Condensation on the outside of windows: why it happens, and why it is usually good news
By Glazing London,
You open the curtains on a cold autumn morning and the view is fogged, but when you wipe the glass from inside, nothing changes. The water is on the outside. It looks like a fault, so it worries people. In almost every case it means the opposite: your windows are keeping the heat in.
The quick answer
Condensation on the outside of windows forms when the outer pane is colder than the dew point of the outdoor air. On a clear, still night the glass loses heat to the sky, the air near the window is damp, and by morning dew has settled on the outer pane, exactly as it does on a car windscreen or the lawn.
Jigsaw Homes, a social landlord that fits energy-efficient windows, describes it as purely cosmetic and a sign the glazing is working: so little heat passes through the unit that the outer pane stays as cold as the air outside.
Why it happens more with new glazing
- Low-emissivity glass reflects heat back into the room instead of letting it reach the outer pane.
- Argon or krypton gas in the cavity slows heat moving across the gap.
- Warm-edge spacer bars cut the heat that used to creep round the edge of older units.
Put those together and the outer pane barely knows the heating is on. That is why the same house can go from "never had it" to "every frosty morning" the week new windows go in.
When you will see it
- Autumn and spring mornings, after a clear night with little wind
- Windows facing open ground, gardens or water, where the air is damper
- Ground-floor and garden-facing windows more than upper floors on a busy street
- The lower part of the pane first, where the coldest air settles
It normally burns off during the morning, faster on windows that face east or south.
Three kinds of condensation, and what each means
| Where the water is | What it means | What to do |
|---|---|---|
| Outside the glass | The unit is insulating well | Nothing: it clears by itself |
| Inside the room | Too much moisture indoors | Ventilate, run extractor fans, dry laundry outside |
| Between the panes | The sealed unit has failed | Replace the glass unit |
Indoor condensation and failed units are covered in our full guide to window condensation causes and fixes, and mist trapped between the panes is explained in misted double glazing: what to do.
When it is worth a call
External dew is never a reason to replace a window. Call us if the mist is between the panes, if a unit has cracked, or if a window will not close properly and lets damp air in round the frame. We replace misted double glazed units in the existing frame across London. Ring 07915 903485 or send us a photo on WhatsApp, and we will tell you whether it needs anything at all.
Questions people ask
Why do my new windows have condensation on the outside but my old ones never did?
Old single glazing and early double glazing leak heat, which warms the outer pane enough to keep dew off. Modern units with low-emissivity glass and gas-filled cavities keep that heat indoors, so the outer pane sits close to the outdoor temperature and collects dew on cold, damp mornings.
Should I wipe the condensation off the outside?
There is no need. It clears by itself once the sun reaches the glass or the air warms up, usually once the morning warms up. Wiping just smears any dust on the glass.
Is condensation on the outside of triple glazing normal too?
Yes, and you may see it more often than with double glazing, because triple glazing lets even less heat reach the outer pane.
When is condensation on a window a reason to call a glazier?
When it is between the panes. Mist trapped inside a sealed unit means the seal has failed, and it will not wipe off from either side. The glass unit needs replacing, usually in the existing frame.
Can I stop external condensation?
Not without making the window less efficient, and there is no good reason to. Some people find it fades sooner on windows that get morning sun, and trimming back plants that hold damp air against the glass can help a little.
Sources
- External condensation on new energy efficient windows, Jigsaw Homes