During a heatwave, roller shutters can make a real difference indoors.
The main rule is simple: block direct sun before it reaches the glass, because windows heat up quickly once sunlight enters.
In the hottest hours, it is usually better to keep roller shutters fully closed, especially on windows exposed to direct sun. This creates a shaded barrier outside the glass and slows down indoor heating.
Leaving the slats slightly open can help if you need a little airflow or daylight without exposing the room to strong sun. This works best on windows that are not facing direct sunlight, or when the outside air is not hotter than the room.
Keep windows closed when the outside air is hotter than the indoor air. Opening them in the middle of the day can bring heat inside instead of cooling the room.
Close shutters and windows before the sun hits the facade. This is especially useful on south- and west-facing windows.
Open windows when the outside air becomes cooler than indoors. Create cross-ventilation if possible, then close again in the morning.
For most hot days, close roller shutters before the sun reaches the windows. Use small slat openings only for controlled airflow or light, and ventilate mainly at night when the air is cooler.