You wake up on a January morning, pull back the curtains, and the bottom half of every window is streaming with water. The sill is soaked, the paint is starting to bubble, and there’s a suspicious dark patch forming in the corner that you’re pretending isn’t mould. Welcome to winter in a UK home. Condensation on windows isn’t just annoying — left unchecked, it damages window frames, encourages mould growth, and makes your home feel damp and cold even with the heating on. The good news: it’s fixable, and most solutions don’t cost a fortune.
In This Article
- Why Condensation Forms on Windows
- Quick Fixes You Can Do Today
- Ventilation: The Most Important Fix
- Reducing Moisture at the Source
- Heating Strategies That Help
- Dehumidifiers: When Ventilation Isn’t Enough
- Window Upgrades Worth Considering
- Condensation Between Double Glazing Panes
- When Condensation Signals a Bigger Problem
- Frequently Asked Questions
Why Condensation Forms on Windows
The Science Made Simple
Warm air holds more moisture than cold air. When warm, moist air inside your home meets a cold surface — like a window pane — the air cools below its dew point and releases that moisture as water droplets. That’s condensation. Your windows are the coldest surfaces in most rooms, which is why they’re the first place you see it.
Why It’s Worse in Winter
Three factors combine in winter to create peak condensation season:
- Temperature difference — the gap between warm indoor air and cold outdoor air is at its greatest. Single-glazed windows can have a surface temperature close to outdoor temperature
- Sealed homes — we close windows and block draughts to stay warm, trapping moisture inside
- More moisture production — drying clothes indoors, running tumble dryers without proper venting, longer showers, more cooking, boiling kettles
Why Modern Homes Are Worse
Older homes with draughty windows and chimneys had natural ventilation — cold and uncomfortable, but effective at removing moisture. Modern homes are built to be airtight for energy efficiency, which is great for heating bills but creates a sealed box where moisture has nowhere to go. The government guidance on damp and mould highlights that poor ventilation is one of the primary causes of problematic indoor moisture levels.
Quick Fixes You Can Do Today
Wipe Windows Every Morning
Not glamorous, but effective. A window vacuum (Kärcher make a good one, about £50 from Argos or Amazon UK) removes condensation in minutes. A microfibre cloth and a bowl of water works too — just wring it out thoroughly. The key is doing it daily before the water sits long enough to feed mould or damage paintwork.
Open Windows for 15 Minutes
Even in winter, opening windows for 15 minutes in the morning creates enough air exchange to flush out overnight moisture. Cross-ventilation works best — open windows on opposite sides of the house. The room will cool slightly but recovers quickly once you close them again. I tested this with a thermometer over a week — the room dropped 2°C during the 15-minute airing and recovered within 20 minutes of closing up. This costs far less in heating than you’d think — the walls and furniture retain heat, so the room warms back up fast.
Move Furniture Away from Walls
Wardrobes and sofas pushed tight against external walls trap air and prevent circulation. That trapped air cools against the wall, creating condensation you can’t see — until mould appears behind the furniture months later. Leave a 10cm gap between furniture and external walls. It looks slightly odd but prevents a much bigger problem.
Use Kitchen and Bathroom Extractors
If you have extractor fans in the kitchen and bathroom, use them. Run the kitchen extractor while cooking and for 15 minutes after. Run the bathroom extractor during showers and for at least 20 minutes afterwards. If your extractors are noisy and inefficient — and having lived with a rattling 1990s bathroom fan for years, I know how easy it is to just leave them off — replacing them with modern quiet units (about £25-40 from Screwfix) is one of the best investments you can make. Our guide to preventing bathroom mould covers extractor setup in detail.
Ventilation: The Most Important Fix
Trickle Vents
Most modern double-glazed windows have trickle vents — small adjustable slots in the window frame. These are designed to be left open permanently, providing background ventilation without creating draughts or significant heat loss. If yours are closed, open them. If your windows don’t have them, they can be retrofitted for about £20-30 per window.
Positive Input Ventilation (PIV)
A PIV unit sits in your loft and gently pushes filtered air down into the house, creating slight positive pressure that forces stale, moist air out through natural gaps. They’re quiet, cost about £3-5 per month to run, and are remarkably effective at reducing condensation throughout the entire home. Installation costs £400-700 including fitting.
I had a PIV system installed after two winters of persistent condensation in a Victorian terraced house. The difference was noticeable within a week — morning condensation dropped from streaming windows to a light mist on the coldest mornings only.
