How much can I save by replacing single glazing with double glazing?

Replacing single glazing (U-value 5.0) with A+ rated double glazing (U-value 1.2) can save a typical UK home £100-£300 per year on heating bills, depending on the number and size of windows. The Energy Saving Trust estimates a detached house could save up to £235 per year. Homes with many large windows or in colder regions will see the biggest savings.

Is triple glazing worth the extra cost in the UK?

Triple glazing (U-value ~0.8) provides about 33% better thermal performance than A+ double glazing (U-value ~1.2). However, the additional cost saving is relatively small — typically £30-£80 per year more than good double glazing. Triple glazing is most worthwhile in very exposed locations, north-facing windows, or for noise reduction. The payback period for the extra cost over double glazing is often 20+ years.

What U-value should new windows achieve under UK Building Regulations?

Under Approved Document L (2021), replacement windows in England must achieve a U-value of 1.4 W/m²K or better, or alternatively the whole window must have a Window Energy Rating of band B or better. For new-build properties, the target U-value is 1.2 W/m²K. Scotland requires 1.2 W/m²K for all replacement windows.

How is heat loss through windows calculated?

Heat loss through windows is calculated using the formula Q = U × A × ΔT, where Q is heat loss in watts, U is the U-value of the glazing (W/m²K), A is the total window area (m²), and ΔT is the temperature difference between inside and outside (°C). This calculator uses the UK average temperature difference of ~8°C over a typical 2,000-hour heating season (October to April).

Do windows affect my EPC rating?

Yes, window glazing type significantly affects your Energy Performance Certificate (EPC) rating. Upgrading from single to double glazing can improve your EPC by one or two bands. This matters because rental properties in England and Wales must achieve at least an EPC band E (with a target of band C by 2035 for rented homes). Better windows also increase property value.

TradeCalcs

Window Energy Savings Calculator — Glazing Upgrade Savings

Calculate how much energy, money and CO&sub2; you could save by upgrading your windows. Compare single, double and triple glazing performance for UK homes.

Your Windows

Number of Windows

Total windows being upgraded

Average Window Size

Choose a preset or enter a custom size

Glazing Comparison

Current Glazing

What you have now

Upgrade To

What you plan to install

Energy Cost

UK gas price cap rate is ~24.5p/kWh (adjust to your tariff)

How We Calculate This

This calculator estimates the energy and cost savings from upgrading your windows by comparing the heat loss through your current glazing with the heat loss through the upgraded glazing.

The formula

Heat loss (W) = U-value × Window area × ΔT

Annual energy (kWh) = Heat loss (kW) × Heating hours per year

Annual saving = (Current kWh − New kWh) × Energy cost per kWh

U-values used

Single glazing: 5.0 W/m²K
Old double glazing (pre-2002): 3.0 W/m²K
Modern double glazing: 1.4 W/m²K
A+ rated double glazing: 1.2 W/m²K
Triple glazing: 0.8 W/m²K

UK assumptions

The calculation uses a UK average temperature difference of 8°C over a heating season of approximately 2,000 hours (roughly October to April). CO&sub2; savings are based on the gas heating emission factor of 0.183 kg CO&sub2;/kWh, as most UK homes use gas central heating. The default energy cost of 24.5p/kWh reflects the current UK gas price cap rate.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.