Condensation on your windows: what it's telling you

As the weather turns colder, many of us wake up to wet windows. Where the condensation forms tells you a lot about what's going on.

On the inside of the glass

This is the most common kind. Warm, moist air from cooking, showers, drying clothes and breathing meets the cold glass and turns back into water.

It usually points to too much moisture and not enough ventilation, rather than a problem with the window. Things that help:

• open trickle vents, if your windows have them
• use extractor fans when cooking and showering
• open a window briefly each day to let moist air out
• avoid drying clothes on radiators where you can
• keep the home evenly heated

If it's heavy and persistent, or you're seeing black mould, it's worth getting advice, as damp can affect your health and your home.

Between the panes

If condensation forms between the two panes of a double-glazed unit, the seal around that unit has failed. Moist air has got in and can't get out.

The window frame is often fine. In many cases only the sealed glass unit needs replacing, which costs much less than a new window. Ask a specialist to look at it.

On the outside of the glass

Condensation on the outside, usually on clear, cold mornings, can actually be a good sign. Modern energy-efficient glass keeps heat inside, so the outer pane stays cold and dew forms on it, just as it does on a car windscreen. It clears as the day warms up.

When to think about new windows

Condensation alone isn't usually a reason to replace windows. It might be time to look into it if you also have:

• several failed sealed units
• rotten or damaged frames
• draughts you can't fix with new seals
• single glazing that makes rooms hard to heat

Our guide to choosing new windows explains what to look for.