Cross ventilation

Cross ventilation is a natural phenomenon where wind, fresh air or a breeze enters upon an opening, such as a window, and flows directly through the space and exits through an opening on the opposite side of the building (where the air pressure is lower).

A commonly used technique to remove pollutants and heat in an indoor environment, cross ventilation can also decrease or even obviate the need for an air-conditioner and can improve indoor air quality.

The phenomenon occurs when openings in an environment (including vehicles) or building (houses, factories, sheds, etc) are set on opposite or adjoining walls, which allow air to enter and exit, thus creating a current of air across the interior environment.

[3] Windows or vents positioned on opposite sides of the room allow passive breezes a pathway through the structure, which circulate the air and provide passive cooling.

If there are windows on both sides in a building, cross ventilation is appropriate where the width of the room is up to five times the floor-to-ceiling height.

If openings are only one side then wind-driven ventilation is more suited for structures where the width is around 2.5 times the floor to ceiling height.

[5] Cross ventilation relies on many factors, such as the tightness of the establishment, wind direction and how much wind is available, its potential travel through chimneys, vents and other openings in the home.

Moreover, cross ventilation is generally only suitable for narrow buildings.

The contrasting height of the openings (walls, sill, panels or furniture) ordered by the space also immediately influence the level and velocity of ventilation.

[1] Cross ventilation works well in climates with hotter temperatures, where the system allows continual changes of the air within the building, refreshing it and reducing the temperature inside the structure and also when the window on the windward side of the building is not opened as much as the one on the leeward side.

[6] An opened window that faces a prevailing wind and is conjugated with another window on the opposite side of a building will supply natural ventilation for fresh air.

[7] Besides windows, other openings like brise soleils, doors, louvers or ventilation grills and ducts can also work as effective ventilation openings, though an awning window provides the least effectivity.

The wind surrounding building structures is important when it comes to assessing the air quality and thermal comfort indoors since both air and heat exchange rely heavily on the wind pressure on the exterior of the building.

For the best airflow, the windward windows of the occupied space should not be opened as much as those on the leeward side.

[8] Disadvantages of wind-driven ventilation include capricious wind speeds and directions (which may create a strong unpleasant draft), and the polluted air from the outside that may tarnish the indoor air quality.

is a local pressure drag coefficient for the building, defined at the location of the upstream opening;

is a local pressure drag coefficient for the building, defined at the location of the downstream opening;

For rooms with single opening, the calculation of ventilation rate is more complicated than cross ventilation due to the bi-directional flow and strong turbulent effect.

As observed in the equation (1), the air exchange depends linearly on the wind speed in the urban place where the architectural project will be built.

CFD (Computational Fluid Dynamics) tools and zonal modelings are usually used to design naturally ventilated buildings.

Windcatchers can assist wind-driven ventilation by guiding air in and out of structures.