Natural ventilation

Natural ventilation systems rely on pressure differences to move fresh air through buildings - pressure differences can be caused by wind or the buoyancy effect created by temperature differences or differences in humidity. 

In either case, the size and placement of openings in the building is critical - useful to think of a natural ventilation system as a circuit, with equal consideration given to supply and exhaust. 

Wind causes a positive pressure on the windward side and a negative pressure on the leeward side of buildings. To equalize pressure, fresh air will enter any windward opening and be exhausted from any leeward opening. 

Buoyancy ventilation may be temperature-induced (stack ventilation) or humidity induced (cool tower). The two can be combined by having a cool tower deliver evaporatively cooled air low in a space, and then rely on the increased buoyancy of the humid air as it warms to exhaust air from the space through a stack.

NB. cool towers should be used in conjunction with stack ventilation of the space in order to ensure stability of the flow. 

Buoyancy results from the difference in air density:  cool air is heavier than warm, and dry air is heavier than humid at the same temperature. 

Within a cool tower, the effect of temperature and humidity are pulling in opposite directions (temperature down, humidity up). 

Within a room, heat and humidity given off by occupants and other internal sources both tend to make air rise. The stale, heated air escapes from openings in the ceiling or roof and permits fresh air to enter lower openings to replace it. 

  

Stack effect ventilation is effective in winter, when indoor/outdoor temperature difference is at a maximum but not so effective in summer (wind or humidity drivers would be preferred) because it requires that the indoors be warmer than outdoors (not desirable in summer). However, a chimney heated by solar energy can be used to drive the stack effect without increasing room temperature.

NB. Cool tower ventilation is only effective where outdoor humidity is very low. 

Solar Chimney (or thermal chimney)

= a way of improving the natural ventilation of buildings by using convection of air heated by passive solar energy. A simple description of a solar chimney is that of a vertical shaft utilizing solar energy to enhance the natural stack ventilation through a building.

Windcatcher

(1) Downward in flow of air due to direct wind entry

The badgir = an architectural feature to cool the inside of the dwelling, often used in combination with courtyards and domes as an overall ventilation / heat management strategy. 

The malqaf =  essentially a tall, capped tower with one face open at the top. This open side faces the prevailing wind, thus 'catching' it, and bringing it down the tower into the heart of the building to maintain air flow, thus cooling the interior of the building. This is the most direct way of drawing air into the building, but importantly it does not necessarily cool the air, but relies on a rate of air flow to provide a cooling effect. 
This use of the malqaf or windcatcher has been employed in this manner for thousands of years, as detailed by contemporary Egyptian architect Hassan Fathy.

(2) Upward flow of air due to a wind assisted temperature gradient

 NB. qanat = underground canal. The open side of the tower faces away from the direction of the prevailing wind so air is drawn upwards using the Coandă effect, similar to how opening the one facing towards the wind would pull air down into the shaft.

As there is now a pressure differential on one side of the building, air is drawn down into the passage on the other side. This hot air is brought down into the qanat tunnel, and is cooled by the combination of coming into contact with the cold earth (as it is several meters below ground, the earth stays continuously cool) as well as the cold water running through the qanat. The air is therefore cooled significantly, and is then drawn up through the windcatcher by the same Coandă effect. This brings cool air up through the building, cooling the structure overall, with the water vapour from the qanat having an added cooling effect.

(3) Upward flow of air due to a solar produced temperature gradient

Finally, in a windless environment or waterless house, a windcatcher functions as a solar chimney. 

It creates a pressure gradient which allows less dense hot air to travel upwards and escape out the top. This is also aided by the day-night cycle, trapping cool air below; temperature in such an environment cannot drop below the nightly low temperature. 

When coupled with thick adobe that exhibits good heat transmission resistance qualities, the windcatcher is able to chill lower level spaces in mosques and houses (e.g. shabestan) in the middle of the day to frigid temperatures (windcatchers in Persian architecture have been routinely used as a refrigerating device for ages). The evaporative cooling effect is strongest in the driest climates.

General design considerations

• Maximize wind-induced ventilation by siting the ridge/long facade of a building perpendicular to the summer winds.
• Naturally ventilated buildings should be narrow.
• Each room should have two separate supply and exhaust openings.
•  Locate exhaust high and inlet low - to maximize stack effect. 
• Orient windows across the room and offset from each other to maximize mixing within the room while minimizing the obstructions to airflow within the room.
• Window openings should be operable by the occupants.
• Provide ridge vents (an opening at the highest point in the roof that offers a good outlet for both buoyancy and wind-induced ventilation).
• Consider the use of clerestories or vented skylights.
• good for night time thermal comfort in houses to vent heated/warm air that rises, and allow heat to be radiated into the cold; also a good outlet for wind driven ventilation
•  Allow for adequate internal airflow
• Consider the use of fan-assisted cooling strategies
• Determine if the building will benefit from an open- or closed-building ventilation approach.
• A closed-building approach works well in hot, dry climates where there is a large variation in temperature from day to night. A massive building is ventilated at night, then, closed in the morning to keep out the hot daytime air. Occupants are then cooled by radiant exchange with the massive walls and floor.
• An open-building approach works well in warm and humid areas, where the temperature does not change much from day to night. In this case, daytime cross-ventilation is encouraged to maintain indoor temperatures close to outdoor temperatures.
• Use mechanical cooling in hot, humid climates.
• Try to allow natural ventilation to cool the mass of the building at night in hot climates.
• Open staircases provide stack effect ventilation, but observe all fire and smoke precautions for enclosed stairways.
• Increase air supply intake by ensuring no outside obstruction (such as vegetation or site objects) nor inside obstruction (such as furniture and interior partition) obstruct inlet openings
• Have at least a 3m height between floor and ceiling (?)
• Window areas should not be excessive and be protected by exterior shading devices
• Reduce the possibility of wall warming by the sun:
• light-coloured building exteriors
• trees/shrubs to provide shading and evaporative cooling
• grass and other groundcover to keep ground temperatures low
• ponds and fountains to enhance evaporative cooling

Many of the above either increase the air flow or lower the heat gain so that the natural ventilation can effectively cool the spaces in the building. 

Mechanical cooling and ventilation systems will be used to supplement the natural ventilation. By lowering the heat gains, the less air flow will be required to remove the heat, thus there will be a lesser need for mechanical cooling systems.