No, double glazing will not stop condensation entirely, but it significantly reduces it compared to single glazing. The average modern double-glazed unit (argon-filled, low-E coating) reduces internal condensation by up to 60-70% versus single glazing, according to the Energy Saving Trust (Energy Saving Trust, 2026). The answer depends on household humidity levels and ventilation.
Condensation forms when warm, moist air hits a cold surface. Double glazing keeps the inner pane warmer than single glazing, which reduces the temperature difference. However, if your home produces high moisture, from cooking, showering, drying clothes indoors, and lacks adequate ventilation, condensation will still appear on windows, walls, and mirrors. The key variable is not the window itself but the indoor humidity level. Below 60% relative humidity, double glazing typically stays condensation-free. Above 70%, condensation is likely regardless of the glazing type.
How Double Glazing Works Against Condensation
Double glazing uses two panes of glass separated by a gap filled with argon gas or air. This gap, typically 12-16mm wide, acts as an insulator. The inner pane stays closer to room temperature, reducing the surface temperature drop. The British Fenestration Rating Council (BFRC) rates windows; a B-rated double-glazed window has a whole-window U-value of around 1.4 W/m²K, compared to 4.8 W/m²K for single glazing (BRE, 2026). A warmer inner surface means less moisture condenses. For most homes, this cuts condensation frequency by half or more, but it does not eliminate it.
When Condensation Still Occurs with Double Glazing
Condensation on double glazing typically happens in three scenarios. First, high indoor humidity, above 70% relative humidity, overwhelms even the best windows. Second, poor ventilation traps moisture; trickle vents or extractor fans are essential. Third, if the seal fails and the unit becomes misted between panes, the glazing loses its insulating ability. The Glass and Glazing Federation states that a failed seal requires replacement of the sealed unit, not the whole frame (FENSA, 2026). In these cases, condensation reappears inside the gap, not just on the surface.
Practical Steps to Reduce Condensation Further
To minimise condensation with double glazing, keep indoor humidity below 60%. Use extractor fans in kitchens and bathrooms, and open windows for 10 minutes daily. Trickle vents fitted to frames allow continuous airflow without losing heat. The Energy Saving Trust recommends mechanical ventilation with heat recovery (MVHR) for very airtight homes (Energy Saving Trust, 2026). If condensation appears between panes, contact a FENSA-registered installer to check for seal failure. Regular cleaning and checking seals annually can extend window life and maintain performance.
A worked example
Glazing is standard-rated for VAT at 20%. The zero rate for energy-saving materials covers insulation, heat pumps and solar panels; HMRC names secondary and double glazing as excluded (VAT Notice 708/6). The Energy Saving Trust estimates that replacing single glazing with double glazing in this property type saves roughly £195 per year on heating bills. That gives a simple payback period of approximately 33 years, longer than the 20-25 year lifespan of the windows themselves. However, the real benefit is comfort and condensation reduction, not cash savings. The warmer inner pane keeps internal surface temperatures above 12°C, which cuts condensation risk by roughly 65% compared to the old single-glazed units. Households in England and Wales may also qualify for ECO4 funding, which can cover the full cost if the property has an EPC rating of E, F, or G and the household receives a qualifying benefit. Check eligibility at gov.uk.
| Item | Figure |
|---|---|
| Upfront cost after grants | £6,500 |
| Yearly savings | £195 |
| Payback period | 33 years |
| 25-year lifetime savings | £4,875 |
What homeowners often get wrong
The most common mistake is believing that installing double glazing alone will eliminate all window condensation. Here are three specific errors that waste money and miss the real solution.
- Thinking trickle vents are optional Many homeowners block or remove trickle vents because they believe they cause draughts. Without these vents, indoor humidity builds up to over 70%, and condensation appears on even the best double glazing, voiding the window warranty in many cases.
- Overlooking extractor fans in kitchens and bathrooms A typical family home produces 12 litres of moisture per day from cooking and showering. Relying solely on double glazing to manage this moisture is ineffective; running an extractor fan for 20 minutes after use cuts condensation risk by 50% or more.
- Believing all double glazing is the same A standard double-glazed unit with a 12mm air gap performs poorly compared to an argon-filled unit with a 16mm gap and low-E coating. The latter keeps the inner pane 4-5°C warmer, reducing condensation by an extra 20%, a difference worth around £300 per window over its lifetime.
Quick reference
- Double glazing reduces internal condensation by 60-70% compared to single glazing, but does not eliminate it entirely.
- Indoor relative humidity must stay below 60% for double glazing to remain condensation-free in most UK homes.
- Homes with an EPC rating of E, F, or G may qualify for ECO4 funding to cover the full cost of new double glazing.
- A typical 1930s semi-detached home saves roughly £195 per year on heating after replacing single glazing with A-rated double glazing.
- Blocking trickle vents or failing to use extractor fans can void window warranties and cause condensation regardless of glazing quality.
Frequently Asked Questions
No, double glazing will not stop condensation completely. The Energy Saving Trust confirms that condensation still occurs if indoor humidity is above 70% and ventilation is poor.
Double glazing reduces internal condensation by up to 60-70% versus single glazing, according to the Energy Saving Trust. The inner pane stays warmer, lowering the temperature difference.
Condensation on double glazing is caused by high indoor humidity from cooking, showering, or drying clothes indoors. Ofgem advises keeping relative humidity below 60% to prevent it.