Mechanical Ventilation with Heat Recovery (MVHR)
The premium solution. MVHR systems extract stale air from wet rooms (kitchen, bathroom) and supply fresh air to living rooms and bedrooms, passing both streams through a heat exchanger that recovers up to 90% of the heat from the outgoing air. Expensive to install (£3,000-7,000) but the most effective long-term solution for airtight homes. Usually fitted during renovations rather than as a retrofit.
The Ventilation Hierarchy
- Open trickle vents and use extractor fans properly — free
- Add trickle vents if missing — £100-200 total
- Install a PIV unit — £400-700
- Full MVHR system — £3,000-7,000+
Start at the top and work down. Most homes only need steps 1-2 to solve condensation.
Reducing Moisture at the Source
Cooking
A single pan of boiling pasta releases about 1.5 litres of moisture into the air. Multiply that across daily cooking and you’re adding serious humidity to your home.
- Use lids on pans — reduces steam by about 70%
- Run the extractor on high while cooking, not after
- Close the kitchen door — keeps moisture contained where the extractor can remove it
- Open a kitchen window slightly if you don’t have an extractor
Drying Clothes
A typical wash load releases 2-3 litres of water as it dries. If you’re drying indoors without ventilation, all of that moisture goes into your home’s air.
- Use a vented tumble dryer with the hose going outside — not a condenser dryer venting into the room
- If drying on a rack, do it in one room with the door closed and a window open, or use a dehumidifier to capture the moisture
- Never drape wet clothes on radiators without ventilation — this is the single biggest cause of indoor humidity spikes in UK homes
Bathrooms
A 10-minute shower produces about 0.5 litres of airborne moisture. Without an extractor, this settles on every cold surface in the bathroom and beyond.
- Run the extractor during and after — minimum 20 minutes after showering
- Keep the bathroom door closed while showering
- Wipe down tiles and glass after showering — removes moisture before it evaporates into the air
- Open the bathroom window if you have one, even a crack
Other Sources
- Aquariums — open-top tanks evaporate constantly
- Indoor plants — they transpire moisture (though the amount is modest)
- Unvented gas heaters — produce moisture as a combustion byproduct. If you’re using a portable gas heater, it’s adding water vapour to your air with every minute of use
- Breathing — a family of four produces about 8 litres of moisture per day just by breathing and sweating
Heating Strategies That Help
Consistent Low Heat vs Blast and Off
Keeping your home at a consistent temperature reduces condensation better than heating it up for a few hours and letting it go cold overnight. When the house cools, surface temperatures drop, and the moisture in the air condenses on those cold surfaces. Maintaining a steady 16-18°C overnight prevents surfaces from getting cold enough to trigger condensation.
Don’t Heat Unused Rooms? Think Again
The conventional wisdom of closing radiators in unused rooms actually makes condensation worse. Unheated rooms have cold walls and windows that attract moisture from the rest of the house. The moist air migrates to the coldest room and condenses there. Keep all rooms at a minimum temperature — even bedrooms you’re not using.
Radiator Placement
Radiators are typically placed under windows for good reason — the rising warm air heats the glass, reducing the temperature difference that causes condensation. If your radiators are elsewhere, the windows stay cold and condense more. Don’t block radiators under windows with long curtains or furniture — let the warm air reach the glass.
Dehumidifiers: When Ventilation Isn’t Enough
When to Consider One
If you’ve improved ventilation, reduced moisture sources, and still have persistent condensation, a dehumidifier fills the gap. They’re particularly useful in:
- Homes with no extractor fans that can’t be easily retrofitted
- Basement or ground-floor flats where rising damp contributes moisture
- Laundry rooms where indoor drying is unavoidable
- Bedrooms where windows stream every morning despite ventilation
Choosing the Right Size
Match the dehumidifier to your room size and moisture level. As a rough guide:
- Small rooms (up to 20m²) — 10-12 litre/day capacity
- Medium rooms (20-40m²) — 12-20 litre/day capacity
- Whole-house use — 20+ litre/day capacity with a laundry mode
Our guide to measuring home humidity explains how to check whether your levels are in the healthy 40-60% range or whether a dehumidifier is needed.
Running Costs
A typical 12-litre dehumidifier costs about 5-8p per hour to run at current UK electricity rates. Running it for 6-8 hours daily in winter adds roughly £10-15 to your monthly electricity bill. Compare that to the cost of mould remediation or window frame replacement and it’s excellent value.
Window Upgrades Worth Considering
Secondary Glazing
Adding a second pane of glass or acrylic inside your existing window frame creates an insulating air gap that keeps the inner surface warmer. This is cheaper than full double glazing replacement (£100-200 per window vs £300-800) and can be done in rented properties with removable magnetic systems. It won’t eliminate condensation entirely but reduces it noticeably.
Double Glazing
If you have single-glazed windows, upgrading to double glazing is the most effective structural fix. Modern double-glazed units with argon gas filling and low-emissivity coatings keep the inner glass surface warm enough to prevent most condensation. Costs £300-800 per window depending on size and material.
Triple Glazing
The inner pane surface temperature with triple glazing stays close to room temperature even in freezing conditions. Condensation is virtually eliminated. However, the cost premium over double glazing (roughly 30-50% more) is hard to justify purely for condensation control — it makes more sense as part of a broader energy efficiency upgrade.
Window Film
Low-cost insulating film applied to the glass surface raises the surface temperature slightly and can reduce condensation. Products like 3M Indoor Window Insulator Kit cost about £10-15 per window. They’re not as effective as secondary glazing but they’re cheap, easy to apply, and can make a meaningful difference on problem windows.
Condensation Between Double Glazing Panes
What It Means
If condensation appears between the two panes of a sealed double-glazed unit — not on the inside surface — the seal has failed. Moisture has entered the gap between the panes, and no amount of ventilation or heating will fix it. The unit needs replacing.
Repair or Replace?
- The sealed unit (the glass) can often be replaced without changing the entire window frame. This costs £75-150 per unit, far less than a full window replacement
- DIY repair kits exist (they drill a hole, inject desiccant, and reseal) but results are mixed and rarely last more than a couple of years
- Full window replacement is only necessary if the frame is also damaged or if you’re upgrading from older double glazing to more efficient units
Warranty
Most double-glazed sealed units come with a 10-year warranty against seal failure. Check your paperwork before paying for replacements — you may be covered.
When Condensation Signals a Bigger Problem
Rising Damp
If condensation is worst on ground-floor walls rather than windows, you may have rising damp. Signs include tide marks on walls, peeling wallpaper at low levels, and salt deposits on brickwork. Rising damp requires professional diagnosis — many damp-proofing companies offer free surveys, though be aware that some oversell treatments. Get at least three quotes and an independent assessment.
Penetrating Damp
Water entering through the building fabric — cracked render, failed pointing, leaking gutters — adds moisture that shows up as condensation elsewhere. If you notice damp patches on walls that correspond to external features (downpipes, window sills, roof junctions), investigate the external cause before treating the internal symptoms.
Structural Ventilation Issues
Some homes, particularly those with retrospective cavity wall insulation, develop ventilation problems that standard advice can’t fix. If you’ve tried everything above and condensation persists, consider a professional ventilation assessment. A qualified surveyor can identify whether the building itself needs structural ventilation improvements.
Frequently Asked Questions
Is condensation on windows bad for your health? Condensation itself isn’t directly harmful, but the mould it encourages is. Black mould (Stachybotrys) produces spores that can trigger asthma, allergies, and respiratory problems — particularly in children, elderly people, and those with existing health conditions. The NHS recommends keeping indoor humidity below 60% to prevent mould growth.
Will a dehumidifier stop condensation on windows? It helps a great deal but may not eliminate it entirely. A dehumidifier reduces the overall moisture level in the air, which reduces the amount of water available to condense. For best results, combine a dehumidifier with improved ventilation and reduced moisture sources.
Should I keep trickle vents open in winter? Yes. Trickle vents are designed to provide continuous background ventilation with minimal heat loss. Closing them traps moisture inside and worsens condensation. The small amount of heat lost through open trickle vents is far less costly than dealing with mould and damp damage.
Why do some rooms get condensation and others don’t? Condensation concentrates in rooms with the most moisture production (kitchen, bathroom) or the coldest surfaces (north-facing rooms, unheated bedrooms). Rooms with good ventilation and consistent heating rarely have problems, even in winter.
Does condensation mean my windows are faulty? Not if the condensation is on the inside surface of the glass. That’s a ventilation and humidity issue, not a window fault. Condensation between the panes of double glazing does indicate a failed seal and needs repair. Condensation on the outside of windows is actually a sign that your glazing is working well — it means the outer pane is cold because heat isn’t escaping through it.