As the UK swelters through another heatwave — with the Met Office forecasting 'above average' temperatures continuing into August — millions of people are throwing open their windows as temperatures climb. But experts warn that instinct could be backfiring.
Opening windows between 11am and 6pm, when outside air is often hotter than indoor air, allows heat to flow in, acting like a convection oven, according to specialists. Instead, the Energy Saving Trust (EST) advises keeping windows firmly shut during the hottest part of the day, especially on sides facing direct sunlight, and closing curtains or blinds to block solar heat.
“Opening windows between 11am and 6pm can actually heat homes; experts recommend keeping them shut and using layered curtains.”
But not all curtains are equal. Kate O'Brien, Style Consultant at Swift Direct Blinds, has named 'window stacking' as an ideal method for protecting homes from excessive heat. The approach involves layering two or three different coverings — such as thermal curtains, blinds and sheer fabrics — to control indoor temperatures.
"We often associate 'thermal' with retaining the heat, but the barrier that they create also blocks heat and sunlight in the warmer months – as well as stopping already cool air from escaping the home," O'Brien said. She recommends light-coloured thermal fabrics as they absorb considerably less sunlight than darker options.
Shutters or blinds offer daytime flexibility: slats can be tilted to redirect sunlight or closed entirely to trap cool air. For rooms you want to keep light and airy, sheer curtains filter sunlight without plunging the room into darkness, O'Brien added.
The EST recommends opening windows when outside air is cooler than indoors — such as late evening, overnight or early morning — to allow cooler air to circulate and remove built-up heat. Opening windows on opposite sides of the house creates a cross-breeze, while opening internal doors improves airflow.
The UK Health Security Agency (UKHSA) advises limiting use of ovens and other heat-generating appliances during the hottest parts of the day and switching off unused electrical devices. Fans can help, but the UKHSA warns they shouldn't be relied on once indoor temperatures exceed 35C, making overnight ventilation crucial.
While it may seem counterintuitive, keeping windows and curtains closed on the sunny side during peak hours — before ventilating overnight — remains one of the most recommended strategies to keep indoor temperatures down.