What is Natural Ventilation?
Natural ventilation uses wind and buoyancy — not fans or ducts — to move fresh air through a building. Architects design window placement, stack height, and cross-ventilation paths to cool and refresh interiors passively, cutting energy use.
Natural ventilation is airflow driven by wind pressure and the stack effect (temperature-driven buoyancy) rather than mechanical fans, using operable openings positioned to draw fresh air in and push stale air out.
- •Driven by wind pressure
- •Needs openings on two opposite walls
- •Works best with strong prevailing wind
- •Ideal for narrow building plans
- •Driven by temperature difference
- •Needs vertical height between inlet and outlet
- •Works even with no wind
- •Ideal for atriums and tall spaces
Try it: interactive calculator
Step-by-step worked examples
An atrium has openings 4 m apart in height, indoor air is 5°C warmer than outdoor (293 K outside), Cd=0.65, effective area 2 m². Find the airflow rate.
Q = Cd·A·√(2·g·h·ΔT/T) Q = 0.65×2×√(2×9.81×4×5/293) Q = 1.3×√(1.339) Q ≈ 1.3×1.157 ≈ 1.50 m³/s
A single-sided room has one window and low outdoor wind. Explain why stack ventilation still works.
No wind → cross-ventilation pressure ≈ 0 Warm indoor air rises and exits near the top of the window Cooler outdoor air enters near the bottom Net result: buoyancy-driven exchange even in still air
Doubling the opening area A, how does airflow rate change (all else equal)?
Q ∝ A (linear relationship in the formula) Doubling A → Q doubles Example: A=2→Q=1.5 m³/s; A=4→Q=3.0 m³/s (same Cd, h, ΔT, T)
Flashcards
Quick quiz
Q1.Natural ventilation driven by temperature differences is called…
Q2.Cross-ventilation requires…
Q3.In Q = Cd·A·√(2gh·ΔT/T), doubling h (height between openings) makes Q…
Q4.Which space type benefits most from stack ventilation?
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Common mistakes
Natural ventilation only works when it's windy. — Correct: Stack (buoyancy) ventilation works even in still air, driven by temperature differences alone.
More window area always means proportionally more airflow, at any height difference. — Correct: Airflow depends on both opening area AND height difference (stack) or wind speed (cross) — not area alone.
Natural ventilation can replace mechanical ventilation everywhere. — Correct: It works well in mild climates and specific building types, but humid/extreme climates or dense urban sites often still need mechanical systems.
Opening any two windows on the same wall creates cross-ventilation. — Correct: True cross-ventilation needs a pressure difference, usually openings on opposite or adjacent facades with different wind pressures.
FAQ
What is natural ventilation?
Natural ventilation is the passive movement of air through a building driven by wind pressure (cross-ventilation) and buoyancy (stack effect), without mechanical fans.
What is the formula for stack ventilation airflow?
Q = Cd·A·√(2gh·ΔT/T), where Cd is the discharge coefficient, A is opening area, h is height between openings, ΔT is the temperature difference, and T is outdoor absolute temperature.
What are examples of natural ventilation strategies?
Cross-ventilation with opposing windows, stack ventilation via atriums or solar chimneys, single-sided ventilation, and wind towers are common examples.
How do you calculate natural ventilation airflow?
Use the stack effect formula for buoyancy-driven flow, or wind-pressure equations for cross-ventilation; both depend on opening size, height/wind speed, and temperature/pressure differences.