Double Glazing Savings Calculator

Double glazing replaces single-pane windows with sealed units containing two panes of glass separated by an insulating gap, typically 16mm of argon gas. The effect on heat loss is substantial. A single-glazed window has a U-value of around 5.0 W/m2K, while standard double glazing achieves 1.4 W/m2K, and triple glazing reaches 0.8 W/m2K. Lower is better, and units fitted in the 1990s sit well behind anything sold today at 2.5 to 3.0 W/m2K.

Building Regulations Part L sets the floor for replacements, which must reach a U-value of 1.4 W/m²K or better, or carry a Window Energy Rating of Band B or above. The WER scale runs from A++ down to G in the same way as a fridge label, and the best A++ units land at 0.8 to 1.0 W/m²K. For most houses an A or A+ window is the sensible balance of cost and performance, because the premium paid for the very top band takes a long time to come back.

The cost of double glazing depends on window count, size, frame material and whether you need new frames or just replacement glass units. uPVC is cheapest at £300 to £600 per window, timber runs £400 to £800 and aluminium £500 to £900. A typical 3-bed semi with 10 windows costs £4,000 to £8,000 for full uPVC double glazing, while premium aluminium frames push that to £6,000 to £12,000 and timber-look composite to £8,000 to £18,000. Large bay windows are priced separately at £1,200 to £2,500 each, patio doors at £900 to £2,500 and bi-fold doors from £2,500 to £6,000. Quotes vary wildly between national chains and local installers, so gather 3 or more before signing.

Energy savings from replacing single glazing with double glazing are estimated at £100 to £200 per year by the Energy Saving Trust, and moving from 1990s units to modern A++ glass saves £80 to £200. On energy alone the payback period is long, at 20 to 40 years, so the case rarely rests there. Acoustic glass reduces noise by 35 to 50%, condensation largely disappears, laminated glass and multi-point locks improve security, and the windows change how the house looks. In the worked example, 10 uPVC windows at £550 each cost £5,500 and save £150 a year, a payback of 36.7 years set against a property value uplift of £2,000 to £5,000.

Protected buildings follow different rules. A listed building at Grade I, II* or II cannot take standard uPVC windows, and any change needs listed building consent, which is free, administered by the council and takes 8 to 12 weeks. The usual answers are timber sash windows with slim-profile double glazing at roughly £1,500 to £3,000 per window, or secondary glazing, which keeps the original single-glazed window and adds an internal pane for £600 to £1,500 while preserving the appearance. Restoring sash windows and adding secondary glazing falls in the same bracket. In conservation areas, front-facing windows visible from the highway need planning permission, and an Article 4 direction can restrict every window on the building.

Certification is not optional. Since 2002 every new window fitted in a domestic property has had to comply with Building Regulations, and the installer must belong to FENSA, CERTASS or another DCLG-approved Competent Person Scheme in order to self-certify that compliance. Membership also brings a 10-year insurance-backed guarantee, and the cost sits inside the installation price rather than on top of it. Without the certificate the work is a Building Regulations breach, which surfaces later when you sell the house or apply for a mortgage, so check the installer's registration before the contract is signed.