Door Undercut Free Area, Transfer Airflow, and Noise

A door gap will pass almost any airflow if you push it hard enough, and the result is a whistle occupants notice immediately -- so the practical ceiling is a face velocity around 300 fpm, and above that the answer is more free area rather than more pressure. A 36 in door with a 3/4 in undercut is 27 sq in, 0.188 sq ft, which at 300 fpm is 56 cfm: adequate for a private office with a small diffuser. Now ask the same door for 250 cfm to a conference room. The velocity goes to 1,333 fpm, more than four times the threshold, and the door will whistle. Work it the other way and the undercut needed is 3.33 in -- not a door anyone will accept, and one that destroys the door's fire, smoke, and sound performance at the same time. THE CORRECT ANSWER IS A TRANSFER GRILLE with 0.83 sq ft of free area, or a ducted return; and if sound privacy matters, a lined transfer boot rather than a plain grille. That is the arithmetic that decides between an undercut and a grille, and it is the reason rooms get starved or turned into whistles: a room needing a few hundred cfm of transfer air needs free area measured in square feet, not square inches, and no realistic undercut provides it. There is a second consequence worth flagging. The same gap is a sound path, so an undercut sized for airflow undoes much of the door's acoustic rating, which in an office or an exam room is a privacy problem rather than an airflow one. A free-area and velocity calculation. It does not address fire and smoke doors, where an undercut is limited by the door's listing and by NFPA 80 clearance requirements and where a transfer opening is generally not permitted at all, because cutting a rated door voids its label. It does not evaluate acoustic performance, address smoke control or pressurization, or account for the pressure difference the transfer path actually operates under, which determines the real flow rather than an assumed face velocity, and it does not size the room's supply or return. The adopted mechanical code, NFPA 80 for rated doors, and the mechanical designer govern.

free area = door width x undercut height; airflow = free area x face velocity; the undercut a required airflow needs = required cfm / (door width in ft x velocity) x 12; a transfer grille needs required cfm / velocity of free area.

The free-area transfer relation by name, with a practical face-velocity ceiling around 300 fpm before the gap becomes audible, and NFPA 80 named for rated door clearances. The mechanical designer governs the transfer path.

Area and velocity arithmetic on the user's own door and airflow figures; no manufacturer or code table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: door_width_in, undercut_in, required_cfm, max_velocity_fpm, free_area_sqin, free_area_sqft, cfm_at_velocity, velocity_at_required_fpm, undercut_needed_in, grille_free_area_sqft

• Noise sets the limit, not physics a face velocity around 300 fpm before the gap is audible; above it, add free areamechanical design practice
• Hundreds of cfm need square feet no realistic undercut provides it; that is a transfer grille or a ducted returnmechanical design practice
• The gap is a sound path too an undercut sized for airflow undoes much of the door's acoustic ratingacoustic practice