Keeping Your Home Cool in a Heatwave, No AC
The instinct in a heatwave is to open windows for “fresh air” — and that instinct is exactly how homes end up hotter than the street. Without AC, you can’t make coolness; you can only capture it when it exists and defend it when it doesn’t. That turns the whole game into a question of timing.
The rhythm: fortress by day, wind tunnel by night
During a heatwave, outdoor air is hotter than your home for most of the day and cooler for a few night hours. The strategy writes itself:
- Day: everything shut, everything shaded. Your home defends its stored coolness.
- Night: everything open, maximum airflow. Your home dumps the day’s heat and restocks.
Most heatwave misery comes from running this backwards — airing at 2 p.m. because it “feels stuffy”, then closing at night out of habit. If you’re unsure whether a given moment is open-time or shut-time, that’s precisely the heatwave open-or-shut question: open only when outside is cooler and no more humid, in absolute terms.
Daytime: shut everything, shade everything
From mid-morning, windows shut and shades down on every sunny façade — exterior shutters or blinds if you have them, since stopping sun before the glass is worth far more than any curtain. Keep interior doors closed to sun-baked rooms so their heat doesn’t migrate.
Then stop making heat indoors. The oven is the obvious culprit, but dishwashers, dryers, gaming rigs and halogen lamps all pay into the same account. Cold meals and delayed appliance runs are worth real fractions of a degree by evening.
Night: flush the heat out
When the outside air finally drops below the inside temperature — often late in the evening, since walls keep radiating stored heat for hours — open wide. Not a crack: wide, and on opposite sides of the home, so wind can push air through rather than just stirring it. If there’s no wind, a fan in a window blowing the right direction can move impressive volumes; a roof window adds stack effect, letting hot air escape high while cooler air enters low.
Keep the flush running as long as outside stays cooler — typically until early morning. Then shut and shade again before the sun reverses the balance, not after you notice it’s warm.
Watch the water, not just the temperature
Here’s the trap hiding inside the night routine: temperature isn’t the only thing coming through the window. Muggy night air can hold more water than your indoor air even while being slightly cooler — and humid air makes the same temperature feel hotter, because your sweat stops evaporating. Trade two degrees for a swampful of moisture and the night feels worse, not better.
Don’t trust relative humidity to warn you. Hot afternoon air at “35% RH” often carries more actual water per cubic metre than cool air at “90%” — the classic dry-air trap. The comparison that matters is absolute humidity: open when the outside air is cooler and holds no more water than yours. Otherwise, keep it shut and let the fan move indoor air across your skin instead.
Let the app watch for you
The whole strategy compresses into one recurring decision: is outside air, right now, cooler and drier than mine? Open/Shut answers it continuously for your home — temperature and absolute humidity, both façades of the day — and tells you the moment to switch between fortress and wind tunnel. Free, private, no account.