Heat escapes from windows and doors primarily because they are the weakest thermal link in the building fabric, accounting for up to 25% of total heat loss in a typical UK home (Energy Saving Trust, 2026). Poor insulation, air leakage, and the inherent properties of glass and frames all contribute to this energy drain.
The rate of heat loss depends on the type of glazing, the frame material, and the quality of installation. Single-glazed windows lose heat roughly twice as fast as double-glazed units, while poorly sealed doors can let in draughts that bypass insulation entirely. The answer also varies by property age, older homes with timber frames and single glazing suffer far greater losses than modern builds with certified energy-efficient products.
Conduction through glass and frames
Glass is a poor insulator. A single-glazed window has a typical U-value of around 5.7 W/m²K, meaning it loses heat rapidly to the colder outside air (BRE, 2026). Double glazing reduces this to about 2.8 W/m²K, while triple glazing can achieve 1.5 W/m²K or lower. Frames also matter, aluminium frames conduct heat more than uPVC or timber, which offer better thermal performance. Even with modern glazing, the frame-to-glass edge seal can act as a thermal bridge if not properly designed.
Draughts and air leakage
Unsealed gaps around window and door openings allow warm air to escape directly. The Energy Saving Trust estimates that draught-proofing windows and doors can save a typical household £60–£90 per year on heating bills (Energy Saving Trust, 2026). Key leak points include worn weatherstripping, gaps between sash frames and casings, and poorly fitted thresholds. Even a 3mm gap around a door can let in as much air as an open window, dramatically increasing heat loss.
Radiative heat transfer through glass
Warm objects and surfaces inside a room emit infrared radiation. Glass is transparent to this radiation, so heat radiates directly through the pane to the colder outdoors. Low-emissivity (low-E) coatings on modern double glazing reflect this infrared energy back into the room, reducing radiative loss by up to 50% compared to uncoated glass (MCS Certified, 2026). Without such coatings, even well-sealed windows lose significant heat through radiation alone.
A worked example
A typical 1930s semi-detached house in Manchester with four single-glazed timber sash windows and an unsealed back door will lose around £410 per year through those openings alone. 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 this cuts heat loss by 25% across the whole house, saving £185 annually on heating bills. With a payback period of around 35 years, many homeowners instead prioritise draught-proofing and secondary glazing, which together cost under £800 and save £100 per year. If the household qualifies for ECO4 funding, the full cost of replacement windows may be covered, reducing payback to zero. A full replacement set with triple glazing would achieve a U-value of 1.5 W/m²K, compared to the original 5.7 W/m²K, a 74% reduction in conductive heat loss.
| Item | Figure |
|---|---|
| Upfront cost after grants | £6,500 |
| Yearly savings | £185 |
| Payback period | 35 years |
| 25-year lifetime savings | £4,625 |
What homeowners often get wrong
The most common mistake is believing that double glazing alone solves all window heat loss. Here are three frequent errors that cost UK homeowners money and comfort.
- Ignoring the frame material Many people assume all double glazing performs the same. Aluminium frames conduct heat up to 1,000 times faster than uPVC or timber, so a poorly framed double-glazed unit can still have a U-value above 3.0 W/m²K, wasting £80 per year more than a thermally broken frame.
- Skipping professional draught-proofing A single gap of 2mm around a door lets in enough cold air to cancel out the benefit of loft insulation. Fitting a brush strip and threshold seal costs £30 but saves £60 annually, yet 40% of UK homes still have unsealed gaps around windows and doors.
- Overlooking trickle vents Sealing windows completely to stop draughts can trap moisture, leading to condensation and mould growth. Building Regulations require trickle vents or mechanical ventilation in most new installations, and blocking them voids the warranty and risks damaging window frames within two years.
Quick reference
- Single-glazed windows lose heat at 5.7 W/m²K, double glazing at 2.8 W/m²K, and triple glazing at 1.5 W/m²K or lower.
- Draught-proofing windows and doors saves a typical UK household £60 per year and costs under £100 to install.
- The BUS grant covers up to £7,500 for a heat pump, but only if your home has at least 100mm of loft insulation and some double glazing.
- Replacing single glazing with A-rated double glazing cuts window-related heat loss by 50% and saves £185 annually on a semi-detached house.
- Fitting trickle vents is a legal requirement for replacement windows under Part F of Building Regulations, skipping them risks invalidating your warranty.
Frequently Asked Questions
Up to 25% of total heat loss in a typical UK home escapes through windows and doors, according to the Energy Saving Trust. This figure can be higher in older properties with single glazing or poor seals.
Double glazing reduces heat loss significantly, with a typical U-value of 2.8 W/m²K compared to 5.7 W/m²K for single glazing (BRE, 2026). However, heat can still escape through frames and gaps if not properly installed or maintained.
Draught-proofing your doors can save a typical household £60 per year, as advised by the Energy Saving Trust. Install brush strips or weather seals around the frame and use a door sweep at the bottom to block air leaks.
