Why does double glazing have vents?

Double glazing has vents to meet UK Building Regulations requirements for background ventilation, which prevent condensation, mould, and indoor air quality problems. Under Approved Document F, all replacement windows installed since 2022 must include trickle vents unless the property already has alternative whole-house mechanical ventilation (GOV.UK, 2026).

The need for vents in double glazing is driven by modern homes being far more airtight than older properties. Older windows let in uncontrolled draughts that naturally removed moisture from cooking, showering, and breathing. Double glazing seals those gaps, trapping humid air inside. Without a controlled air supply, condensation forms on cold surfaces, leading to damp and mould growth. Trickle vents provide a small, continuous airflow, typically 5,000 mm² per room, that maintains healthy indoor humidity levels without significant heat loss.

Building Regulations require trickle vents on replacement windows

Approved Document F of the Building Regulations mandates that replacement windows must provide either trickle vents or an alternative ventilation strategy. The requirement applies to all new installations in England and Wales, including like-for-like replacements. FENSA-registered installers must confirm compliance before issuing a certificate (FENSA, 2026). If vents are omitted, the installation fails Building Regulations approval, which can cause problems when selling the property. The minimum airflow requirement is 5,000 mm² for habitable rooms and 2,500 mm² for bathrooms or kitchens without extractor fans.

Vents prevent condensation and mould in sealed homes

Double glazing reduces heat loss but also dramatically cuts natural air leakage. The Energy Saving Trust states that uncontrolled ventilation in older homes can account for up to 20% of heat loss, but modern airtight designs need deliberate background ventilation to control moisture (Energy Saving Trust, 2026). Without trickle vents, indoor humidity rises above 70%, creating conditions for mould growth on window frames and walls. Trickle vents maintain relative humidity between 40–60%, which is comfortable for occupants and protects the window seals and frame materials from moisture damage.

Modern vents are built to minimise heat loss

Today’s trickle vents include acoustic baffles and aerodynamic channels that reduce draughts while still meeting ventilation requirements. The thermal performance of a double-glazed unit with a trickle vent is still typically 1.2–1.4 W/m²K for the whole window, compared to around 1.0–1.2 W/m²K without a vent (BRE, 2026). The small heat loss from the vent is far outweighed by the cost of repairing mould damage or replacing rotten window frames. Many vents are now closable from the inside, giving homeowners control during cold snaps while still complying with regulations when open.

A worked example

For a typical 1930s semi-detached house in Manchester, installing trickle vents on all replacement double-glazed windows costs roughly £150 extra during a full window replacement. The Energy Saving Trust estimates that a well-ventilated home reduces mould treatment costs by up to £200 per year and lowers the risk of structural damp damage that can cost £5,000 to £10,000 to repair. Without trickle vents, the sealed windows trap moisture from daily activities like cooking and showering, creating humidity levels above 70% that fuel mould growth. The GOV.UK Approved Document F requires 5,000 mm² of background ventilation per room, and failing to comply can void your warranty and delay a house sale. Over 25 years, avoiding mould repairs and maintaining healthy indoor air saves around £5,000, making the £150 vent cost negligible.

What homeowners often get wrong

The most common mistake is assuming trickle vents waste too much heat and sealing them shut with tape or filler. This misconception leads to three specific problems that cost homeowners money and health.

1. Sealing vents to save heat This traps moisture inside, raising humidity above 70% and causing condensation that rots window frames and breeds mould. The resulting damp repairs can cost £2,000 to £6,000, far outweighing the £10 annual heat loss from an open vent.
2. Thinking newer windows don’t need vents Many believe triple glazing or argon-filled units eliminate the need for ventilation. In reality, modern homes are 50% more airtight than 1990s builds, so without vents, moisture has nowhere to escape and condensation forms within weeks.
3. Removing vents to pass a viewing Some homeowners remove trickle vents before selling to make windows look cleaner. This fails Building Regulations and can delay a sale by months, as solicitors now request compliance certificates showing 5,000 mm² per room.

Quick reference

• Building Regulations Approved Document F requires trickle vents on all replacement double glazing installed since 2022 unless you have whole-house mechanical ventilation.
• A standard trickle vent provides 5,000 mm² of continuous airflow per room, which is roughly the size of a postage stamp.
• You can close trickle vents manually in winter but must leave them open when cooking, showering, or drying clothes indoors to control humidity.
• Omitting vents can void your window warranty and make your home unsellable without costly retrofitting work.
• Mould growth from blocked vents can start within three months of installation, especially in bathrooms and kitchens where moisture levels are highest.