A traditional Iranian solar cooling design using a wind tower

Passive cooling is a building design approach that focuses on heat gain control and heat dissipation in a building in order to improve the indoor thermal comfort with low or no energy consumption.[1][2] This approach works either by preventing heat from entering the interior (heat gain prevention) or by removing heat from the building (natural cooling).[3]

Natural cooling utilizes on-site energy, available from the natural environment, combined with the architectural design of building components (e.g. building envelope), rather than mechanical systems to dissipate heat.[4] Therefore, natural cooling depends not only on the architectural design of the building but on how the site's natural resources are used as heat sinks (i.e. everything that absorbs or dissipates heat). Examples of on-site heat sinks are the upper atmosphere (night sky), the outdoor air (wind), and the earth/soil.

Passive cooling is an important tool for design of buildings for climate change adaptation – reducing dependency on energy-intensive air conditioning in warming environments.[5][6]


Passive cooling covers all natural processes and techniques of heat dissipation and modulation without the use of energy.[1] Some authors consider that minor and simple mechanical systems (e.g. pumps and economizers) can be integrated in passive cooling techniques, as long they are used to enhance the effectiveness of the natural cooling process.[7] Such applications are also called 'hybrid cooling systems'.[1] The techniques for passive cooling can be grouped in two main categories:

Preventive techniques

This ancient Roman house avoids gaining heat. Heavy masonry walls, small exterior windows, and a narrow walled garden oriented N-S shade the house, preventing heat gain. The house opens onto a central atrium with an impluvium (open to the sky); the evaporative cooling of the water causes a cross-draft from atrium to garden.

Protection from or prevention of heat gains encompasses all the design techniques that minimizes the impact of solar heat gains through the building's envelope and of internal heat gains that is generated inside the building due occupancy and equipment. It includes the following design techniques:[1]

Modulation and heat dissipation techniques

The modulation and heat dissipation techniques rely on natural heat sinks to store and remove the internal heat gains. Examples of natural sinks are night sky, earth soil, and building mass.[11] Therefore, passive cooling techniques that use heat sinks can act to either modulate heat gain with thermal mass or dissipate heat through natural cooling strategies.[1]


A pair of short windcatchers (malqaf) used in traditional architecture; wind is forced down on the windward side and leaves on the leeward side (cross-ventilation). In the absence of wind, the circulation can be driven with evaporative cooling in the inlet. In the center, a shuksheika (roof lantern vent), used to shade the qa'a below while allowing hot air rise out of it (stack effect).[13]

Ventilation as a natural cooling strategy uses the physical properties of air to remove heat or provide cooling to occupants. In select cases, ventilation can be used to cool the building structure, which subsequently may serve as a heat sink.

These two strategies are part of the ventilative cooling strategies.

One specific application of natural ventilation is night flushing.

Night flushing

A courtyard in Florence, Italy. It is tall and narrow, with a fountain spouting very thin streams of water at the bottom, and upper rooms opening onto it. Night flushing of the courtyard happens automatically as the night air cools; evaporative cooling cools it further and can be used to create drafts and change the air during the day. Windows can be left open around the clock.

Night flushing (also known as night ventilation, night cooling, night purging, or nocturnal convective cooling) is a passive or semi-passive cooling strategy that requires increased air movement at night to cool the structural elements of a building.[15][16] A distinction may be made between free cooling to chill water and night flushing to cool down building thermal mass. To execute night flushing, one typically keeps the building envelope closed during the day. The building structure's thermal mass acts as a sink through the day and absorbs heat gains from occupants, equipment, solar radiation, and conduction through walls, roofs, and ceilings. At night, when the outside air is cooler, the envelope is opened, allowing cooler air to pass through the building so the stored heat can be dissipated by convection.[17] This process reduces the temperature of the indoor air and of the building's thermal mass, allowing convective, conductive, and radiant cooling to take place during the day when the building is occupied.[15] Night flushing is most effective in climates with a large diurnal swing, i.e. a large difference between the daily maximum and minimum outdoor temperature.[18] For optimal performance, the nighttime outdoor air temperature should fall well below the daytime comfort zone limit of 22 °C (72 °F), and should not have low absolute or specific humidity. In hot, humid climates the dirunial temperature swing is typically small, and the nighttime humidity stays high. Night flushing has limited effectiveness and can introduce high humidity that causes problems and can lead to high energy costs if it is removed by active systems during the day. Thus, night flushing's effectiveness is limited to sufficiently dry climates.[19] For the night flushing strategy to be effective at reducing indoor temperature and energy usage, the thermal mass must be sized sufficiently and distributed over a wide enough surface area to absorb the space's daily heat gains. Also, the total air change rate must be high enough to remove the internal heat gains from the space at night.[17][20] There are three ways night flushing can be achieved in a building:

These three strategies are part of the ventilative cooling strategies.

