Why painting your home white could help you survive heatwave

LONDON: At a seminar on building cooling strategies in the late 1990s, I vividly remember hearing that "in 30 years' time, the climate of London will feel like Marseille's today". That warning stuck with me. Back then, it sounded both alarming and oddly appealing.

Three decades on, it no longer feels theoretical. As a Londoner of Mediterranean origin, I've lived through the shift. When I co-wrote The Architecture of Natural Cooling, I drew not only on professional expertise but also on childhood memories of white walls, shady courtyards and shuttered windows.

These ancient techniques - once suited to the Mediterranean - now hold lessons for modern Britain, where heatwaves are becoming the new normal.

One of the simplest and most effective ways to cool a building is to change its colour. White surfaces reflect sunlight rather than absorb it, and studies show that painting roofs white or adding some other type of reflective coating can reduce the internal temperatures by more than 1°C and sometimes more than 4°C. They can even lower the surrounding outdoor temperatures by up to 2°C.

That might not sound like much, but across a whole city it can make a real difference, helping to counter the urban heat island effect (where human-made surfaces absorb heat and mean a city is hotter than the surrounding countryside) and keep homes more comfortable during the hottest hours of the day.

The success of these strategies, however, comes with a caveat. The more low-energy "passive" strategies - shutters, white buildings, ventilation and so on - we adopt in combination, the more likely they are to work effectively. A white roof, for example, is more effective if windows stay shut during the hottest hours, with shutters or external shades to keep the sun out.

If you close the windows, you will be better off with heavyweight walls and floors because the materials store coolness from the night air and release it through the day. That's one reason Mediterranean homes often stay comfortable for longer, even in extreme heat.

Night-time ventilation also plays a key role - at least if the air outside actually cools down after dark. In cities like London or Manchester, with a strong urban heat island effect, reflective roofs and avoiding the waste heat generated by air-conditioning units become even more crucial.

What about winter?
Some people may worry that a white roof might make their home colder in winter. But this is a very marginal problem, especially if the roof is well-insulated. How much you'll need to heat your home is driven by the ability of your home's outer shell to retain the heat that is already inside, rather than its ability to prevent heat from coming from outside.

In northern climates, winter sunlight is weak and often blocked by clouds. If, in a cold climate with sunny skies, you want to harness solar energy for warmth, it's more effective to let sunlight in through double-glazed windows than to rely on darker building materials.

A practical upgrade
Repainting your house white is not excessively expensive, at least compared to the big overall costs involved in heating and maintaining a home. Many homeowners, especially in suburban residential areas in the UK, already choose white finishes when refurbishing.

On flat or low-pitched roofs, reflective coatings can be applied at relatively low cost. For steeply pitched roofs, it is not possible to apply coats of paint as they would soon wear away and look terrible, requiring regular repainting. Tile roofs also need to "breathe" and let moisture out - paint could block this process, leading to damp problems.

The best option is to replace dark shingles or slate tiles with more reflective clay tiles that reduce the roof's surface temperature. This is a more time-consuming and expensive option, with costs in the UK starting from about GBP 125 per square metre of roof.

The climate is changing, and there's no getting away from it. Yet sometimes the best solutions aren't hi-tech or expensive.

A coat of white paint, combined with a few other simple design strategies, could help keep Britain's homes cooler, cheaper to run and better prepared for the climate changes and high energy prices expected in the decades ahead.