Yazd city is one of the oldest continuously inhabited cities in Iran and is a world heritage site. It is one of the largest cities of the world built almost entirely out of adobe. The skyline of city of Yazd is defined by “badgirs” rising from the city’s buildings. Badgirs or wind-catchers are towers with openings at the top that help in cooling a building through natural and environmentally friendly methods that capture the wind that blows over the buildings.
watch this video about the dessert city Yazd and its adobe built natural ventilation system:
The badgir essentially creates a draft which prevents hot air from accumulating. The badgir tower rises high enough for it to catch the breeze that blows across the top of buildings, trap the breeze, and then direct the trapped breeze down through a chimney into a building. The way in which the draft through a building is created is by the section facing the breeze catching and compressing the air and directing it down through the building, into the building’s lowest level, say a basement if the building has one. Since the breeze flowing over the top of a building can come from various directions and can even change direction frequently, many badgirs were created with four or more faces on each side of a square (or polygonal) chimney-like tower. In other words, badgirs come with one, two, four and even five faces.
The up and down chimney shaft itself is divided into the same number of sections as the badgir face. For instance, if the top opening of the badgir has four faces or openings, the chimney shaft itself must also be divided into four sections. Dividing a square chimney into four gives us four triangle shaped shafts that run up and down the chimney.
However, if the badgir merely traps a current of wind, the draft created within the building will be mild. To increase the draft flow rate, the openings of the badgir that are opposite to side facing the wind, create a suction effect that suck out the air from a building.
If there is no wind, the south face of the badgir tower heats up and since hot air rises, the air in the southern section will rise up sucking the air down from the cooler section that faces north.
At night, The tower cools down first and the cold air being heavier than warm settles down through the chimney into the building, pushing out the warm air through the windows, creating a draft and cooling down the building at the same time. As the mud bricks don’t conduct heat readily, while they may take time to cool down at night, they will also take time to heat up during the day. If there is a large pool of water in the house, that too will cool down and become a heat sink absorbing heat during the day.
Stage two creates the wind action that a fan working in conjunction with an exhaust fan would create. To enhance the cooling, in those buildings with owners that had the position and wealth to afford such a system, the badgir and water storage systems were combined to create the effect of a water cooler. This is accomplished by passing the draft created by the badgir over a pool of water – say a small pool or reservoir in the basement. If a home was fortunate enough to afford this system and construction, the basement room was converted into a gathering room with ledges that served as seats and beds for an afternoon nap.
Stage four is to make the badgir works in conjunction with an underground water channel, kariz. The temperature of the flowing water in a kareez was generally lower than in a standing pond and a long kareez channel provided a larger cooling surface area as well. This system created a temperature drop as much as 200C.
In this stage the microclimate created by a badgir is enhanced by a courtyard that includes a pond and garden. If the section of the badgir facing the direction from which the breeze is coming is blocked, and if instead the incoming air is taken in from a shaded courtyard with a pool and a garden, the incoming air will be somewhat cooler, and the courtyard itself will benefit from the induced draft. This system was and is exploited by the town’s wealthiest residents.
These text informations have been taken from: