Condensation on windows - causes and solutions

When moisture on windows is simply physics, and when it is a signal that something is wrong with the window.

Moisture or water droplets on window glass are annoying, but also widely misunderstood. Why does it happen, and when is it just physics rather than a sign that something is wrong with the window?

What is condensation and why does it form?

Condensation occurs when warm, moist air meets a cold surface - in this case the window glass. Moisture in the air turns into droplets on the coldest surface.

This is completely normal physics, but how often it happens and where it appears can tell you very different things.

Condensation on the inside of the window

This is the most common form of condensation, and it usually points to:

High humidity in the room

Cooking, showering, drying laundry indoors, or simply poor ventilation can create high humidity that condenses on the coldest surfaces - usually windows.

Insufficient thermal insulation

Older or lower-quality windows with a poorer U-value let the glass stay colder, which increases the chance of condensation forming.

What to do:

Ventilate rooms regularly, especially after showering or cooking
Provide constant, low-level air exchange (trickle ventilation)
If the problem persists, review the thermal quality of the windows

Condensation between the panes of the glazing unit

This is a more serious sign. If condensation or misting appears between the panes (not on the inner surface), the seal of the glazing unit has failed.

What it means: the air or gas fill between the panes has escaped and moist air has entered the unit. Cleaning or ventilating will not fix this - the glazing unit needs replacing.

Condensation on the outside of the window

Less common, but just as normal - it usually forms early in the morning, when the outer glass surface is cooler than the surrounding air. It actually indicates good insulation: the window keeps heat inside so well that the outer pane stays cool.

How to tell normal condensation from a problem

Situation
Cause
Action

Condensation inside, frequent
High humidity / poor insulation
Ventilate, review window quality

Condensation between panes
Failed seal
Glazing unit replacement needed

Condensation outside, rare
Normal effect, good insulation
No action needed

Frequently asked questions

Does condensation mean the windows are faulty?

No. Condensation on the inner surface is not a defect - it is physics. Moisture settles on the coldest surface in the room. Condensation between the panes, however, does mean the glazing unit has failed.

Why did the old windows not have condensation?

Old windows were less airtight and air circulated through the gaps. That meant bigger heat losses, but also natural ventilation. New windows are energy efficient but need active airing.

Does triple glazing condense more?

Triple glazing insulates better, so the inner surface is warmer and condensation forms less often than on double glazing. Outer condensation may appear more often - that is a good sign.

What should I do if condensation is between the panes?

That means the seal has failed. Contact your window supplier - it is usually a warranty case and the glazing unit is replaced.

What does this problem cost if ignored?

Persistently high humidity can cause mould on walls and window sills, damage finishes and affect health. It is better to act early.

Seeing persistent condensation that will not go away?

Book a consultation - we will assess whether the issue is ventilation, window quality, or whether replacement is needed.