The Windcatchers

Passive Cooling Strategies

Windcatchers are diverse in form and configuration and supply natural ventilation and cooling for traditional buildings in hot regions. Photo Reference: Aida Zare

Windcatchers or Baadgir ( in Persian) are one the most effective and ancient passive cooling systems based on convection and natural ventilation. It has been used in hot climates for centuries in some regions like Iran, Egypt, India and Africa. There is a lot of diversity in the design of a windcatcher. The form and configuration, height, and cross section of a windcatcher vary according to the climatic condition of each area, especially the local wind characteristics; its altitude, speed and direction. Short one sided windcatchers of a windy area can cover the cooling needs of a house simply, while a tall four sided windcatcher is needed to response to harsh climatic conditions of a hot city like Yazd in Iran.

Function

The function of windcatchers can generally be in three different ways. For the regions with the favorable prevailing wind, their openings should be designed in windward side to direct air flows to the indoor spaces. The opening of a windcatcher can be on the leeward side in order to suck the indoor air by the negative airflow pressure. This strategy can be helpful in the regions where the prevailing winds contain unwanted dust or any pollution. Directing airflow by a windcatcher can work by using buoyancy forces from temperature gradients. This would be useful in the areas that don’t catch enough airflows or breezes, so the stack effect will be caused by the temperature difference. 

 

There are three main ways to catch or vent the airflow through a windcatcher. Photo Reference: Ghiabaklu,Z. 2014.

In these three strategies windcatchers decrease the Indoor air temperature and improve its quality using convection cooling. While most of the time the windcatchers act as a part of an evaporating cooling system. Putting water pots beneath the channel of the windcatchers or near their openings is one of the strategies that was used to moderate the humidity and air temperature. There are also some examples of special rooms with a pool in between, that are placed beneath a windcatcher. The room has several windows to the other parts of the building on each side and provide a nice cool and humid breeze for them. In some cases the channel of the windcatcher is connected to an underground tunnel or even reach to Qanat (underground water canal) and benefits geocooling and evaporative cooling to minimize the air temperature, increase the humidity and velocity of the airflow and improve the performance of the windcatcher.

The Elements of a Windcatcher

 

The elements of a windcatcher. Photo Reference: Aida Zare

Each windcatcher usually has a roof, a box, a neck, some partitions and a channel. The roof of a windcatcher is designed in different types of forms like: flat, slope or curved. Curved roof can improve the suction capacity of the channel.

Beneath its roof is the box of the windcatcher that directs the airflows and can be designed in various forms from square, rectangle, octagonal or even circular. Deciding for the form and direction of the box and its openings is very effective in the function of a windcatcher and needs an accurate knowledge and analysis of the local wind conditions. The opening partitions act like radiator fins, they have aesthetic values while avoiding entering burglars or birds into the channel of the windcatcher.

The neck or stem of a windcatcher connects the box of the windcatcher to the roof of the building. The longer the windcatcher is, the longer is its neck. Depending on the favorable wind latitude, this part varies from a very high shaft to a very short part or can be even eliminated from the body of a windcatcher in some cases.
The channel of a windcatcher is a vertical shaft that conducts the airflows from up to down or vice versa. It is usually built with thermal mass like adobe, so that the temperature of outdoor wind decreases while passing it. Main partition or division blades have different shapes like: +, H, I, or × shape. They divide the channel into several parts and increase the air speed according to the Bernoulli effect.

Material, Texture and color of windcatchers

Like other parts of a traditional building windcatchers are constructed by local material. The common windcatchers of hot and arid climate are built by earth material like adobe and brick. The characteristics of these materials like their high U value improve the function of the windcatcher. Finishing layers of  windcatchers usually are covered by straw and clay plaster. The light color of the plaster reflects the sunlight and decreases the heat gain by radiation.
In hot and humid climates, windcatchers are plastered by lime, clay and stucco. The color of this plaster is white so they can decrease the temperature of airflow before entering the indoor spaces. The penetrability of this material is very helpful to decrease the extra humidity too.

 

A two story windcatcher in Aghda Caravanserai, near Yazd city in Iran is made of adobe and brick. Photo Reference: Sobhan Zare Mohazabiye

Integrity

Windcatchers played a very important role in providing thermal comfort in hot climates without consuming any non renewable energy. They are a part of a cooling system of buildings and are designed by the architects. The form and function of a windcatcher is completely adapted to the climatic condition of the region and also matched by the other parts of the building. Hence they are an inseparable element of a whole and present an integrated beauty for a single house. Meanwhile in some cities where this passive strategy is used in each building, they turn to a symbol for that city and make a specific cultural landscape for it. They are a nice example of a detail (here a cooling system) in a building that can effect and make an elegant feature for a city.

 

One sided windcatchers of Ardakan city near Yazd. Each house usually has several windcatchers. They create a unique view for this city while responding to the harsh climatic condition of the context. Photos Reference: Sobhan Zare Mohazabiye

 

References:

Ghiabaklou, Zahra. (2014). Fundamentals of Buildings Physics 4: Passive Cooling. ACECR Publication.  Amirkabir University.

Kheirkhah Sangdeh, Parham, Nasrollahi, Nazanin. (2022). Windcatchers and their applications in contemporary architecture. Energy and Built Environment: Volume 3, Issue 1.

Mahmoodi Zaranid, Zahra. (2014). Baadgir Symbol of Iranian Architecture. Yazda Publication. Tehran.