During a heatwave, leaving windows open all day can make your home hotter.
The best rule is simple: open windows when the air outside is cooler than indoors, then close them before outdoor temperatures rise.
Early morning is usually one of the best times to ventilate your home. If the outdoor air feels cooler than the air inside, open several windows to let trapped heat escape.
If possible, open windows on opposite sides of the home to create cross ventilation.
Once it becomes warmer outside than inside, close your windows. Keeping them open during the hottest part of the day can allow more hot air into the house.
Close curtains, blinds or shutters too, especially on windows receiving direct sunlight.
When outdoor temperatures begin to fall, reopen the windows. Evening ventilation can help release heat that has built up indoors during the day.
However, do not rely only on the time. During severe heatwaves, evenings can remain very warm. Only open windows when it is cooler outside than inside.
Morning: Open windows while it is cooler outside.
Daytime: Close windows and block direct sunlight.
Evening: Reopen windows when outdoor temperatures drop.
Fans can make you feel cooler by moving air, although they do not actually lower the room temperature. In the evening, a fan near an open window may also help improve airflow.
During a heatwave, windows should not stay open all day. The key is timing: if it is hotter outside than inside, keep windows closed. If it is cooler outside, open them and ventilate.