Condensation on windows: what it tells you about your home

Every autumn the same scene plays out: mist reappears on the glass in the morning, then black marks settle in the corner of the frame. Many people conclude their windows are faulty. That is almost always wrong — and an expensive mistake, because it leads to replacing what works.

Condensation follows simple physics, and that simplicity is good news: it means there are only three possible levers.

Why water appears on the glass

Indoor air contains water vapour. The warmer it is, the more it can hold. When that air meets a cold surface it cools locally, its capacity to hold vapour drops, and the excess settles as droplets. That is the dew point.

The pane is simply the coldest surface in the room: that is why water shows there first. It plays the part of the indicator, not the culprit. If the condensation appeared instead in a wall corner or behind a wardrobe, the diagnosis would be the same — but the damage would stay invisible for longer.

How much vapour does a household produce?

The orders of magnitude below are those commonly used in building physics. They explain why an occupied home produces several litres of water as vapour every day.

What changed in modern homes

An old, unrenovated house leaked everywhere. That parasitic air cost a fortune in heating, but it removed vapour continuously without anyone having to think about it.

Replacing windows, insulating the roof and sealing gaps removed those leaks — a good thing in energy terms. But the vapour is still produced. If no organised ventilation takes over, it stays indoors. That is why condensation so often appears right after insulation works, in a home that never had the problem before.

Identifying the real cause

Before committing to any work, these observations steer the diagnosis reliably:

• Mist appears in one room only → local problem: missing or blocked extraction, a permanently closed door, an unheated room
• Mist appears everywhere, mainly in the morning → insufficient whole-home air renewal
• Black marks follow a sharp vertical or horizontal line on a wall → structural thermal bridge; ventilation alone will not be enough
• Damp rises from the base of walls with blistering plaster → suspected rising damp, an entirely different subject
• The problem started after building works → increased air-tightness with no compensating ventilation
• The problem is seasonal and disappears in spring → normal condensation behaviour, to be addressed through ventilation

What does not help

Some common responses are useless at best and counterproductive at worst.

An electric dehumidifier treats the symptom in one room, consumes electricity continuously, and never removes the vapour anywhere except into its own tank. It earns its place in a cellar; it does not replace ventilation in an occupied home.

So-called anti-mould paint hides the mark for a season. The fungus returns because the condition that lets it grow has not changed.

Blocking ventilation grilles because they cause a draught is the most frequent and most damaging reaction: it removes precisely the air supply that let the extraction work.

The approach we recommend

In a significant share of the cases we handle, the problem disappears at step 4. That is exactly why measuring first pays for itself: it avoids installing an expensive system where a blocked grille explained everything.

1. 1Measure before deciding: record existing extract airflow, room by room.
2. 2Log relative humidity and CO₂ over several days in the affected rooms, overnight included.
3. 3Check air inlets: façade grilles, door undercuts, transfer grilles.
4. 4Fix what is free or cheap first: reopen a grille, clean a valve, change a filter.
5. 5Re-measure after the fix, and only then consider a heavier investment.