How does stack effect ventilation work?

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.

How does cross ventilation work?

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.

What opening area percentage is needed?

As a rule of thumb, ventilation openings should be 2-5% of the floor area for cross ventilation and 3-8% for single-sided ventilation. Building Regulations Approved Document F requires minimum opening areas based on room use.

When should I use mechanical ventilation instead?

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).

What is the discharge coefficient?

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.

TradeCalculator

Natural Ventilation Calculator

Calculate opening areas for stack effect and cross ventilation

Ventilation Type

Room Volume

Internal Temperature

External Temperature

How We Calculate This

Enter the room volume, internal and external temperatures, and stack height (distance between low and high openings). For stack effect, the calculator uses Q = Cd x A x sqrt(2gH x deltaT/Ti) to find the required opening area. For cross ventilation, Q = Cd x A x v x effectiveness factor. The required area is shown along with the achievable air changes per hour and the opening as a percentage of floor area.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.