When to open a window

The advice compares **absolute** humidity — grams of water per cubic metre — rather than temperature or the relative-humidity reading. A warm, muggy day is the trap: air that feels inviting can carry far more water than the house holds, and opening up then makes the damp worse. So a window is suggested when the air outside is genuinely pleasant and no wetter than indoors, or when the house is damp and the outdoor air is drier.

Nothing is suggested while it rains, when the outdoor air is unhealthy to draw in — the unit's filters then beat an open window — or when a humidity reading is missing on either side. Silence means the comparison could not be made honestly, not that the air is fine.