The Morning Rule: When to Shut Everything
You aired all night, the flat is finally cool, the morning feels gentle — so the windows stay open while you make coffee. By 9:30 that pleasant breeze has quietly switched sides: it’s now delivering heat into your home. Summer comfort hinges on catching that exact moment.
The crossover: when outside overtakes inside
All night, outdoor air was cooler than your rooms, so every open window worked in your favor — that’s the whole engine of night cooling. But on a hot day the outdoor temperature climbs fast after sunrise, while your walls hold the night’s coolness. At some point the two lines cross: outside becomes warmer than inside.
From that instant, ventilation reverses its job. The same open window that cooled you at 6 AM heats you at 10 AM. The morning rule is simply: shut everything at the crossover, not at some fixed hour. On a scorching day that can be 8 AM; on a milder one, closer to noon. Your home’s makeup matters too — a heavy building that banked a cool night crosses later than a light flat that warms with the street. That difference is the subject of thermal inertia.
Why shutting early beats shutting late
The mistake is rarely dramatic — it’s twenty “just a bit longer” minutes here and there. But the heat you let in mid-morning doesn’t leave at lunch; it soaks into walls and furniture and comes back at you all evening. Shut ten minutes too early and you lose almost nothing. Shut an hour too late and you can spend the whole evening paying it back.
When in doubt, close. A shut home coasts on stored coolness; an open one imports the day in real time. And if you’re also tempted by the “dry” afternoon air your weather app advertises, don’t be — that’s the 3 PM dry air trap, and it brings in water along with the heat.
Shutting means shutters too
Glass lets radiant heat pour in even through a closed window. So the morning rule has a second half: as the sun reaches each façade, drop the shutters or exterior blinds on that side — east in the morning, south midday, west in the afternoon. Blocking sunlight before it hits the glass is worth far more than any curtain behind it; the full ranking is in shutters, blinds, curtains: what blocks heat.
The picture by mid-morning on a hot day: windows shut, shutters down on the sunny sides, a dim and quiet home holding its cool like a thermos.
The evening mirror
The same crossover happens in reverse. As the outdoor temperature falls past your indoor temperature — and the outdoor air is no wetter than yours, in absolute terms — everything can open again, wide. Evening, night, early morning: harvest coolness. Morning crossover: lock it in. Two decisions a day, correctly timed, outperform any amount of fiddling in between.
Let the app watch for you
The crossover shifts with the weather, the season, and your building — which is why a fixed “close at 9” rule keeps failing. Open/Shut tracks outdoor temperature and absolute humidity against your home’s state and tells you the moment the balance flips, in both directions. Free, private, no account — just open, or shut, at the right time.