What causes water droplets on the inside of a window?
When warm, moisture‑laden air meets a surface that is colder than the dew‑point temperature, the vapour condenses into liquid. Glass conducts heat; any pane colder than the room can become a condensation surface, producing a thin film that can fog the view, drip onto the sill and damage the frame over time.
Indoor humidity, temperature and glazing performance
In the UK, indoor relative humidity (RH) usually ranges from 30 % in winter to 60 % in summer. Above 60 %, condensation risk rises by about 40 %. Low‑performance glazing exacerbates this; a single‑pane window with a U‑value of 5.8 W/m²·K can cool to around 12 °C while the room stays at 20 °C, dropping below the indoor dew point at 70 % RH.
Outside condensation – a different symptom
Double‑glazed or low‑U‑value units may show condensation on the outer pane. On a calm night with 2 °C outside and 80 % RH, the exterior glass can fall to ‑1 °C, below the outdoor dew point, creating a fine film. The interior pane stays clear because it is warmed by the room.
Typical indoor activities that raise RH
Cooking, long showers and indoor drying of laundry can push RH to 70 % or higher. Each 10 % increase adds roughly 0.5 °C to the dew‑point, making condensation more likely on uninsulated glass.
How condensation affects window longevity
Repeated wetting of frames encourages rot and mold; unchecked moisture can shorten a window’s life by 15‑20 % and compromise indoor air quality.
Regular cleaning of the interior pane with a gentle, non‑acrylic cleaner helps remove mineral deposits that can trap moisture. Inspect frames for cracks and seal any gaps with a weather‑stripping product to reduce condensation spots.
Mitigation strategy 1 – control indoor humidity
Improving ventilation is the simplest first step. Opening windows for 5‑6 minutes after a shower can lower RH by 10‑15 %. The Building Research Establishment recommends exhaust fans of at least 100 L/s per bathroom to remove moist air efficiently.
For more detailed guidance on airflow rates, see our home ventilation tips guide.
In addition to exhaust fans, installing a small, continuous ventilation system such as a heat‑recovery ventilator (HRV) can maintain low RH without significant energy loss.
Mitigation strategy 2 – use a dehumidifier
Portable dehumidifiers set to keep RH between 40‑55 % can reduce condensation risk by up to 30 %. Choose a unit sized for the room’s volume; a 30‑litre‑per‑day capacity suits most 20 m² bathrooms.
While dehumidifiers consume electricity, the energy cost is offset by the reduced heating requirement during winter. Choosing a unit with a built‑in humidity sensor and timer improves efficiency.
Mitigation strategy 3 – upgrade glazing
Replacing single‑glazed units with double‑ or triple‑glazed windows that have low‑emissivity (Low‑E) coatings cuts U‑values to 1.0‑1.5 W/m²·K. Under UK Building Regulations Part L, new homes must not exceed 1.6 W/m²·K. Low‑E glazing can cut heat loss by up to 50 % compared with single glazing, keeping interior surface temperatures above the dew point in most winter scenarios.
Explore our energy efficient windows section for performance data and rebate information.
Professional installation of low‑E glazing ensures proper sealing and alignment, and most manufacturers offer a 25‑year performance guarantee.
Mitigation strategy 4 – choose breathable window treatments
Heavy curtains or internal blinds trap moist air against the glass. Sheer curtains or blinds with airflow gaps allow moisture to circulate, reducing surface wetness.
When selecting curtains, opt for natural fibres such as cotton or linen, which are breathable and less prone to trapping moisture.
Condensation risk table
| Condition | Indoor RH | Glass U‑value (W/m²·K) | Typical interior glass temp (°C) | Condensation likelihood |
|---|---|---|---|---|
| Single‑pane, winter heating | 70 % | 5.8 | 12 | High |
| Double‑glazed Low‑E, moderate heating | 55 % | 1.3 | 16 | Low |
| Triple‑glazed, high efficiency | 45 % | 0.9 | 18 | Very low |
Using the table, homeowners can compare their current setup against risk thresholds and plan upgrades accordingly.
Putting it all together – a condensation‑proof home
A practical approach combines three layers of defence: maintain indoor RH at 40‑55 % through ventilation and dehumidification, upgrade to glazing meeting the 1.6 W/m²·K limit, and select window dressings that allow air movement. When applied together, heating demand can drop by up to 15 % and mould risk is markedly reduced.
Regulatory and financial context
UK Building Regulations Part L limits new‑build window U‑values to 1.6 W/m²·K. ENERGY STAR and the Green Homes Grant (when available) provide rebates for compliant high‑efficiency glazing. Installing a compliant exhaust fan also satisfies Approved Document F ventilation rates, helping maintain code compliance.
Staying informed about changes to building codes and available incentives allows homeowners to take advantage of new opportunities for efficiency gains.
