Wind-driven airflow through a building is about Q = Cv x A x V, where A is the inlet area, V the outdoor wind speed and Cv an effectiveness near 0.5-0.6 when the wind blows straight at the opening and 0.25-0.35 when it comes in at a diagonal. A 0.5 square metre inlet in a 1.5 metre per second wind gives about 0.375 cubic metres per second, 26 air changes an hour in a 52 cubic metre room.
Part of Passive Solar Building Design
Sizing and placing openings on opposite walls so outdoor wind carries air through a building, and the limits of that effect.
Place Openings on Opposite Walls
The carrier’s rule: put operable openings (windows, shutters, vents) on opposite walls, so wind entering one side exits the other. Make the outlet slightly larger than the inlet. When the wind is calm, put inlets low and outlets high so the stack effect adds to the flow (see Stack Effect & Thermal Chimneys).
How Much Air?
A common design estimate (see the natural-ventilation chapter of the ASHRAE Handbook, Fundamentals) is:
Q = Cv x A x V
with Q the airflow in cubic metres per second, A the free area of the inlet in square metres, V the wind speed in metres per second outside, and Cv an effectiveness factor. Cv is roughly 0.5-0.6 for wind perpendicular to the inlet and roughly 0.25-0.35 for wind at an angle. Real values vary with surroundings: trees, neighbours and hills change the wind a house sees. Outdoor wind near the ground is lower than the wind speed at weather stations; measure it with Wind Measurement.
Does a Bigger Outlet Help?
Two openings in series behave like one smaller opening. The effective area is A = 1 / sqrt(1/A_in^2 + 1/A_out^2). With an inlet of 1 m2:
| Outlet area | Effective area | Share of inlet area |
|---|---|---|
| 1.0 m2 | 0.71 m2 | 71% |
| 1.5 m2 | 0.83 m2 | 83% |
| 2.0 m2 | 0.89 m2 | 89% |
| 3.0 m2 | 0.95 m2 | 95% |
So an outlet larger than the inlet increases flow, with diminishing returns once it is about twice the size. The carrier’s slightly larger outlet is a sound rule; the largest gain is the first increase. A small inlet also makes the air faster as it enters, which feels cooler to the people in its path.
What You Need
- Openings with free area you can measure — the open part, not the whole window.
- A wind speed estimate — from a few weeks of measurement.
- A floor plan — to see what blocks the path.
Method: Lay Out the Flow
Step 1: Mark the prevailing summer wind on the plan. See Prevailing Wind Patterns.
Step 2: Put the inlet where the wind strikes the wall and the outlet on the opposite or sheltered side.
Step 3: Clear the path Interior walls and doors across the path stop the flow. Leave doorways open or add vents high over doors.
Step 4: Put the inlet where the people are Moving air only cools the skin where it passes.
Worked Example
Room 5 m x 4 m x 2.6 m, V = 52 m3. Inlet free area 0.5 m2, outdoor wind 1.5 m/s.
- Wind straight at the inlet, Cv = 0.5: Q = 0.5 x 0.5 x 1.5 = 0.375 m3/s = 1,350 m3/h = 1,350 / 52 = 26 air changes per hour.
- Wind at a diagonal, Cv = 0.25: Q = 0.25 x 0.5 x 1.5 = 0.19 m3/s = 675 m3/h = 13 air changes per hour.
If the window screens reduce the free area by a third, the flow falls by about a third in proportion. The results show that wind can change the air in a room more than ten times an hour, but only while the wind blows; on a calm night the flow is nearly zero.
Common Mistakes
- Openings on adjacent walls only — the air takes a short corner path.
- Outlet smaller than the inlet — it throttles the flow.
- Counting on wind at night — it often drops after sunset; design for stack effect too.
- Big openings on the sunny side — they admit heat; face them away from the sun or shade them.
Safety
Through-draughts spread fire and smoke
Wind through a house feeds a fire and carries sparks and embers. Close shutters when there is a fire nearby, and keep forges, kitchens and smoky work downwind of the inlet. Do not tie ventilation openings open with children or the sick lying in a draught for hours in cold weather.
What’s Next
- Learn next: Stack Effect & Thermal Chimneys — airflow without wind
- Combine with: Ventilation & Summer Cooling — fitting it into the daily cycle