Open or shut? During a heatwave, everyone answers this question with instinct, and instinct is wrong. We open the windows when the weather feels nice, while the air outside is hotter than inside. We let in air that feels dry, while it actually carries more water. Open/Shut ends the guessing.

ONE GLANCE, ONE ANSWER
A full-screen verdict, updated in real time: OPEN or SHUT, with the reason in one sentence. No charts to interpret, no thresholds to configure.

PHYSICS THAT IS ACTUALLY RIGHT
Relative humidity is misleading: hot afternoon air can show a low percentage while holding more water than your living room. Open/Shut compares absolute humidity, the grams of water per cubic metre, indoors and out. Airing never trades a cool room for a damp one.

IT KNOWS YOUR HOME
Describe your windows once: "garden side", "bedroom skylight". When the verdict is OPEN, the app crosses the wind direction with your facades and tells you which openings to use for cross-ventilation, in your own words. It also learns your building's thermal inertia and estimates the indoor temperature between your readings.

IT PLANS AHEAD
An hourly timeline shows the next favorable window over the coming hours, and a notification arrives at the exact moment conditions flip. Open at the right time, not when you happen to think about it.

SUMMER AND WINTER
In summer, flush out the heat as soon as the evening air turns cooler. In winter, air strategically to fight condensation and mould without throwing your heating out the window.

PRIVATE BY DESIGN
No account, no backend, no tracking. Your location is only used to fetch your local forecast. Free, with no subscription.

Open/Shut. An open-and-shut case.