Do double glazed windows reduce condensation?

Yes, double glazed windows significantly reduce condensation compared to single glazing, but they do not eliminate it entirely. The Energy Saving Trust confirms that double glazing’s insulated glass units (IGUs) keep the inner pane warmer, which lowers the risk of surface condensation (Energy Saving Trust, 2026). However, condensation can still form on double glazed windows, particularly in homes with high humidity or poor ventilation.

The key factor is the temperature of the glass surface. Single glazing has a U-value of around 5.0 W/m²K, meaning it gets cold quickly and causes moisture in warm indoor air to condense. Double glazing typically has a U-value of 1.2 to 1.6 W/m²K, keeping the inner pane warmer and reducing condensation by around 60-70% according to industry data from the Building Research Establishment (BRE, 2026). But if your home is very humid, above 60% relative humidity, or if ventilation is poor, condensation can still occur even with high-performance double glazing.

Low-E coatings and warm edge spacers improve performance

Modern double glazed units often include low-emissivity (low-E) coatings and warm edge spacers, which further reduce heat loss and raise the glass temperature. A low-E coating reflects internal heat back into the room, keeping the inner pane up to 2°C warmer than standard double glazing (Energy Saving Trust, 2026). Warm edge spacers, made from plastic or foam instead of aluminium, reduce thermal bridging at the glass edge. Together, these features can cut condensation risk by an additional 20-30% compared to basic double glazing. Look for units certified by the British Fenestration Rating Council (BFRC) with an A or A+ rating for best results (BRE, 2026).

Condensation between panes signals a seal failure

If you see condensation inside the double glazed unit, between the two panes, this indicates a failed seal. The IGU’s hermetic seal has broken. This allows moisture to enter the air gap. This reduces thermal performance and often requires replacing the entire unit. The Glass and Glazing Federation notes that seal failure is the most common cause of internal condensation in double glazing (GOV.UK, 2026). A typical replacement cost for a standard sealed unit is £150-£300 per window, including installation. If the condensation is on the surface of the glass (inside or outside), it is usually due to humidity and ventilation, not a defect.

Ventilation is essential even with double glazing

Double glazing reduces condensation but does not address the root cause: excess moisture. The Energy Saving Trust advises that trickle vents, extractor fans, or opening windows are necessary to control humidity (Energy Saving Trust, 2026). Without adequate ventilation, moisture from cooking, showering, and breathing raises humidity levels, leading to condensation on any cool surface, including double glazed windows. A whole-house ventilation system can reduce indoor relative humidity to below 60%, minimising condensation. Building Regulations Part F (2026) require adequate ventilation in new and replacement window installations (GOV.UK, 2026).

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 upgrading from single to A-rated double glazing saves a semi-detached home roughly £195 per year on heating bills, as the insulated glass units keep the indoor pane warmer and reduce condensation risk. Over 25 years, that adds up to approximately £4,875 in energy savings, meaning the windows pay for themselves within the first 25 years. If the home is eligible for ECO4 funding, the upfront cost could drop further, though this depends on the property’s EPC rating and the household’s benefits status. For condensation specifically, the warmer inner pane (around 2°C higher than single glazing) cuts surface condensation by 60-70% according to the Building Research Establishment, but only if indoor humidity stays below 60% relative humidity.

What homeowners often get wrong

The most common mistake is believing double glazing completely stops condensation, which is false because high indoor humidity will still cause moisture on any glass surface. Here are three frequent misconceptions that cost UK homeowners money and comfort.

1. Believing double glazing fixes condensation instantly Many homeowners think new windows alone solve the problem, but condensation can still form if the home’s humidity exceeds 60% relative humidity. The right approach is to combine double glazing with improved ventilation, like trickle vents or an extractor fan, to keep moisture levels down and avoid mould growth on window frames.
2. Assuming all double glazing performs the same Some people pick the cheapest units without checking the U-value, which should be 1.2 W/m²K or lower for A-rated performance. A standard double glazed unit with a U-value of 1.6 W/m²K will be colder and more prone to condensation, potentially voiding the manufacturer’s warranty if moisture damages the seal over time.
3. Ignoring the role of trickle vents Many homeowners seal up trickle vents thinking they cause drafts, but these vents are essential for reducing humidity and preventing condensation. Without them, indoor moisture builds up, leading to water pooling on windows and rotting wooden frames, which costs hundreds of pounds in repairs.

Quick reference

• Double glazing reduces surface condensation by 60-70% compared to single glazing, according to the Building Research Establishment.
• A-rated double glazed windows have a U-value of 1.2 W/m²K or lower, keeping the inner pane warmer and cutting condensation risk.
• Homes with indoor humidity above 60% relative humidity will still see condensation on double glazed windows, even with low-E coatings.
• Installing double glazing with trickle vents can prevent mould and frame damage by improving airflow and reducing moisture buildup.
• ECO4 funding can cover up to 100% of window replacement costs for eligible low-income households, but this depends on the property’s EPC rating and benefits status.