Quick Answer — How Much Ventilation Does a Terrarium Need?
There is no published standard for terrarium ventilation, and anyone quoting one is guessing. What can be calculated exactly is the consequence: how fast air moves through the enclosure, and how much water that air carries out. Once you know the water loss in millilitres per day, the ventilation question answers itself — you need enough air exchange to keep the air fresh and little enough that you can replace what evaporates.
Airflow comes from the stack effect: warm air inside rises and leaves through the upper vent, drawing room air in through the lower one. Three things control it — the smaller of the two vent areas, the vertical distance between them, and the temperature difference.
Three enclosures at their default settings:
- 45 × 45 × 60 cm, 20 cm² vents, 40 cm apart, 24 °C at 85% over a 21 °C room at 45% — 12.5 air changes per hour, one complete change every 4.8 minutes, 324 ml of water lost per day
- 30 × 30 × 45 cm, 6 cm² vents, 30 cm apart, 23 °C at 90% over a 20 °C room at 50% — one change every 6.1 minutes, 81 ml per day
- 90 × 45 × 45 cm, 120 cm² vents, 30 cm apart, 28 °C at 60% over a 21 °C room at 40% — one change every 0.94 minutes, 2,238 ml per day
That last row is the lesson. A large, warm, heavily vented enclosure loses well over two litres of water a day, which is why screen-top setups need a fogger or an automated misting system rather than a spray bottle.