Daily Code Talk #40 — IMC 406: Ventilation of Uninhabited Spaces (406.1)

🧠 Plain English Highlights• 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/ft² of horizontal area and auto-enable when relative humidity (RH) > 60% in the space. (OA = outdoor air)🧮 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 (ft²) × 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.🗺️ Where to Show It • M-001: “IMC 406.1: Uninhabited spaces ventilated by NAT-IBC or MECH (Exhaust ≥ 0.02 cfm/ft²; 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.🚧 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/ft² at external static (long duct runs, screens, and hoods not accounted).🔧 Field Tips• 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.Want a 1-page Crawl/Attic Vent Card (IBC natural opening checklist + mechanical calc sheet + RH sequence you can paste on M-603)? Comment IMC406 and I’ll share it.