IMC 309 is short, but it controls an important design assumption: spaces intended for human occupancy need heat. Portable space heaters do not count as the compliance strategy.
Plain-English Summary
Interior spaces intended for human occupancy must be capable of maintaining at least 68 F at a point 3 feet above the floor on the design heating day. The code allows active or passive heating systems, but the requirement has to be met by the building design, not temporary plug-in heaters.
Exception Check
The key exceptions are spaces where the primary purpose is not associated with human comfort and Group F, H, S, and U occupancies. Do not apply the exception just because a space is normally unoccupied; confirm the occupancy and intended use.
Permit Proof Chain
Space use -> occupancy classification -> whether human occupancy and comfort apply -> design heating day basis -> active or passive heating system -> capacity/load documentation -> thermostat/control location -> exception basis if heat is omitted.
Masterbuild QA Lens
The risk is a shell, storage, back-of-house, or support space being shown without heat because nobody expected comfort complaints, while the occupancy or use still triggers the 68 F requirement.
Drawing / Submittal Check
On schedules and floor plans, identify which spaces are heated, which are exempt, and how the heating system satisfies the design day. If heat is omitted, state the exception basis cleanly.
Field Tip
During existing-building reviews, look for converted storage rooms, offices carved out of warehouses, and tenant back rooms. The new use can trigger heating even when the old space did not need it.