To close your windows or not to close your windows: that is the question. In a heatwave, the best thing to do is to keep your windows and blinds closed during the day. That's because it's likely to be hotter outside than it is inside, and you want to prevent hot air and sunlight from getting into your home. At nighttime, you can open your windows to let the cooler air filter through your house. Makes a lot of sense, right?