Natural Ventilation Calculator

Stack effect relies on temperature difference between inside and outside. Warm indoor air is less dense and rises, exiting through high-level openings, drawing cool air in through low-level openings. The greater the height difference and temperature difference, the stronger the effect.

Cross ventilation uses wind pressure differences on opposite sides of a building to drive airflow. Openings on the windward side allow air in, while openings on the leeward side allow air out. It is most effective when openings are on opposite walls and the building depth is less than 5x the floor-to-ceiling height.

For purge ventilation, Approved Document F 2021 (England) requires an openable area of at least 1/20 (5%) of the floor area for hinged or pivot windows opening 30° or more, or 1/10 (10%) for windows opening 15–30°. As a CIBSE design rule of thumb (Guide B2 / AM10), cross ventilation typically needs an effective openable area of roughly 2–5% of floor area, while single-sided ventilation needs about three times as much. Always confirm the specific requirement for the room use against Approved Document F.

Mechanical ventilation may be needed when: natural ventilation cannot provide adequate airflow, noise from outside prevents opening windows, air quality is poor (pollution, industrial areas), security concerns prevent open windows, or heating energy loss from ventilation must be minimised (MVHR).

The discharge coefficient (Cd) accounts for friction and turbulence as air passes through an opening. Typical values: sharp-edged orifice 0.61, hinged window 0.60-0.65, sliding window 0.65, louvre 0.35-0.50. Lower Cd means a larger opening is needed for the same airflow.