How to Stop Condensation on Windows: The UK Homeowner Guide

Wiping the windows every morning is such a familiar part of British winters that most people treat it as normal. It is not. Persistent window condensation is the clearest early warning that a home is holding more moisture than it can shed, and it is the single most common route to a mould problem in UK housing.

The encouraging part is that condensation is almost always fixable, and usually without major work. This guide explains the mechanism, then the changes that genuinely make a difference.

Why Condensation Forms

Air holds moisture, and how much it can hold depends on its temperature. Warm air holds a great deal; cold air holds very little. When warm, moist indoor air touches a cold surface, it cools — and once it drops below its dew point, it can no longer hold that moisture, which condenses into liquid water on the surface.

Window glass is almost always the coldest surface in a room, which is why condensation appears there first. It is not a window defect. The window is simply the place where an existing moisture problem becomes visible.

How Much Moisture a Household Actually Produces

The volumes surprise most people. A typical family of four generates somewhere in the region of ten to fifteen litres of water vapour indoors every day, from ordinary activity:

• Breathing and perspiration — roughly a litre per person overnight, which is why bedroom windows are worst in the morning
• Drying laundry indoors — up to five litres from a single load, by far the largest single contributor
• Cooking — around three litres a day, more with frequent boiling or steaming
• Bathing and showering — one to two litres per person per day
• Unvented tumble dryers — every litre removed from clothes ends up in the room air

In a well-ventilated older house, much of that escaped through draughts. In a modern, well-sealed home, it stays put — which is why condensation problems have become more common as insulation and glazing have improved.

Condensation on the Inside vs the Outside

These are completely different phenomena, and confusing them causes needless worry.

Condensation on the inside of the glass is the problem this guide addresses. Indoor moisture is meeting cold glass.

Condensation on the outside of the glass is good news. It means your glazing is insulating so effectively that almost no heat is reaching the outer pane, leaving it cold enough for outdoor dew to form — exactly as it does on a car windscreen. It needs no action.

Condensation between the panes of a sealed unit is a third case entirely: the perimeter seal has failed and the unit has lost its inert gas fill. This cannot be cleaned or dried and the sealed unit needs replacing.

Six Changes That Actually Work

1. Stop Drying Laundry on Radiators

This is the highest-impact change available to most households, and it is free. A wet load contains several litres of water, and drying it indoors puts every drop into your air. If you have no alternative, dry it in one room with the door closed and a window open or an extractor running, rather than spreading it through the house.

2. Ventilate Properly, Not Constantly

Short and wide beats narrow and permanent. Opening windows fully for five to ten minutes each morning exchanges the humid air without significantly cooling the building fabric — the walls and furniture retain their heat, so the fresh dry air warms up quickly. Leaving a window on the latch all day cools the structure while exchanging air far less efficiently.

Also check your trickle vents, the small slotted vents in the window frame. They are frequently painted shut or closed by occupants who assume they are draughts. They are there deliberately, and they should be open.

3. Use Extractor Fans Correctly

Building Regulations Part F sets intermittent extraction rates of 30 litres per second for kitchens and 15 litres per second for bathrooms. Two practical points matter more than the numbers:

• Run the fan for 15 to 20 minutes after you finish cooking or showering, not just during. Overrun timers do this automatically
• Keep the door closed while it runs, so the fan pulls moisture out rather than letting it spread through the house

A fan that rattles but does not hold a sheet of paper against the grille is not moving meaningful air and needs cleaning or replacing.

4. Keep Heating Low and Steady

Intermittent bursts of high heat are worse than consistent background warmth. Surfaces need to stay above the dew point, and a wall that cools right down between heating cycles will condense every time the heating comes back on. A steady 18 to 21 degrees in occupied rooms, including a little warmth in rooms you use less, is far more effective than heating hard for two hours a day.

Avoid unflued bottled gas and paraffin heaters entirely. They release roughly a litre of water vapour for every litre of fuel burned, directly into the room.

5. Improve Air Circulation Against Cold Surfaces

Furniture pushed hard against an external wall creates a cold, still pocket where condensation and mould thrive unseen. Leave a 50mm gap behind wardrobes and sofas on external walls, and avoid overfilling built-in cupboards on those walls.

6. Consider Mechanical Ventilation

Where behavioural changes are not enough — often in flats, or homes with limited window openings — a positive input ventilation unit gently introduces filtered air from the loft, displacing humid air continuously and holding indoor humidity down. Continuous mechanical extract ventilation is an alternative. Both are established solutions for persistent condensation in UK housing.

When Condensation Has Become a Mould Problem

Mould germinates where surface relative humidity remains above roughly 80 percent for sustained periods. It appears first in the coldest, least ventilated spots:

• Window reveals and silicone seals
• Corners of external walls, especially at ceiling level
• Behind furniture on external walls
• Inside wardrobes and cupboards against outside walls
• Cold bridges around lintels and wall ties

Black speckling is typically Cladosporium or Aspergillus. Wiping it away treats the appearance only — if the surface keeps reaching dew point, it returns within weeks, and repeated cycles drive spores deeper into porous plaster and sealant.

Extensive or recurring growth needs professional mould remediation, which addresses the contamination properly and, critically, identifies the moisture source so it does not simply come back. See our guides on whether black mould is dangerous and professional removal versus DIY.

If You Rent

Condensation and mould in rented homes are frequently blamed on tenant behaviour when the underlying cause is inadequate ventilation, failed extractor fans, or missing insulation — all of which are the landlord's responsibility.

Under Awaab's Law, social landlords in England face binding deadlines to investigate and remedy reported damp and mould hazards. Report problems in writing, keep copies, and photograph the affected areas with dates. Our guide to Awaab's Law and tenant rights sets out the process.

When It Is Not Just Condensation

Not all damp is condensation, and misdiagnosis wastes money. Seek a professional assessment if you see:

• A tide mark rising up a ground-floor wall — possibly rising damp
• An isolated damp patch that worsens after rain — likely penetrating damp from a defect outside
• Damp appearing suddenly in one area — possibly a plumbing leak
• Damp that persists through summer — condensation is strongly seasonal, so year-round damp points elsewhere

Get Help With Persistent Damp and Mould

Flash Restorations provides professional mould remediation and moisture diagnosis across the UK. Our IICRC-certified technicians identify the actual moisture source rather than simply cleaning the surface. Call 0800 123 4567 to arrange an assessment.