/ Field knowledge · IMC 406
“Ventilated per code” is not a calculation.
IMC 406.1: crawl spaces and attics are ventilated naturally per IBC openings, or mechanically with both supply and exhaust. A note that says “ventilated per code” with no math is the comment.
Educational 2024 IMC 406.1. Confirm the adopted FBC Mechanical edition and IBC opening rules. This is not a vent-area design.
- Choose a path. NAT-IBC openings (sizes, locations, screens, envelope/fire coordination) or MECH-406.1.
- Mechanical path. Exhaust ≥ 0.02 cfm/ft² of horizontal area, plus a matching supply path (ducted OA or transfer with makeup strategy).
- RH control. Auto-enable when relative humidity > 60% in the space. DCT coordination example: stop at 55–58% (deadband). Sensor in the space, not on a cold exterior surface.
- Show it. M-001 states the path. M-101 shows crawl/attic zones, intake/exhaust, air paths, backdraft dampers, insect screens. M-601 lists cfm, supply path, RH setpoints.
Only exhaust, no supply
Exhaust without a makeup path is the redline. Fan cfm sized to a catalog number instead of 0.02 cfm/ft² at external static (duct, screens, hoods) is the next one.
Source: DCT 406 · transfer: 501.4 · tool: attic / crawl check.
What to send
Attic or crawl floor area, whether openings exist, and whether a fan is proposed. Photos of existing vents help on a replacement.
If the set says “ventilated per code”
Send the attic or crawl area and whether this is natural openings or a fan. Masterbuild will say whether 406.1 math is even on the drawings.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval.