It usually means the temperature outside is below 5 (and we don't run the heating overnight, so the temperature drops and helps the process along). I have idly contemplated having the sealed units replaced (but I'm not sure the energy-saving, or my effort with a cloth, would justify the cost and hassle) – we did have the window in my office done when we moved and that's the only one that remains condensation-free. That said, the gardeners did have the unit in one of the patio doors replaced when they strimmed a stone through it, and that's still a bit shit (but I suspect they bought the cheapest unit possible).