Passive Cooling Systems For Sustainable Architecture: A Guide To The Best Options

A passive cooling system keeps a building comfortable using its design rather than machines: shading keeps the sun’s heat out, ventilation and night flushing carry heat away, and thermal mass, evaporation and planting absorb or remove the rest. The EU’s BUILD UP portal defines it as an approach that uses building design and materials to keep a comfortable temperature without mechanical or electrical devices. Which strategies work depends heavily on climate. This guide explains each one, shows where it suits with a climate-applicability table, and sets out what the evidence says about how effective passive cooling really is.

It is written for architects and students who need to choose strategies for a real project. Understanding the local climate, the surroundings and the way a building will be used is the starting point for passive design and for sustainability more broadly.

With mechanical air conditioning we can regulate almost any building artificially, but the energy and emissions cost of doing so is large. Passive design uses natural conditions to regulate temperature and energy use first, so that any active system that remains can be smaller. Passive cooling is one piece of a larger toolkit, and our introduction to sustainable architecture shows how it fits with net-zero energy goals and passive house design.

What is the meaning of passive cooling?

Passive design uses natural elements to reduce the reliance on artificial climate control. A building’s layout, materials, openings and orientation are arranged to improve comfort and cut energy use, which makes buildings more eco-friendly when the approach is widely adopted. Passive cooling is the part of passive design aimed at overheating: it reduces heat gains and removes heat without mechanical systems. It can be designed into new buildings and, to a lesser degree, retrofitted to existing ones.

Passive cooling strategies

Different strategies tackle different parts of the problem. Some stop heat getting in, some move heat out, and some store it and release it later. Most good designs combine several of them into one passive cooling system.

Shading

The oldest and most reliable method is to stop the sun’s heat reaching the interior. A well-designed shading system can admit daylight and preserve privacy while preventing overheating and glare. External shading is generally more effective than internal blinds, because it stops solar radiation before it passes through the glass. A building can also shade itself through its form, with deep reveals, overhangs and courtyards.

Window shades can be horizontal, vertical, recessed, seasonal or fixed. Horizontal overhangs suit facades facing the midday sun, while vertical fins or adjustable shades suit low morning and evening sun. The glazing and frame also matter, and deciduous trees can provide seasonal shade: the US Department of Energy recommends trees that shade windows and walls in summer as one of the simplest ways to keep a home cool.

Convection and natural ventilation

Cooling by convection relies on two forces: wind pressure, and the buoyancy of warm air, which rises and is replaced by cooler air. Natural ventilation uses these forces to move air through a building without fans. Openings such as clerestory windows, roof ventilators, wind scoops and windcatchers can be arranged to exploit them. Moving air also cools people directly by increasing evaporation from the skin, which is why air movement is the main cooling tool in hot, humid climates.

Cross ventilation

Cross ventilation lets air flow through a building from openings on one side, such as windows, to openings on the opposite side. It works best in narrow plans, with openings facing the prevailing breeze and an outlet at least as large as the inlet. It improves comfort and air quality, but it only cools the building when the outdoor air is cooler than the indoor air, so in hot-dry climates it is mainly a night-time strategy.

Stack effect and solar chimneys

The stack effect occurs when warm air rises and leaves through high openings, drawing cooler air in through low openings. The taller the vertical distance between inlet and outlet, and the larger the temperature difference, the stronger the flow. Atriums, stairwells and double-height spaces can act as stacks. The US Department of Energy notes that openings at higher levels let rising hot air escape.

A solar chimney strengthens the stack effect by heating a column of air with the sun, so that it rises faster and pulls fresh air through the building. For background on how buildings harness the sun in other ways, see our overview of active and passive solar systems. Solar chimneys are also called thermal chimneys or thermosiphons.

Thermal chimneys can be combined with evaporative or earth cooling of the incoming air. They can also help with heating, in a similar way to a Trombe wall: if the top exterior vents are closed and high interior vents are opened, the warmed air circulates into occupied spaces instead of escaping.

Windcatchers

The windcatcher is one of the oldest forms of passive cooling system. Windcatchers (badgir in Persian) have been used for centuries in Iran, the Gulf and North Africa. They are towers, traditionally of brick or stone, with openings at the top. Openings facing the wind catch the breeze and drive it down into the building, while openings on the leeward side draw warm air out by suction. Many traditional examples pass the air over water, such as a pool or an underground channel, so that evaporation cools it further.

How much cooling a windcatcher delivers depends on wind speed, humidity and the design. Claims that windcatchers lower indoor temperatures by around 10 °C are common, but the highest figures usually come from computer models of windcatchers combined with evaporative cooling. One 2018 modelling study in the journal Energies reported about 11 °C of cooling for a 4 m windcatcher with wetted columns. Treat such figures as the potential of a well-designed system in dry heat, not a guaranteed result.

Insulation

Insulation is usually discussed for heating, but it also slows heat flowing into a building in hot weather, especially through roofs exposed to strong sun. It works best as continuous insulation that wraps the building without thermal bridges. Thermal bridges create cold spots where moisture can condense in heated buildings, leading to mould and decay. In cooling climates, insulation must be combined with shading and ventilation, otherwise heat gained through windows and from occupants can become trapped inside.

Night cooling (night flushing)

Night cooling uses cool night air to remove the heat a building has absorbed during the day. Windows, vents and doors are opened at night, the air flushes heat out of exposed floors and walls, and the cooled structure then absorbs heat the following day. The UK reference site Designing Buildings states that night purging can reduce daytime internal temperature rises by around 3–6 °C in the UK, needs significant exposed thermal mass, and works only where the day–night temperature range is large, with an absolute minimum of about 5 °C. Security, noise and outdoor air quality can rule it out in some locations.

