High UV and windows on the sunny side

A UV index of 6 or more is high. Those hours are also when sun on glass dumps heat into the room.

UV itself is about skin and eyes outdoors. Indoors, the ventilation problem is the heat through the pane. The two often arrive together: clear sky, high sun, hot glass.

Ventilate Pro closes windows on the sunny side when UV is 6 or higher. Shade-side windows can still open if outdoor air is cooler. That is the same idea as protecting your windows from the sun: you do not treat every façade the same.

A top-floor flat feels this first. So does a street façade with no trees, which is often the hot side of a heat island.

Shade the glass if you can

Exterior shutters beat interior curtains. Once sunlight has passed through the glass, most of its energy is already in the room. Blocking it outside is the bigger win. UV advice in the app is about which windows to keep shut, not a substitute for sunscreen.

The sunny-side checklist

• Close sun-facing windows when UV is 6 or higher
• Leave shade-side windows free to follow the temperature advice
• Block sun on the glass before it reaches the room

This is not medical advice. For time outdoors, follow your local UV and heat guidance.