Indoor versus outdoor temperature: when to open windows
Compare indoor and outdoor temperature instead of following the clock. Open when outside is cooler, then close before outdoor heat climbs again.
The most useful heatwave rule is also the simplest: open when the air outside is cooler than the air inside; close when it is warmer.
The difficult part is catching the crossover. It rarely happens exactly at sunset, and a forecast for the whole city cannot tell you what the air beside each window feels like.
Compare like with like
Use an indoor thermometer away from direct sun, the oven, electronics, and the stream from a fan. For outdoors, use a shaded local reading rather than a thermometer sitting on a sunny balcony or behind hot glass.
Then compare the two temperatures:
- Outside is warmer: keep windows closed. Shade sun-facing glass and use the fan to cool people, not to pull hot air indoors
- Outside is slightly cooler: begin with a smaller opening and check whether the room responds
- Outside is clearly cooler: open wide where safe and create a path from one side of the home to another
- Outside warms above indoors again: close and protect the cooler air you gained
There is no universal hour. A shaded courtyard may become useful before a street that has stored heat all afternoon. A top-floor room may remain warmer than the rest of the home long after dark.
The evening crossover
After sunset, outdoor temperature usually starts falling faster than indoor temperature. Walls, floors, and furniture release stored heat slowly, so indoor air lags behind. When the outdoor line drops below the indoor line, ventilation changes from adding heat to removing it.
That is the start of the cooling phase described in the three phases of heat. Open as much as safety allows, keep interior doors open, and use the right fan direction to speed up the exchange.
The morning crossover
Morning reverses the process. Outdoor air warms while the home may still hold some of the cool gained overnight. Close before the outdoor reading rises above the indoor one.
East-facing windows need attention first because direct sun can heat the glass before the general forecast looks hot. Close those windows and lower their shade; other façades can remain open while their local air is still cooler.
Why the forecast can be wrong for your window
A weather app measures or models outdoor air for an area. Your window sits in a microclimate shaped by shade, height, paving, traffic, courtyards, and warm walls.
Treat the forecast as the trend and your indoor reading as the decision point. If the forecast says 23 °C while the room is 27 °C, outdoor air is promising—but a sun-baked street or balcony may still be warmer for a while.
Temperature leads, but it is not the only safety signal. Very poor air, high pollen for someone who is sensitive, or humid air can justify a smaller or shorter opening. See when cooler air is not worth letting in.
What if outside never becomes cooler?
Do not keep windows wide open just because it is night. On a tropical night, the outdoor minimum may stay too high to remove much heat.
Keep daytime shading strict, cool people with moving air and water, and use any brief period when the local outdoor reading does fall below indoors. Ventilate uses indoor temperature, local weather, sun exposure, and your layout to rank that decision window by window.