As temperatures in Wales and parts of the UK are set to reach 32°C this weekend, the question of whether to open or close windows during a heatwave has resurfaced. Experts across the board agree: keep windows, doors, blinds, and curtains closed during the hottest parts of the day to minimise solar heat gain and maintain a cooler indoor environment.
Why Closing Windows Works
Tina Loveland, Senior Designer at Appeal Shading, explains: "Once sunlight has passed through a window and converted to heat inside a room, you're fighting a losing battle. Blinds and shutters intercept that heat before it reaches your living space, which is proven to actually reduce heat build-up." She notes that a quality fitted blind on a sun-facing window can cut solar heat gain by 40-70%, with light-coloured or reflective fabrics working best by bouncing energy back through the glass. Dark blinds, by contrast, absorb heat and can worsen the situation.
Conservatories are particularly vulnerable, often reaching 40°C or above during a heatwave without roof blinds, radiating heat downwards into the house. Keeping conservatory blinds closed is essential.
When to Open Windows
Opening windows during a heatwave feels intuitive, but experts warn against it when outdoor temperatures are higher than indoor ones. Tina Loveland states: "If it's hotter outside than inside, opening your windows will warm your home up, not cool it down. During a UK heatwave, outdoor temperatures can stay above 30°C from mid-morning until early evening. If your home is sitting at 24°C or 25°C, cracking a window will actually just let hotter air in."
NHS and Public Health England guidance aligns with this: keep windows in sun-exposed rooms closed during the hottest part of the day. The recommended approach is to treat your home like a cool box—sealed and shaded until the temperature outside drops. Tina advises opening windows and blinds once outdoor temperatures fall below indoor levels, typically from around 9pm or 10pm in peak summer, and creating a cross-draught by opening windows on opposite sides of the house to flush out accumulated heat.
Additional Expert Tips
Max Wilson, co-founder of Pocket Storage, suggests treating your house like a fridge between 10am and 6pm by keeping everything on the sunny side closed. He warns: "Opening your windows will either cool or warm up your house depending on how warm it is outside. The major mistake most homeowners make is opening all their windows and doors for the entire day. The problem is, opening windows and doors when it is warmer outdoors is no different from opening a door to an oven."
Wilson also highlights that UK houses are built to conserve heat, with insulating materials, furniture, and walls absorbing heat all day and releasing it at night. He recommends clearing clutter from windows and doors to improve air circulation and ensuring a clear path between opposite windows to create airflow.
John Klee, a heating technology expert at BestHeating, suggests a simple trick: placing a bowl of ice in front of a fan can circulate a cool breeze rather than redistributing warm air. Edward Simon, director at Direct Fabrics, advises using internal doors deliberately—closing off sun-exposed rooms during the day to prevent heat drifting into cooler spaces, and opening them at night to clear stored heat. He also recommends reducing indoor heat sources during peak hours, such as oven cooking, tumble drying, or running multiple heat-producing appliances, to avoid tipping a warm home into an uncomfortable one.