Key takeaways include: keep indoor RH between 40‑55 %, upgrade to low‑U glazing, maintain proper ventilation, and use breathable window treatments. These steps work together to minimise condensation and improve comfort.
If you need more detailed advice, consider consulting a qualified professional. You may also review the resources linked above.
Understanding Dew Point and Temperature Gradient
The dew point is the temperature at which air becomes saturated and water vapour condenses. In practice, the interior surface of a window will often drop below this temperature if the window is poorly insulated. The temperature difference between the room and the glass, often called the temperature gradient, is the key driver of condensation. Even a few degrees of cooling can push the glass surface below the dew point, especially during winter when the room temperature is lower than the outdoor temperature.
For example, if a room is kept at 20 °C and the indoor RH is 65 %, the dew point is around 12 °C. A single‑pane window that cools to 11 °C will see condensation, whereas a double‑glazed unit that stays above 13 °C will remain dry.
Common Indoor Activities That Increase Humidity
- Cooking – simmering sauces or boiling water releases moisture; a kitchen vent or extractor fan can remove 200–300 L/s of humid air.
- Showers and baths – a 10‑minute hot shower can raise room RH by up to 20 % if ventilation is absent.
- Drying laundry indoors – drying a load of clothes can add 0.5–1 g of water per litre of air; a 5 m² room will see RH rise by roughly 5 % within 30 minutes.
- Household chores – activities like washing dishes, brushing hair, or even breathing in a densely populated room contribute incremental moisture.
Awareness of these activities allows homeowners to time ventilation or use of a dehumidifier more effectively.
Case Study: A Typical Two‑Storey House
Jane owns a 75 m² detached house with three single‑pane windows. During the winter, the internal RH regularly reaches 75 % after cooking and evening showers, and condensation appears on all window sills. She installed a 120 L/s exhaust fan in each bathroom, a 30‑litre‑per‑day dehumidifier in the kitchen, and replaced the windows with double‑glazed Low‑E units. After six months, indoor RH stabilised around 48 %, condensation ceased, and the heating bill dropped by 8 % compared with the previous year.
Step‑by‑Step Guide to Installing a Heat‑Recovery Ventilator (HRV)
- Assess your home – Identify the rooms that generate the most moisture (bathrooms, kitchens). Verify that the building has adequate structural clearance for ductwork.
- Select the unit – Choose an HRV with a recovery efficiency of 55 % or higher. Size it to at least 30 L/s per occupied room for a typical 20 m² space.
- Plan the duct layout – Run supply ducts from the HRV to the living area and exhaust ducts to the exterior, ensuring the paths are as straight as possible to minimise pressure loss.
- Install the unit – Mount the HRV on a wall or in the loft, following the manufacturer’s instructions for mounting height and vent placement.
- Seal the system – Use duct sealant or foil tape to seal junctions. Check that the intake and exhaust flanges are properly gapped to avoid backdraft.
- Set the controls – Program the thermostat to maintain the room temperature at 20 °C and the dehumidifier to keep RH between 40‑55 % during peak activity periods.
- Test and calibrate – After the first week, measure indoor RH in several rooms. Adjust the HRV speed or add supplementary ventilation if required.
Professional installation ensures the HRV is correctly sealed and the heat‑exchange unit is protected from dust and moisture, extending its service life.
Cost‑Benefit Analysis: Energy Savings vs. Investment
Installing a Low‑E double‑glazing unit typically costs £200–£400 per window, while a complete set of 10 windows can reach £2,000–£3,000. The pay‑back period depends on the heating system and local energy prices, but many homeowners observe a return of 2–3 years via reduced heating costs.
A 30‑litre‑per‑day dehumidifier costs about £150 upfront and consumes 0.5 kWh per day. With electricity at £0.30/kWh, the daily cost is £0.15, equating to £54 per year. If heating savings amount to £150 annually, the net benefit is £96 per year.
Combining both measures often yields a pay‑back of 3–4 years and a steady reduction in mould-related health complaints.
Frequently Asked Questions
- Can I simply close windows to keep heat inside? Closing windows reduces heat loss but increases indoor RH, making condensation more likely.
- Will a dehumidifier affect indoor air quality? Modern dehumidifiers come with built‑in filters that can capture dust and allergens, improving overall air quality.
- Is it necessary to replace all windows? Targeting the most exposed or poorly insulated windows provides the greatest benefit without a full replacement.
- How often should I check my ventilation system? Inspect ductwork and fans annually to ensure they remain free of dust and that seals stay intact.
Conclusion: Building a Condensation‑Free Home
Preventing window condensation requires a holistic approach that balances moisture control, thermal performance, and airflow. By monitoring indoor RH, installing appropriate ventilation, adopting dehumidifiers when necessary, upgrading glazing, and selecting breathable window treatments, homeowners can maintain comfortable indoor conditions while protecting their property and reducing energy consumption.
For additional guidance on choosing the right window technology and ventilation systems, refer to our detailed product reviews and installation resources on the site.
