2.4.1 Driving forces of natural ventilation
Natural ventilation is driven by stack effect (temperature differences) and wind pressure.
Warm air is lighter than cold air. That causes the stack effect, which means that warm air inside a building will rise. The warm air will leave the building at the top through leakages, stack ducts or open windows and be replaced by cold air entering the building at ground level. The higher the building, the more powerful the stack effect. For the stack effect to work efficiently, there must be air passages through the building.
These can be stairways in combination with windows at both ground level and roof level that can be easily opened at the same time. Due to their position in the roof, VELUX roof windows maximise the ventilation potential of the stack effect. See section 2.4.3 for an example of stack effect.
Wind (wind pressure)
When a building is exposed to wind, air will enter the building at the windward side and leave through openings at the leeward side. The wind pressure is higher on the windward side than on the leeward side. This will drive air from the windward side of the building through the building to the leeward side. The shape of the building and the surrounding landscape or buildings have an impact on the air flow. The magnitude of the pressure difference generated by wind pressure is determined automatically as part of a simulation in tools like the VELUX Energy and Indoor Climate Visualizer. Typical values can be found in standards (e.g. BS5925:1991, DIN19466:2009). See section 2.4.3
for an example of wind driven natural ventilation.