Natural ventilation with windcatchers in Iran

Looking at a climate chart for Yazd in central Iran makes me wonder why people wanted to build a town there in the first place. Located in the middle of the desert with very little rainfall, the summer temperatures are scorching hot with large amounts of solar radiation, while large differences in day and night temperatures can make nights uncomfortably cold. And yet the city has been inhabited for at least 3000 years, and during this time the city and its inhabitants have adapted to the harsh climate by creating a built environment that uses passive techniques to create a more comfortable microclimate in the city. Covered streets, courtyards, domed roofs, thick walls, water elements and seasonal variations in the use of indoor and outdoor spaces are some of the techniques that make traditional Iranian desert cities and houses more comfortable to live in – all without the need for air conditioning or electricity. But this article focuses on one particularly effective and sophisticated technique, namely the Iranian windcatcher, which is a ventilation device known locally as badgir.

Buildings need to be ventilated in order to provide fresh air for the people inside. In hot climates air flow is also important for thermal comfort reasons because it makes hot temperatures more bearable. Before mechanical ventilation devices buildings had to rely on natural forces to create ventilation, and the same principles can be used to naturally ventilate buildings today. The Iranian windcatcher is an example of a device that uses these principles to naturally ventilate and cool buildings, and it does so in many different ways, depending on the design and the weather. The windcatcher is essentially a tower with one or more vertical shafts with openings facing different directions.

Windcatchers in Yazd
Windcatchers such as these are a common sight in Yazd in Iran.

Controlling wind to create comfort

As the name suggests, the windcatcher works best during windy weather. When wind hits a building, it creates pressure differences around the building which in turn creates air flow through the building. A positive pressure is created on the windward side and a negative pressure on the leeward side, meaning air flows in from the windward side and out through the leeward side. In urban environments wind is often blocked by other buildings, and this is certainly true in traditional Iranian desert towns in which buildings are clustered together to provide shading and protection from the sun. The windcatcher on the other hand is located above the buildings where there are much less obstructions. At this height the windspeed and thus also the wind pressure is higher, which means there is more potential for creating air flow through the building.

Windcatcher in Yazd
This Austrian guy I met in Yazd nearly had his beard pulled out by the power of the windcatcher.

When there is a prevailing wind direction, the windcatcher can consist of just one shaft facing the prevailing wind direction, but most windcatchers I saw while in Iran had multiple shafts with openings facing different directions. If all of these are open, air will flow in from the windward side and out from the leeward side. This can create a lot of air flow through the tower, but there is a risk that air will simply enter from one shaft and immediately exit through another, meaning there will be little air flow inside the main part of the building. This problem can be avoided by having a control system that makes it possible to close some shafts, depending on the current temperature and wind direction.

Windcatchers in Meybod
One-sided windcatchers such as these can work well when there is a prevailing wind direction but they have less functionality than windcatchers with multiple shafts and openings.

If only the windward side is open, air will flow into the house through the tower and out through lower openings. This may seem like the logical thing to do, but in some cases it can actually be more beneficial to have the leeward side open instead. If only the leeward side of the windcatcher is open, the tower will draw air out of the house, meaning air will flow in though lower openings. This can be more beneficial earlier in the day when shaded streets and courtyards are still cooler than the outdoor air above the house, thanks to the large mass of the thick walls that are common in traditional Iranian buildings. In the night the outdoor temperature drops considerably and then it is more beneficial to have the windward side open so that the cooler night air can come in and cool down the structure of the building.

House in Kashan with a windcatcher
The courtyards of traditional Iranian houses such as this one in Kashan improve thermal comfort together with the windcatcher.

Windcatchers and evaporative cooling

The cooling effect of windcatchers generally relies on the perceived cooling effect of air flow, but combining the windcatcher with water elements can actually lower the temperature of the air. Evaporation of water requires energy, and this energy comes from the water itself which cools down the water and consequently also the surrounding air. The added humidity can also improve comfort in the otherwise very dry climate. A simple way to connect windcatchers with water is to have a fountain under the windcatcher.

Another way to use water is connecting the windcatcher to an underground water channel. This type of water channels are common in Yazd and the surrounding towns because there is a network of underground aqueducts that were used to provide water to these desert cities. The air in the aqueduct is cooler than the outdoor air and if there is an opening between the aqueduct and the house so that air can flow from one to the other, a windcatcher can be used to draw air out of the aqueduct and through the house. The aqueduct is usually accessed from a basement and especially the basement becomes very cool this way.

Windcatcher in Yazd
The cooling effect of windcatchers can be enhanced by combining them with water elements such as this fountain located directly below a windcatcher.

The solar chimney and the potential of windcatchers

The Iranian windcatcher functions even in the absence of wind. It does this by using the heat of the sun to create temperature differences and taking advantage of a phenomenon known as the stack effect. Hot air is lighter than cold air and because of this hot air rises. The structure of the windcatcher is very exposed to the sun which means the structure and the air inside heats up during the day. This creates a temperature and pressure difference between the top and bottom of the tower and since hot air rises, air will flow up through the tower. In this case the windcatcher works like a chimney, ventilating hot air out of the building while cooler air is drawn in from lower openings. The taller the windcatcher is the higher the pressure difference between the top and bottom is and the more air flow there will be.

This same principle is used today in modern solar chimneys where the effect is enchanced through the use of materials that absorb more solar heat. There are many other ways in which windcatchers could be improved for modern use. Traditional windcatchers are not perfect but by understandig how the windcatchers work, we can use the same principles to passively ventilate and cool buildings even today, without the need for air conditioning.

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