There are numerous benefits to using night flushing as a cooling strategy for buildings, including improved comfort and a shift in peak energy load.[22] Energy is most expensive during the day. By implementing night flushing, the usage of mechanical ventilation is reduced during the day, leading to energy and money savings.

There are also a number of limitations to using night flushing, such as usability, security, reduced indoor air quality, humidity, and poor room acoustics. For natural night flushing, the process of manually opening and closing windows every day can be tiresome, especially in the presence of insect screens. This problem can be eased with automated windows or ventilation louvers, such as in the Manitoba Hydro Place. Natural night flushing also requires windows to be open at night when the building is most likely unoccupied, which can raise security issues. If outdoor air is polluted, night flushing can expose occupants to harmful conditions inside the building. In loud city locations, the opening of windows can create poor acoustical conditions inside the building. In humid climates, night flushing can introduce humid air, typically above 90% relative humidity during the coolest part of the night. This moisture can accumulate in the building overnight leading to increased humidity during the day leading to comfort problems and even mold growth.

Radiative cooling

Main article: Radiative cooling

The infrared atmospheric window, frequencies in which the atmosphere is unusually transparent, is the large blueish block on the right. An object that is fluorescent in these wavelengths can cool itself to below ambient air temperature.
Radiative cooling energy budget in Iranian Architectural element, yakhchāl

All objects constantly emit and absorb radiant energy. An object will cool by radiation if the net flow is outward, which is the case during the night. At night, the long-wave radiation from the clear sky is less than the long-wave infrared radiation emitted from a building, thus there is a net flow to the sky. Since the roof provides the greatest surface visible to the night sky, designing the roof to act as a radiator is an effective strategy. Types of radiative cooling strategies that utilize the roof surface include direct and indirect,[11] and fluorescent.

Evaporative cooling

A salasabil (currently dry) in the Red Fort in Delhi, India. A salasabil is designed to maximize evaporative cooling; the cooling, in turn, may be used to drive air circulation.

Main article: Evaporative cooling

This design relies on the evaporative process of water to cool the incoming air while simultaneously increasing the relative humidity. A saturated filter is placed at the supply inlet so the natural process of evaporation can cool the supply air. Apart from the energy to drive the fans, water is the only other resource required to provide conditioning to indoor spaces. The effectiveness of evaporative cooling is largely dependent on the humidity of the outside air; dryer air produces more cooling. A study of field performance results in Kuwait revealed that power requirements for an evaporative cooler are approximately 75% less than the power requirements for a conventional packaged unit air-conditioner.[27] As for interior comfort, a study found that evaporative cooling reduced inside air temperature by 9.6 °C compared to outdoor temperature.[28] An innovative passive system uses evaporating water to cool the roof so that a major portion of solar heat does not come inside.[29]

Ancient Egypt used evaporative cooling;[13] for instance, reeds were hung in windows and were moistened with trickling water.[30]

Evaporation from the soil and transpiration from plants also provides cooling; the water released from the plant evaporates. Gardens and potted plants are used to drive cooling, as in the hortus of a domus, the tsubo-niwa of a machiya, and so on.

Earth coupling

A qanat and windcatcher used as an earth duct, for both earth coupling and evaporative cooling. No fan is needed; the suction in the lee of the windtower draws the air up and out.

Main article: Ground-coupled heat exchanger

Earth coupling uses the moderate and consistent temperature of the soil to act as a heat sink to cool a building through conduction. This passive cooling strategy is most effective when earth temperatures are cooler than ambient air temperature, such as in hot climates.

In conventional buildings

Further information: Green building

There are "smart-roof coatings" and "smart windows" for cooling that switches to warming during cold temperatures.[32][33] The whitest paint formulation can reflect up to 98.1% of sunlight.[34]

See also


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