During a UK heatwave, you should close windows and doors during the day once outdoor air becomes hotter than indoor air. This prevents hot air from entering and helps retain cooler indoor temperatures. Most British homes are designed to retain heat, so opening windows when it is hotter outside will allow warm air to enter and raise internal temperature further [1].

The strategy works in two phases. During the hottest part of the day, keep windows and doors closed and use blinds, curtains, or external shutters on sun-facing windows to block direct sunlight and solar heat gain. Reflective or light-coloured blinds are more effective than standard dark curtains because they reflect rather than absorb solar radiation. You can also seal obvious gaps around doors and windows with draught excluders to prevent hot air infiltration.

Once outdoor temperature drops below indoor temperature—typically in the evening or at night—open windows to allow cooler air to circulate and flush out stored heat. Creating a cross-breeze by opening windows on opposite sides of the house makes this "night purge" more effective. Close windows again before dawn when outside air begins warming.

Electric fans can help increase comfort and air circulation, but only when indoor temperature is below approximately 35°C; above that threshold, fans may be ineffective or unsafe as they can increase dehydration by blowing hot air over the body. Reduce internal heat generation by avoiding ovens, tumble dryers, and unnecessary electronics during the hottest hours. If you have air conditioning, operate it with all windows and doors closed to maximize efficiency. For vulnerable populations—elderly people, infants, and those with health conditions—monitor carefully and seek cooler public spaces such as libraries or supermarkets if your home becomes dangerously hot.