How to Stop Condensation on Windows
Condensation appears when warm, moisture-laden indoor air meets a cold glass surface. In the UK the ideal indoor relative humidity (RH) is 30-50%. When RH rises above 60% the risk of mould, wood rot and poor indoor air quality increases sharply. Guidance from the WHO links high humidity to asthma and respiratory infections. Condensation is a visible warning that either humidity is too high, the glass temperature is too low, or both.
Three common patterns help diagnose the problem:
- Interior condensation. Moisture forms on the room-side of the pane. This is the most frequent issue and usually points to excess indoor moisture or insufficient heating.
- Exterior condensation. Water droplets appear on the outside glass. This is harmless, often caused by clear night skies and low outdoor humidity. It indicates the window is insulating well.
- Between-pane condensation. Fog appears between the two panes of a double-glazed unit. A failed seal means the insulating gas has escaped and thermal performance is compromised. This requires professional repair or replacement.
A quick finger test can confirm the location: run a finger across the glass. Moisture on the room side points to indoor humidity, moisture outside needs no action, and fog trapped between panes means the unit should be inspected. Understanding which type you have helps you choose the most proportionate and cost-effective response.
Low-Cost Behavioural Controls
Before investing in hardware, most homes can reduce condensation with simple daily habits that cost nothing and can be applied immediately.
Ventilation
Opening windows for just five minutes each day can exchange up to 20% of indoor air and drop relative humidity by 10-15%. In kitchens and bathrooms, run extractor fans during cooking or showering and continue for 20 minutes afterward to remove residual steam. Where natural ventilation is limited, trickle vents provide constant low-level airflow and have been required on all new double-glazed replacements in the UK since 2022.
Cross-ventilation — opening opposite windows — creates a draught that pulls moist air through the room. A 10-minute burst in the morning or evening can make a noticeable difference, even in winter. For consistent results, avoid blocking vents with curtains or furniture and clean extractor filters regularly to maintain airflow.
Heating
Maintain a steady indoor temperature of 18-21°C. When the thermostat is set lower, the glass surface cools and the dew point is reached more easily. Consistent heating keeps glass surfaces warmer and reduces the likelihood of condensation. According to the Department for Energy Security and Net Zero (DESNZ), steady heating can also help manage heating bills when combined with sensible thermostat scheduling.
Avoid large temperature swings between rooms. Keeping bedroom and living room temperatures within a few degrees of each other limits cold spots where moisture is more likely to settle. Using thermostatic radiator valves can help balance heat without overheating the whole property.
Humidity Management
Install a hygrometer (approx. £10-£15) to monitor conditions accurately. If RH regularly exceeds 60%, a dehumidifier sized for the room (roughly 0.5-1 L/day for a 20 m² bedroom) will bring levels down effectively. Avoid excess moisture where possible: dry laundry outdoors or in a well-ventilated room, use lids on pots while cooking, and limit the number of indoor plants in problem rooms. Closing curtains at night can help reduce cold air settling on the glass, but leave a small gap to allow air circulation.
Targeted Upgrades for Lasting Results
When behavioural changes alone are not enough, structural upgrades provide a greater reduction in condensation and improve overall energy performance and comfort.
Glazing Improvements
Upgrading from single to A-rated double glazing can cut condensation risk by up to 60% and raise the inner pane temperature from 5-8°C to 12-15°C. Low-emissivity (Low-E) coatings add a further 2-3°C by reflecting heat back into the room. Triple glazing offers another 5-7°C gain but at a significantly higher cost and with diminishing returns for condensation alone. For many UK homes, secondary glazing or draught-proof strips achieve similar benefits at a lower price while preserving original frames.
If between-pane fog appears, the sealed unit must be replaced. The cost of a new insulated glass unit (IGU) ranges from £100 to £250 per window, and many manufacturers allow IGU replacement without removing the frame, which limits disruption and cost.
Insulation Enhancements
Cavity-wall insulation and loft insulation reduce heat loss through the building envelope and help keep internal surfaces warmer. The Carbon Trust estimates that loft and cavity insulation together can lower heating bills by 10-15% and lift interior glass temperatures by 0.5-1°C, which can cut condensation incidents by 15-20%. Ensure insulation is installed without gaps to avoid cold bridging around window reveals.
