Should You Open Windows During a Heatwave?
When the air inside feels like soup, every instinct says fling the windows open. During a heatwave, that instinct is usually wrong — and it can leave your home both hotter and damper than before. The right move depends on two numbers, not on how stuffy you feel.
Why opening at the wrong time backfires
Air is a delivery service. Open a window and you take delivery of whatever is outside: its heat and its water. At 4 pm on a heatwave day, the outdoor air is almost always hotter than your indoor air. Opening simply imports heat, and the water vapour riding along with it soaks into your walls, sofa and bedding.
Meanwhile, your home has been quietly playing defence. Walls, floors and furniture warm up slowly — that is thermal inertia — so a shut, shaded home can sit several degrees below the outdoor peak all afternoon. Open at the wrong hour and you throw that head start away in minutes.
The two-number rule
Forget gut feeling. Open your windows only when both of these are true:
- It is genuinely cooler outside than inside (thermometer, not vibes).
- The outdoor air carries less water than the indoor air — measured in absolute humidity (grams of water per cubic metre), not in percent.
If either condition fails, shut everything and wait. Cooler-but-wetter air trades one problem for another; hotter-but-drier air is still hotter. Only cooler and drier air actually improves your home.
The “35% humidity” trap
Weather apps show relative humidity, and it lies to you in summer. A percentage only says how full the air is for its temperature — and hot air is a far bigger sponge. Afternoon air at 32 °C and 35% RH often contains more water than cool evening air at 80% RH. Airing “because the air is dry right now” is how people steam their own homes at the hottest hour of the day. The full story is in relative vs absolute humidity.
A heatwave day, hour by hour
| Time | Move | Why |
|---|---|---|
| Night → early morning | Open wide, create a cross-draft | Coolest, usually driest air of the day |
| Mid-morning | Shut everything | Outside starts beating inside |
| Afternoon | Stay shut, shutters and blinds down | Block solar gain; your walls are your cooler |
| Early evening | Resist the urge | It feels cooler out, but often isn’t yet |
| Late evening | Open again once outside truly drops below inside | Start the night-flush |
The evening line is where most people slip. Right after sunset the air outside can still be warmer than your interior, and often more humid too. Wait for the crossover, then open big — night cooling does the heavy lifting during a heatwave.
What “shut” really means
Shut is more than closed glass. Sunlight through a window heats a room like a slow grill, so daytime defence is windows closed and shaded — shutters outside if you have them, blinds or thick curtains if you don’t. Keep interior doors open so the cool reserve spreads, and save heat-producing chores (oven, dishwasher, laundry) for after dark. Your home becomes a thermos: sealed and dark by day, wide open by night.
Let the app watch for you
The two-number rule is simple; checking it every hour is not. Open/Shut compares the temperature and the absolute humidity inside and outside your home, and tells you the moment the air outside is actually worth letting in — and when to seal back up. Free, no account, just the physics.