A well-documented example is the Queens Building at De Montfort University in Leicester, completed in 1993 by Short Ford Associates with the engineer Max Fordham. Its passive cooling system combines rows of ventilation stacks, exposed brick, block and concrete surfaces, and night-time cooling; design studies predicted that a heavyweight fabric with summer night cooling would keep internal temperatures within a few degrees of the design target.

Evaporative cooling

When water evaporates it absorbs heat, lowering the air temperature and raising humidity. Traditional Persian and Egyptian architecture used this for centuries.

Windcatchers directed air over underground water channels, and courtyard houses used planting, fountains and pools to cool the air. Roof ponds and water curtains work on the same principle. Evaporative cooling depends on dry air: the US Pacific Northwest National Laboratory notes that evaporative coolers work well in dry areas because the gap between air temperature and wet-bulb temperature is large. In humid climates the effect largely disappears and the extra moisture makes conditions worse.

Thermal mass and roof space

Heavy materials such as concrete, brick, stone and earth absorb heat slowly and release it slowly. In a climate with hot days and cool nights, exposed thermal mass absorbs daytime heat and releases it at night, when ventilation can carry it away. Australia’s government guide Your Home notes that thermal mass can help cool homes where the day–night temperature difference is about 6 °C or more. A ventilated roof space or loft also helps: the hottest air collects at the top of the building, away from occupants, and can be vented out.

Traditional adobe buildings show the principle clearly. At Taos Pueblo in New Mexico, multi-storey dwellings of thick sun-dried earth walls absorb the heat of the day and release it during the cool desert night.

Greening

Trees, shrubs and planting moderate air temperature and humidity, add shade and support biodiversity. Vegetation also absorbs solar radiation, provides shade and releases moisture through transpiration. Courtyard gardens, green walls and planted roofs can all contribute; for more on roof planting, read about the benefits of green roofs. Like other evaporative strategies, the cooling effect of planting is strongest in dry heat.

Climate applicability: which strategy suits which climate

The table summarises how each strategy performs in three broad climate types, based on the government and research sources cited in this article. Use it as a first filter when choosing a passive cooling system, then test the design for your site with climate data or energy simulation.

How each strategy works: section diagrams

The four mechanisms that do most of the work in passive cooling are easiest to understand in section. Cross ventilation moves air horizontally from the windward to the leeward side. The stack effect moves it vertically, from low inlets to high outlets. Night flushing moves cool night air across exposed thermal mass. Evaporative cooling passes air over water or planting before it enters the occupied space.

How effective is a passive cooling system? Evidence and limits

Passive cooling can make a large difference, but it is not unlimited:

• It can rival active systems in the right conditions. BUILD UP cites research showing that under specific climatic conditions some passive or quasi-passive methods achieve temperature reductions comparable to active systems.
• Night flushing has measured benefits. The UK figures of 3–6 °C lower daytime temperature rise depend on exposed thermal mass and cool nights.
• Climate sets the ceiling. Evaporative cooling and thermal mass lose most of their effect in humid climates with small day–night swings, where air movement and shading become the main tools.
• Extreme heat needs back-up. BUILD UP concludes that passive cooling alone may not be sufficient in areas hit by strong heatwaves, and recommends combining it with efficient active cooling such as heat pumps. Its effectiveness may also fall as climates warm.

Pros and cons of passive cooling systems

Pros

• Lower energy use and running costs, because a passive cooling system uses little or no electricity once built.
• Better comfort and air quality when ventilation brings in fresh air.
• Smaller active systems where some mechanical cooling is still needed.
• Resilience: shaded, well-ventilated buildings stay more tolerable during power cuts.

Cons

• Strongly climate-dependent: the wrong strategy for the climate does little or harms comfort, as with evaporative cooling in humid air.
• Design constraints: cross ventilation needs narrow plans and opposite openings, and night flushing needs exposed mass and secure openings.
• Noise, security and outdoor pollution can prevent windows being opened.
• Harder to retrofit: form, orientation and mass are difficult to change in existing buildings.
• Occupants must operate openings and shades correctly, or the building will underperform.

Passive cooling in homes without air conditioning

Communities that live without air conditioning show these strategies working together. Many Amish homes, for example, rely on the same passive strategies described above. A Lancaster County visitor centre describes plenty of windows to circulate air and admit cooler night air, shade trees around the house, and separate summer kitchens that keep cooking heat out of the living spaces. Practices vary between communities.

Choose a cooling strategy by its operating conditions

Begin with the outdoor temperature, humidity, night-time conditions, noise and air quality, rather than selecting a technique from a photograph. Night ventilation needs sufficiently cool outdoor air and safe openings; thermal mass needs a way to release stored heat; direct evaporative cooling is constrained by humidity and water quality. Shading should reduce unwanted solar gain before equipment is sized. Write down the conditions in which occupants can use each measure and the fallback for hot, polluted or windless periods. Evaluate the combined design with climate data and an appropriate thermal model instead of adding advertised savings together.

Decision summary and next step

Start every passive cooling system with shading, because heat that never enters needs no removal. Then match the rest to climate: air movement in hot-humid climates; night flushing, thermal mass and evaporative cooling in hot-dry climates; and shading with night ventilation in temperate climates. Check performance for your site with simulation, and plan efficient active back-up for heatwaves.