Condensation on the outside of your windows? That's good news

From late August the first misty mornings arrive, and every year they bring the same worried phone calls: new windows, fitted months ago, suddenly fogged over, on the outside. It looks like a fault. It is actually the windows doing their job, and understanding why saves an unnecessary complaint or, worse, an unnecessary repair bill from someone less honest.

Why efficient glass mists on the outside.

Condensation forms on any surface that is colder than the air around it, the same way a cold drink mists on a summer afternoon. An old, inefficient window leaks enough heat from the house to keep its outer pane slightly warm overnight, so dew never forms on it.

A modern double glazed unit holds the heat inside the room instead. The outer pane stays cold overnight because barely any heat is escaping through the glass to warm it. On a clear, still night at the end of summer, that cold outer pane drops below the dew point and mists over, exactly like the grass and car windscreens around it.

When it appears and when it clears.

External condensation is most common in late summer and autumn, when nights are cool and clear but the air still carries plenty of moisture. You will usually see it in the early morning, often only on the windows facing open sky, and it clears on its own as the sun warms the glass, typically by mid-morning.

It may be patchy, misting some windows and not their neighbours. That is down to tiny differences in exposure, shelter and sky view, not a difference in the glass.

The condensation that is a problem.

Where condensation appears matters far more than how much of it there is. On the outside face: a sign of efficient glass, no action needed. On the inside face, in the room: the room needs more ventilation or less moisture, common in kitchens, bathrooms and bedrooms overnight.

Between the two panes, where you cannot wipe it: that is a failed sealed unit. The gap between the panes is meant to be sealed and dry, and misting inside it means the seal has gone. The unit will not recover, but it can be replaced on its own without changing the frame, which is a far smaller job than a new window.

A quick way to tell the three apart.

Run a finger across the misted glass. If it wipes clear from inside the room, it is internal condensation and the answer is ventilation. If it wipes clear from outside, it is external condensation and the glass is performing well. If it will not wipe from either side, the mist is inside the unit and the sealed unit has failed. That one is for us.