Which side of the glass is my window condensation on?

All three look identical from a metre away, and all three are described the same way — 'condensation on the windows'. They have nothing in common beyond the water, and the wrong diagnosis leads to spending money on the wrong thing entirely.

What you'll need

• A finger
• A cloth

Step by step

1. Wipe the room-facing surface.If the water comes off on your hand, the condensation is inside the room and the glass is simply the coldest surface available for it to form on.
2. Check the outside face if the inside was dry.External condensation looks like a fine even mist across the whole pane, usually in the early morning after a clear night, and often on the upper floors first.
3. Conclude a failed unit if neither wipes.Water or a hazy film between the panes, sometimes with a tide mark or visible streaking, means the perimeter seal has failed and the desiccant is saturated.
4. For internal condensation, look at the room, not the window.Drying washing indoors, showering without extraction, cooking without a lid, unvented tumble dryers and blocked trickle vents are the usual sources. The window is the messenger.
5. Measure the humidity if it persists.A cheap hygrometer. Above about 60 percent relative humidity indoors in winter, you have a moisture generation or ventilation problem regardless of what the windows do.
6. Ventilate rather than seal.Open the trickle vents, run the extractor for twenty minutes after a shower, and air rooms briefly and sharply rather than leaving a window ajar all day.
7. For external condensation, do nothing.It is a sign the glass is losing very little heat to the outside, which is exactly what modern glazing is for. It burns off within an hour or two of sunrise.
8. For a failed unit, price the glass not the window.The sealed unit can be replaced on its own in most frames. The frame, hardware and cill stay. It is a fraction of the cost of a new window.
9. Check the frame drainage while you are there.Persistent water sitting in the bottom of the frame accelerates seal failure, so blocked drain slots shorten the life of the glass.

Tips

• External condensation is more common on north-facing windows and on the top floor, because those surfaces radiate to the coldest part of the sky.
• If only one or two windows in the house stream with water inside, look at what happens in those rooms rather than at those windows.
• Condensation on the inside of a bedroom window most mornings is normal in a cold snap. Water pooling on the cill and mould growing on the seals is not.

Common mistakes

• Replacing windows because of internal condensation — New windows are usually better insulated and better sealed, so the glass is warmer but the air changes drop. Households frequently report condensation moving from the glass to the walls and corners, where mould grows instead.
• Treating external condensation as a fault — It happens because the outer pane is well insulated from the warm interior, which is precisely what you paid for. Calling the installer back cannot achieve anything.

If it doesn't work

Questions people ask