Calculating natural ventilation rate means estimating how much air flows through a space when movement is driven only by wind and temperature differences, without mechanical fans. While natural ventilation can be powerful in some buildings, its rate is less predictable than mechanical ventilation, so calculations are usually approximate.
Two main mechanisms drive natural ventilation: wind pressure and the stack effect. Wind creates positive pressure on windward openings and negative pressure on leeward openings, pushing air through the building. The stack effect occurs when warm indoor air is lighter than cooler outdoor air, creating a buoyancy-driven flow from lower inlets to higher outlets.
For wind-driven flow, a simplified formula for airflow through openings is:
Q = Cw × A × v
where Q is airflow (m3/s), Cw is a discharge coefficient that accounts for opening shape and pressure distribution, A is the effective area of openings (m2) and v is the wind speed (m/s). The effective area considers both inlet and outlet openings; the smaller of the two generally limits the flow.
For stack-driven flow, a commonly used formula is:
Q = Cs × A × √(2gHΔT / T)
where Q is airflow, Cs is a discharge coefficient, A is the opening area, g is gravitational acceleration, H is the vertical distance between inlet and outlet, ΔT is the indoor-outdoor temperature difference, and T is absolute temperature. This equation shows that larger vertical separation, larger openings and larger temperature differences all increase natural ventilation rate.
In practice, engineers often combine wind and stack contributions, but they also recognise the uncertainty in wind direction, speed fluctuations and temperature changes. For industrial buildings, natural ventilation calculations are used to estimate typical or favourable conditions, while safety-critical functions (such as fume control or mine ventilation) usually rely on mechanical fans to guarantee minimum airflow.
To apply these formulas, you must carefully define:
– The size and position of inlet and outlet openings (louvers, roof vents, wall openings).
– The expected range of wind speeds and directions.
– Typical indoor and outdoor temperature differences for the design season.
After obtaining a calculated natural ventilation rate, designers check whether it meets requirements for air changes per hour, contaminant dilution and heat removal. If not, they either increase natural opening areas, improve building geometry or add mechanical fans to supplement natural forces.
In summary, natural ventilation rate is calculated using empirical formulas for wind and stack effects based on opening areas, wind speed and temperature difference. Because actual conditions vary, these calculations are best treated as estimates and often combined with mechanical ventilation in industrial and mining environments where minimum airflow must be guaranteed.
