Windows closed? Windows open? What’s the best way to keep your home cool in the heatwave?

It’s the UK’s hottest day on record, with temperatures that reached above 40C before 1pm. As a number of schools close early and rail passengers are advised to avoid all but essential travel, those of us who can are staying home to keep out of the heat. But, with everyone desperate to keep their homes cool, a heated debate is raging: should windows be open, or closed – and what about the curtains?

As our homes become increasingly hot and stuffy, the temptation is to open a window – but, counterintuitively, Dr Andrew Shea, a senior lecturer in building physics in the University of Bath’s department of architecture and civil engineering, argues this is all wrong.

“If it’s cooler inside than outside, you should keep your windows closed,” he says. Although it’s easy to imagine that flinging your windows wide might invite some hint of a cool and pleasant breeze, in all likelihood, you’ll just be bringing warm air in – making your living space even hotter. “If you have a shaded area at the back of the house, for example, you might be able to open a window there – but generally, you don’t want to open the windows. Just grab on to that last bit of cool air inside,” he says.

Dr Ana Souto, a principal lecturer at Nottingham Trent University’s school of architecture, design and built environment, says her real expertise on keeping cool comes from the fact that she is Spanish. “Close everything,” she advises. “You can open the windows late in the evening and early in the morning – until 9am or so – when it’s cool, but otherwise keep them closed. My place is like a cave, but it works.”

Dr Shea agrees that the time for open windows is overnight. “You can get apps which will tell you the temperature [inside and outside], so you can figure it out more scientifically, but at night-time, you could quite readily throw the windows open and get some cool air in.”

Amin Al-Habaibeh, professor of intelligent engineering systems within the product design team, also at Nottingham Trent University, suggests some exceptions to the windows closed rule: “Heat comes into the house in two ways: solar radiation from the sun, and hot air. We want to limit both, by closing the windows and curtains – but, if it reaches a point where the temperature inside the house is equal to that outside, then you should open the windows, keeping the curtains closed.”

The location of the windows can also have an impact. “Sometimes, on south-facing windows, closing the window and closing the curtain can create a greenhouse effect between the two – and this can become another source of heat,” says Prof Al-Habaibeh. “In this scenario, opening a window can help to reduce the heat dissipation from the south side.”

He also suggests that conservatory windows are always kept open during a heatwave – “otherwise it creates a greenhouse effect – and the door between the conservatory and the house should be kept closed.”

What about other justifications for opening a window – such as the idea that placing a wet towel or cloth in front of the airflow could help? Dr Shea remains sceptical. “It might work in principle, but I wouldn’t do it in these extreme weather conditions,” he says. “The evaporation of water does extract some heat from the air, but the degree of cooling wouldn’t be enough to make a difference. If your house has already reached unbearable temperatures then it might be a last resort.” The upshot? “You’re better off keeping the warm air out altogether.”