How to ventilate your home during a heatwave

Most homes without air conditioning have more room to manoeuvre than people think. The difference between a flat that stays bearable and one that becomes an oven is rarely the building itself. It is timing: which hours you open, which hours you shut, and what you do about the sun hitting the glass.

The one rule underneath everything

Open when the air outside is cooler than the air inside. Close when it is warmer.

That sounds obvious, but it is the opposite of what most people do. The instinct on a hot day is to throw the windows open for fresh air. At 3 pm in a heatwave, that instinct actively heats your home. Every hour a window stands open against 35 °C air, your rooms move closer to 35 °C.

The corollary matters just as much: when the outdoor temperature finally drops below your indoor temperature, usually well after sunset, that is your window of opportunity. It may only last a few hours, and it is the only free cooling you get.

Three things to get right

Shading. Sun coming through glass is a much bigger heat source than warm air leaking in. Blocking it outside the window is far more effective than blocking it inside. See protecting your windows from the sun.

Timing. The pattern over a heatwave is not a single decision but a daily rhythm of keeping heat out, flushing heat at night, and holding the cool you gained. Start with indoor versus outdoor temperature, then see the three phases of heat. If you have air conditioning, make that comparison before you turn it on. If the building remains warm after the forecast cools, follow the after-heatwave recovery guide.

Airflow. Where you put a fan depends entirely on whether your home has windows on one side or on opposite sides. The right placement moves a lot of air. The wrong one just makes noise. See where to place a fan. Layout, courtyards, top floors, and night flush are unpacked in how your apartment layout sets the ceiling.

Air worth letting in. Cooler outdoor air can still be dirty, full of pollen, glaring through a sunny pane, or too sticky. Temperature still leads. See when cooler air is not worth letting in.

Your layout sets the ceiling

A flat with windows on two opposite façades can be flushed of hot air in minutes once outdoor air is cooler, because air has somewhere to go. A single-sided flat, with all its windows on one façade, has no natural through-draught and depends much more on fan placement and patience.

Neither is a lost cause. But knowing which one you have changes what you should do, and it is the first thing to work out before you start moving fans around. More in apartment layouts.

Safety first

None of this replaces medical judgement. If someone is showing signs of heat exhaustion, confusion, or heatstroke, ventilation advice is not the priority. Follow your local health agency’s guidance and get help.