How Much Can Secondary Glazing Save on Energy Bills? A Worked Example in Pounds

"How much will I save?" is one of the first questions people ask about secondary glazing. Most answers you find online are either a vague percentage or a suspiciously round number. This article does it differently: we show a worked example in pounds, with every assumption visible, so you can adjust it for your own home.

The Short Answer

For a typical single-glazed London period home, secondary glazing on every window might save in the region of £100–£250 a year on gas heating, and often more once draught reduction is included. Savings depend heavily on how many windows you have, how draughty they are, how warm you keep the house, and your energy prices.

The Worked Example

Here are our assumptions. Change any of them to fit your home.

• Window area: 15m² of glazing, typical for a Victorian terrace.
• Before: Single-glazed timber sash windows, U-value around 4.8 W/m²K.
• After: Secondary glazing with low-emissivity glass, combined U-value around 1.8 W/m²K.
• Improvement: 3.0 W/m²K.
• Heating season: Around 2,200 degree days, a common figure for southern England.
• Gas price: 6p per kWh, with a boiler around 90% efficient.

Step 1: Heat saved per square metre

2,200 degree days is about 52,800 degree hours. Multiply by the 3.0 W/m²K improvement and divide by 1,000 to get kilowatt hours:

$$52{,}800 \times 3.0 \div 1{,}000 \approx 158 \text{ kWh per m}^2 \text{ per year}$$

Step 2: Across the whole house

$$158 \times 15 \approx 2{,}370 \text{ kWh per year}$$

Step 3: Gas used and cost

Allowing for 90% boiler efficiency, that is about 2,630 kWh of gas. At 6p per kWh:

$$2{,}630 \times £0.06 \approx £158 \text{ per year}$$

That is the saving from the glass alone. It does not include draughts.

Draughts: The Hidden Extra

Old sash windows leak air around the sashes and meeting rails. Every cubic metre of warm air that escapes must be replaced and heated. In a draughty period home, this air leakage can be as important as heat lost through the glass. A well-sealed secondary unit cuts most of it, so the real-world saving is often higher than the glass-only figure above. See our sash window draughts guide.

Adjusting for Your Home

• Smaller flat with 6m² of glazing: Roughly 40% of the example, so around £60–£100 a year.
• Large house with 30m² of glazing: Roughly double, around £300–£500 a year.
• Higher gas price: Savings rise in proportion.
• Electric heating: Electricity costs several times more per kWh than gas, so savings in pounds are much higher.
• Already double glazed: The improvement is far smaller, so secondary glazing is mainly worthwhile for noise.

Payback

On energy savings alone, payback is usually measured in many years, not months. That is why most people choose secondary glazing for a combination of reasons: warmth and comfort, quieter rooms, less condensation, keeping original windows, and in rentals, a better EPC rating. For prices, see our secondary glazing cost guide; for EPCs, see our guide to improving EPC ratings without replacing windows.

Comfort Is Not on Your Bill

Single glazing makes rooms feel colder than the thermostat says, because the cold glass draws heat from your body. Secondary glazing raises the inside surface temperature of the window, so the room feels warmer at the same setting. Many people find they can turn the thermostat down a degree, which brings a further saving of its own.

For the technical background, see understanding U-values. To compare cheaper alternatives, read secondary glazing vs window film, curtains and shutters.

Frequently Asked Questions

Does secondary glazing reduce heating bills?

Yes, especially in single-glazed homes. It reduces heat lost through the glass and cuts draughts around old frames.

How much does secondary glazing save per year?

For a typical single-glazed London terrace, roughly £100–£250 a year on gas from the glass alone, with more from reduced draughts. Your figure depends on window area, energy prices and how you heat the home.

Is secondary glazing worth it just for energy savings?

On energy alone, payback is slow. Most people choose it for comfort, noise reduction and keeping original windows, with the bill savings as a bonus.