Plain English:
406.1: Crawl spaces and attics must be ventilated either (a) naturally per the IBC opening rules, or (b) mechanically with both supply air and exhaust. If mechanical, exhaust >= 0.02 cfm/ft^2 of horizontal area and auto-enable when relative humidity (RH) > 60% in the space. (OA = outdoor air)
Calculation Check: Step-by-Step (mirror on M-601)
Choose path: NAT-IBC openings or MECH-406.1.
If NAT-IBC: Show opening sizes/locations and bug screens; coordinate with envelope and fire/smoke requirements.
If MECH-406.1:
Exhaust cfm = Area (ft^2) x 0.02
Provide matching supply path (ducted OA or transfer from conditioned space with make-up strategy).
RH control: Auto start when RH > 60%; stop at 55-58% (deadband).
Condensation control: Keep air paths and insulation continuous; avoid short-circuiting between supply and exhaust.
Code Path: Where to Show It (sheet map)
M-001: "IMC 406.1: Uninhabited spaces ventilated by NAT-IBC or MECH (Exhaust >= 0.02 cfm/ft^2; auto on at RH > 60%)."
M-101+ (Plans): Indicate crawl/attic zones, intake/exhaust locations, air paths, backdraft dampers, and insect screens.
M-601 (Schedules): List exhaust fan cfm (at 0.2-0.5 in.w.g. as applicable), OA/supply path, controls = RH sensor setpoints, power, and rated environment.
M-603 / M-002 (Sequences): RH enable > 60%, off < 55-58%; fault alarm on sensor failure; winter lockout temp (if used) with condensation check.
Coordination Check: Frequent Redlines
Only exhaust shown-no supply/make-up path.
RH sensor missing, not in space, or not set to 60% enable.
NAT path claimed but no IBC opening math or locations.
Fan cfm sized to nominal, not to 0.02 cfm/ft^2 at external static (long duct runs, screens, and hoods not accounted).
Check: Uninhabited Space Vent Layout
Plan must show exhaust outlet and air intake locations in each uninhabited space with duct size or vent area (for passive systems), fan capacity, and control method (thermostat, humidistat, or continuous).
For crawl spaces, show vent locations on the foundation plan and confirm total vent area meets the code minimum.
Confirm vent openings are shown on the foundation plan and included in the foundation permit scope - not just the mechanical drawings.
Review Risk:
Crawl space vent locations on mechanical drawings not coordinated with the foundation drawings are a common construction gap - the foundation contractor builds solid walls while the mechanical drawings show vents.
Vent openings not in the foundation permit are not installed by the foundation contractor - confirm they are in the foundation scope, not assumed.
Missing total vent area confirmation against the floor area minimum is a routine plan review
Field Tip:
RH sensing: Use replaceable-sensor models rated for high-humidity, dusty spaces; mount away from cold exterior surfaces.
Duct terminations: Add critter guards and backdraft dampers; slope ducts to drain away from equipment.
Energy/comfort: If drawing from conditioned space, confirm heating/cooling penalty is acceptable; consider small balanced OA duct to crawl/attic instead.
Masterbuild QA Lens
Climatic adjustments to ventilation rates are jurisdiction- and ASHRAE climate-zone-specific. Confirm which climate zone the project is in (ASHRAE 169 map, confirmed by authority), whether the project uses natural ventilation adjustments per 406.1, and that any humidity or enthalpy control sequences tied to climate are shown on the control diagrams.
Drawing / Submittal Check
For IMC 406 uninhabited space ventilation (drawing requirements): the plan must show the exhaust outlet and air intake locations in each uninhabited space, with the duct size (or vent area for passive systems), the fan capacity, and the control method (thermostat, humidistat, or continuous). For crawl spaces, show the vent locations on the foundation plan and confirm the total vent area meets the code minimum. Missing vent area calculations and vent locations on foundation plans are a common omission.
Common Review Risk
Crawl space vent locations are commonly specified in the mechanical drawings but not coordinated with the foundation drawings. The mechanical drawings show vents on all four sides - but the foundation drawings show solid concrete foundation walls without openings. Confirm the vent openings are shown on the foundation plan and included in the foundation permit scope. A crawl space vent that is not in the foundation permit is not installed by the foundation contractor.
When To Escalate
Escalate when the crawl space or attic ventilation drawings conflict with the energy code requirements - when the energy code requires an unvented attic or crawl space but the mechanical code requires ventilation. Unvented assemblies require an AHJ determination that the energy code exception is acceptable, and that determination must be in writing before the permit is filed.
Rated Assembly Coordination
Ventilation openings in uninhabited spaces like attics and crawlspaces often penetrate fire-rated floor-ceiling or roof assemblies. Confirm each opening is protected with a listed damper or that the assembly exemption applies and is documented.
Load Assumption Check
Attic and crawlspace ventilation rates depend on floor or ceiling area and the presence of a vapor retarder. Confirm the ratio used (1:150 vs. 1:300) matches the actual vapor retarder installation shown on the drawings.