DCV allowed but never below Ra floor (403.3.1.3). VAV must hold OA at turndown (403.3.1.4).
403.3.2 (R-2/R-3/R-4 <=3 stories): Each dwelling gets OA by exhaust/supply/combo; continuous or >=1 hr/4 hr with average meeting Eq 4-9 (403.3.2.1). 30% reduction only if ducted to bedrooms + living/dining/kitchen and whole-house is balanced. Corridors/common areas >= 0.06 cfm/ft^2 (403.3.2.2). Local exhaust: kitchens 100 int / 25 cont; baths 50 int / 20 cont (403.3.2.3). Label OA controls (403.3.2.4). Fans listed to ANSI/AMCA 210-ASHRAE 51 (403.3.2.5).
404 (Garages): Run continuously or automate with CO+NO₂ detectors listed to UL 2075. Modes: Full-On >= 0.75 cfm/ft^2; Standby >= 0.05 cfm/ft^2 (404.1). Accessory rooms: positive pressure; ventilate per 403.3.1 (404.2).
405 (Controls): If occupied, ventilation runs. If the AC unit provides required OA, controls must automatically maintain required OA during occupancy (405.1).
406 (Uninhabited): Crawl/attic: natural per IBC or mechanical >= 0.02 cfm/ft^2; auto-enable when RH > 60% (406.1).
407 (Healthcare): Ambulatory care & Group I-2 must meet IMC 407 + ASHRAE 170 + NFPA 99; design to the most restrictive (407.1).
Coordination Check: Frequent Redlines
Vbz shown but Ez/Voz or Zp/Ev/D missing.
DCV below Ra floor; VAV not holding OA at low flow.
Intake clearances/screens not per 401.4-401.5.
R projects claiming 30% reduction without ducted, balanced design.
Garages missing NO₂ or UL 2075 citation.
Natural ventilation claimed without 4%/8%/below-grade proof.
Field Tip: Quick Field Tips
Tag diffuser types; when unsure, use Ez = 0.8.
Show transfer-air math for kitchens/toilets used as makeup.
For garages, trend ppm/speed/cfm; fail-safe to Full-On on detector/BAS fault.
In dwellings, show Eq 4-9 duty-cycle calc and label OA controls.
Code Path: IMC Section 401.1 (Scope) -> Section 402 (Natural) or Section 403 (Mechanical) -> Table 403.3.1.1 (Breathing Zone Rates) -> Eq. 4-3/4-4 (System OA) -> Section 404 (Garages) -> Section 405 (Controls) -> Section 406 (Uninhabited) -> Section 407 (Healthcare). Start with occupancy, confirm method, show the math.
Section 403.3.1.1: Vbz = Rp x Pz + Ra x Az, then apply Ez for zone design OA (Voz = Vbz/Ez).
Section 404: CO + NO2 detectors listed to UL 2075 are required when automatic control is used in garages.
Section 407: ASHRAE 170 + NFPA 99 coordination is required for healthcare - the most restrictive governs.
Check: Before You Submit
Confirm Vbz, Ez, Voz, and system OA are explicitly shown on the ventilation schedule or on M-001.
Verify OA intakes meet Section 401.4 clearances and are shown on the floor plan or roof plan with dimensions.
Confirm natural ventilation claims show the 4% openable area calculation and any required 8% credits.
Review Risk:
Showing Vbz but not Voz or system OA - the most common Chapter 4 comment: calculation is incomplete.
DCV systems that do not maintain OA at low sensor readings - controls sequence must show the minimum OA floor.
R-2/R-3/R-4 claiming the 30% engineered reduction (Section 403.3.1.1.2) without a balanced, ducted design.
Masterbuild QA Lens
Chapter 4 is where ventilation math meets permit reality. The most common failure mode is not a wrong number - it is a number that is right but not shown, or shown in a way the reviewer cannot verify. Show the OA path, the Ez basis, the intake locations, the control sequence, and the DCV setpoints all on the drawings.
Drawing / Submittal Check
Confirm that Vbz, Ez, Voz, and system OA are shown explicitly on plans or schedules. Intake locations and clearances should be on the plan view. DCV setpoints, VAV minimum OA, and control sequences go in the sequences sheet or notes. Residential duty-cycle calcs should be on the drawing, not just in the calc file.
Common Review Risk
Showing the breathing-zone airflow (Vbz) but not the zone design OA (Voz) or the system OA. Claiming a 30 percent residential OA reduction without a ducted, balanced design. Missing NO₂ sensor citation for enclosed garages. Natural ventilation claims without openable-area proof.
When To Escalate
Escalate to direct PE review when a zone occupancy basis is unclear, when healthcare or Group I-2 occupancies are involved, when a DCV or VAV design requires specific control verification, or when a residential multifamily project involves complex duty-cycle or airflow balancing.
Healthcare Ventilation Note
For healthcare work, verify the room use, pressure relationship, filtration basis, outdoor air/exhaust relationship, controls, and construction cleanliness expectations before the design is treated as permit-ready.
Ventilation Coordination
For Chapter 4 projects: the ventilation coordination summary on M-001 must confirm that every space with a ventilation requirement (occupancy-based, occupant-load-based, or contaminant-based) has a code-compliant ventilation path. The most common Ch4 coordination gap is a space added during design development that never received a ventilation assignment.