Controlled Mechanical Ventilation
Mechanical ventilation with heat recovery (MVHR) extracts moisture while recovering up to 95% of heat. In tightly sealed or newly retrofitted homes, MVHR is the most reliable way to keep RH within 30-50% without sacrificing energy efficiency. While installation costs vary by property size and layout, the system can reduce heating demand and help avoid costly mould remediation and redecoration over time.
Cost-Benefit Overview
Table 1 summarises typical interventions, approximate costs, and expected condensation reduction.
| Intervention | Typical Cost (UK) | Condensation Reduction | Additional Benefits |
|---|---|---|---|
| Daily 5-minute window opening | £0 | 10-15% humidity drop | Improved air quality, no energy use |
| Extractor fan installation | £150-£300 | 20-30% moisture removal | Removes cooking and bathroom steam |
| Dehumidifier (20 L/day) | £120-£250 | 30-40% RH reduction | Prevents mould, protects furnishings |
| Double glazing (A-rated) | £3,500-£5,500 | Up to 60% condensation reduction | Higher EPC rating, lower heating bills |
| Cavity wall insulation | £500-£1,500 | 15-20% condensation reduction | 10-15% heating bill savings |
| Trickle vents (per window) | £20-£50 + £50-£100 fitting | 30-50% surface condensation drop | Compliance with Part F, minimal heat loss |
| MVHR system | £5,000-£12,000 | 70-80% humidity control | Continuous fresh air, heat recovery |
Payback periods vary. Simple ventilation measures often pay back within a year through avoided repairs and redecoration. Glazing and insulation upgrades typically have pay-back windows of 5-30 years depending on energy prices and incentives. Schemes such as the Home Upgrade Grant, the Energy Company Obligation (ECO) and local authority programmes may offset costs for eligible homeowners.
Putting It All Together – A Homeowner Checklist
- Install a hygrometer and monitor RH for one week. If average RH > 60%, proceed to step 2.
- Run extractor fans during high-moisture activities and for 20 minutes afterward. Open windows for 5-10 minutes daily and check trickle vents are present and unobstructed.
- Keep indoor temperature steady at 18-21°C, particularly in rooms prone to condensation.
- If RH remains above 55%, add a dehumidifier sized for the room. Use a humidifier only if RH falls below 30%.
- Inspect windows for seal failure. If fog is trapped between panes, arrange for IGU replacement.
- Audit insulation. Ensure loft insulation is at least 270 mm and cavity walls are filled where appropriate.
- Evaluate glazing. Consider double-glazed Low-E units if current windows are single-glazed or show poor thermal performance.
- For very airtight homes or persistent issues, explore MVHR for continuous moisture control while retaining heat.
Following this hierarchy — starting with no-cost actions and moving up only when needed — helps avoid unnecessary spend while achieving a condensation-free environment. Record RH readings and actions taken so you can see what delivers the best result for your home.
Monitoring indoor humidity, using extractor fans, and ensuring adequate ventilation are essential steps. A hygrometer can help you track RH, and professional advice is recommended when planning major upgrades such as new glazing or MVHR systems.
Long-Term Considerations
When evaluating upgrades, consider both upfront cost and lifetime energy savings. Simple measures like regular ventilation often deliver immediate moisture reductions, while larger investments such as double-glazing or MVHR should be assessed with an energy-modelling tool or a certified energy assessor. Improved efficiency can also support a higher Energy Performance Certificate (EPC) rating, which is helpful when selling or renting.
Choose certified materials for durability. Look for BFRC certification or the CE mark for glazing, and Low-E glass with argon-filled units where specified. Insulation products should meet relevant British Standards, such as BS EN 13162 for mineral wool, and be installed to manufacturer specifications to avoid cold bridges.
Minor tasks like fitting trickle vents or weather-stripping can be DIY, but full-frame window replacement or MVHR installation typically requires a specialist to ensure compliance with Building Regulations Part F, airtightness, and warranty requirements.
If condensation persists or you are planning a major retrofit, consider seeking independent, qualified advice to assess the most suitable ventilation and glazing options for your property. A consultative review can help you compare options objectively and choose a proportionate solution based on your home’s age, construction and budget.
