The short answer
Where the condensation sits tells you what is wrong. On the inside means too much indoor humidity and not enough ventilation — manage it with airflow, not new windows. On the outside is harmless and actually a sign your glazing is working well (the outer pane stays cold). Between the panes means the sealed unit has failed and lost its seal — that one is a glazing fault, and the unit (not the whole window) usually needs replacing. See misted or blown double glazing repair for fixing failed units.
Condensation on windows worries a lot of homeowners, but the right response depends entirely on where the moisture appears. This guide explains the three types — internal, external and between-the-panes — what each one means, and what (if anything) you need to do about it.
Condensation at a glance
- Inside the glass Indoor humidity — ventilate
- Outside the glass Harmless — glazing working
- Between the panes Failed sealed unit
- Between-pane fix Replace the unit, not the frame
- Driven by Humidity, ventilation & temperature
- Worst rooms Kitchens, bathrooms, bedrooms
The three types of condensation
Internal condensation forms on the room side of the glass when warm, moist indoor air meets the cooler surface. External condensation forms on the outside, usually on clear mornings, when the outer pane is colder than the air. Between-the-panes condensation forms inside the sealed unit and is the only one that signals a glazing fault. Knowing which you have stops you spending money on the wrong fix.
| Where | What it means | What to do |
|---|---|---|
| Inside the glass | Too much indoor humidity | Ventilate, use extractor fans, trickle vents |
| Outside the glass | Glazing working well | Nothing — it clears as the day warms |
| Between the panes | Sealed unit has failed | Replace the glass unit |
Internal condensation: a ventilation issue
If moisture sits on the room side of the glass, the cause is indoor humidity rather than the windows themselves. Cooking, showering, drying clothes indoors and even breathing all add water to the air, which condenses on the coldest surface — often the window. New windows will not solve this; in a draughty old house the air escaped, whereas tighter modern windows can make humidity more noticeable. The fix is ventilation: use extractor fans, open trickle vents, air rooms regularly and avoid drying washing on radiators. Building Regulations Part F covers ventilation provision when windows are replaced.
Between the panes: a failed unit
Condensation, mist or fog between the two panes of glass means the sealed unit has lost its airtight seal, letting moisture in. You cannot wipe it away because it is inside the glass. This is the one type that is a genuine glazing fault. The good news is that you usually only need to replace the failed glass unit, not the whole window or frame — often a fairly affordable repair. See can you replace just the glass. This is general guidance; the right fix depends on your windows’ age and condition.
Compare double glazing quotes
If you have failed units or are weighing up replacement, comparing quotes helps you judge value fairly. Use our service to reach FENSA or CERTASS registered installers in your area.
Frequently asked questions
Why do I get condensation inside my windows?
Internal condensation is caused by indoor humidity meeting the cooler glass — from cooking, showering, drying clothes and breathing. It is a ventilation issue, not a window fault. Use extractor fans, open trickle vents and air rooms regularly to manage it.
Is condensation on the outside of double glazing bad?
No — it is a good sign. External condensation means the outer pane is staying cold while the inner pane stays warm, which shows your glazing is insulating well. It clears on its own as the day warms and needs no action.
What causes condensation between the panes?
Misting between the panes means the sealed glass unit has lost its airtight seal and let moisture in. This is a genuine glazing fault. You usually only need to replace the failed glass unit, not the whole window or frame.
Will new windows stop condensation?
New windows will stop between-the-pane misting from failed units, but they will not stop internal condensation, which is driven by indoor humidity and ventilation. Tighter modern windows can even make internal condensation more noticeable until ventilation is improved.
Sources & further reading
- Energy Saving Trust — condensation, ventilation and energy efficient glazing
- Glass and Glazing Federation (GGF) — condensation and sealed unit guidance
- GOV.UK / Building Regulations Approved Document F — ventilation when replacing windows
- BFRC — glazing performance and U-values
This is general information, not advice for your specific property or installation. The right response depends on your windows and home. Replacement units and windows should be fitted by a FENSA or CERTASS registered window installer. We are an independent information and introduction service, not an installer.