The aim: stop heat getting in, then let it out
Passive cooling means keeping a building comfortable using design and behaviour rather than powered cooling. It rests on three ideas. First, reduce the heat that enters. Second, remove heat that has built up. Third, use materials that absorb heat so the temperature rises more slowly. Each of the three headings in the title maps onto one of these.
It works best in climates where summer heat is moderate and nights cool down. In very hot and humid places, or during extreme heatwaves, passive measures may not be enough on their own, and it is important to look after people at risk of heat illness.
Shade: keep the sun off the building
Most unwanted summer heat arrives as sunshine through windows. Once sunlight passes through glass and strikes a floor or a wall, it turns into heat that stays inside. Blocking it before it reaches the glass is therefore more effective than trying to deal with it inside.
- External shading such as shutters, awnings, external blinds and overhanging eaves works best because the heat is stopped outside. The right design depends on the sun’s angle. A fixed overhang on a window that faces the sun at midday can shade it in summer when the sun is high, while still letting in low winter sun.
- Deciduous planting, such as a leafy tree or climbing plant on a trellis, gives shade in summer and lets light through in winter once the leaves drop. It takes years to establish, and roots can affect foundations if planted too close, so choose carefully.
- Internal blinds and curtains are less effective because the heat has already entered, but light-coloured, reflective or thickly lined ones still help. This is explored further in the article on window insulation, which also covers how the same curtains reduce winter heat loss.
- Reflective or light-coloured surfaces on roofs and walls absorb less sunlight. Where this is feasible, it may help, but benefits depend on climate and building.
Orientation matters. In the northern hemisphere, east- and west-facing windows often cause the most trouble in summer because the sun is low and strong in the morning and evening, and it is difficult to shade it with a simple overhang. In the southern hemisphere the directions reverse.
Airflow: remove heat that has built up
Moving air can take heat out of a house, but only when the outside air is cooler than the inside air.
- Night ventilation. In many climates, the air outside is cooler at night. Opening windows then, ideally on opposite sides of the building, lets cooler air flush out the day’s heat. Security and noise often limit this, so consider lockable window restrictors or vents.
- Cross-ventilation. Openings on opposite or adjacent sides let the wind pass through a space. A single-aspect flat is harder to cool this way.
- The stack effect. Warm air rises, so opening a high window or rooflight along with a low one can draw cooler air in and push warm air out, particularly in a stairwell.
- Closing up by day. If the outside air is hotter than the inside, opening windows brings heat in. A common approach is to keep windows and shades closed during the hottest hours and open them again as the evening cools.
- Fans. Fans do not lower the air temperature, but moving air helps sweat evaporate and can make people feel cooler. A fan generally uses much less electricity than air conditioning, though it is only helpful when someone is in the room. Fans can also be placed in a window to push cooler air in at night.
Draught proofing that suits winter can interfere with summer ventilation if it is overdone, so keep windows and vents that can be opened fully. The article on draught proofing explains how to seal unwanted gaps while keeping deliberate openings.
Thermal mass: slowing the temperature swing
Thermal mass is a material’s capacity to absorb and store heat. Heavy materials such as masonry, concrete, stone and brick take in heat slowly during the day and release it slowly at night. A building with exposed thermal mass can therefore smooth out daily temperature swings, staying cooler by day, so long as the stored heat can be released at night.
That last point is essential. Thermal mass only helps if it gets a chance to cool down, which means night ventilation. If a heavy building is closed up through a heatwave with warm nights, the mass can eventually warm to the point where it holds heat and releases it into rooms in the evening. Covering mass with carpets, plasterboard linings or furniture reduces its effect.
Lightweight buildings, including many modern timber-framed homes and top-floor flats under a roof, heat up and cool down quickly. They rely more on shade and ventilation, and good home insulation in the roof helps limit heat getting in from above.
Reducing internal heat
The house itself generates heat. Cooking, hot water, tumble dryers, dishwashers, lighting and electronics all add to the load. Small changes can help on hot days: cooking in the cool of the morning or evening, using a lid on pans, drying clothes outside, and switching off equipment that is not in use. Efficient lighting gives off less waste heat, as in LED lighting. Choosing more efficient household appliances also means less heat discharged into the room.
Mechanical help when needed
Passive measures can reduce or sometimes remove the need for air conditioning, but not always. If cooling machines are used, a smaller heat load means a smaller system and lower running costs. Some heat pumps can also provide cooling, which is covered in the article on how heat pumps work. For anyone vulnerable to heat, such as older people, young children, or those with certain health conditions, a safe temperature may take priority over saving energy, and following local health guidance during heatwaves is sensible.
A simple order of action
- Shade the windows that get the most sun, preferably outside.
- Keep windows and blinds closed while the day heats up, if the outside air is hotter.
- Open up and ventilate when the air cools, using cross-ventilation where possible.
- Cut down on heat-producing activities at the hottest time.
- Over the longer term, consider roof insulation, external shading and planting.
Results vary with climate, building type and orientation, so what works well in one home may do little in another. Treat these methods as a set of options to test rather than a guarantee of comfort in extreme heat.