The Closed Bedroom Door: Overnight Carbon Dioxide Accumulation for One and Two Sleepers, and the Door Gap or Vent Area That Holds It Below 1000 ppm

Bedroom doors are closed at night for quiet and privacy, and the air inside a closed room is exchanged only slowly. This paper models carbon dioxide accumulation over an eight-hour night in a 36.4 cubic metre bedroom, using a stated sleeping generation rate of 0.0040 litres per second per adult and a stated closed-door infiltration rate of 0.10 air changes per hour. Two sleepers behind a closed door end the night at 4777 ppm, heading for a steady state of 8332 ppm, and spend 7.2 hours above 1000 ppm; one sleeper ends at 2598 ppm. Holding two sleepers at or below 1000 ppm requires 1.36 air changes per hour — a flow of 13.8 litres per second. The remedy is larger than folklore suggests: a standard 12 mm door undercut supplies only 17 percent of that flow at the stated driving velocity, and the full requirement corresponds to a vent free area of 552 square centimetres, or an equivalent door undercut 69.0 mm deep. The numbers size the fix precisely, and correct it. This study is simulated, not measured.