Keeping a home warm & insulated is always a major concern for the average British household, especially in the winter months. A poorly insulated home can add to your energy bills and you’d never know. But did you know that nearly 1/5th of heat in your house is lost through the windows?

We’ve put together some of the most eye-opening stats about heat loss in the average home. Are you ready for your jaw to drop? You can also find a complete write-up of the post below.

Are Your Bills Flying Out Of The Window_20042016

Infographic Write-Up

Heating homes is a major concern for all homeowners, especially in those chilly winter months. And it comprises a major chunk of your power bill. But did you know that nearly 1/5th of heat in your house is lost through the windows?

Here are a few eye-openers for you.

In an Uninsulated House Heat is Lost in the Following Ways

  • 18% is lost through the windows
  • 11% is lost through the floor and the door
  • 12% is lost due to draughts and essential ventilation
  • 26% of heat is lost through the roof
  • 33% is lost through the walls

How Can You Prevent this Heat Loss?

You can prevent this heat exodus through the windows in a couple of different ways:

  • By installing energy-efficient windows
  • By installing window furnishings

Energy-Efficient Windows Will Have Many of the Following Features

  • Double, triple or even quadruple-glazing
  • Low-emissivity (low-E) glass
  • Inert gas, such as argon or krypton in the sealed unit
  • Low conductivity or warm-edge spacer bars
  • Insulated frames and sashes
  • Good air tightness

Glazed Glass

  1. Double-Glazed windows have two sheets of glass with a gap in between, usually about 16mm, to create an insulating barrier that keeps heat in.
  2. Triple-Glazed windows have three-sheets of glass instead of two.

The typical lifetime of double-glazing is around 20 years, however this can vary from 10-35 years based on quality of materials, installation and where the windows are situated.

Over time, the gas within the panes leak out. When around 25% of the gas has evaporated, the thermal performance of the windows reduces. Replacing the windows or installing a secondary glazing would be a good idea then.

There are three types of secondary glazing:

  • Transparent Film
  • Temporary Secondary Glazing
  • Semi-Permanent Secondary Glazing

Low Emissivity Glass

The most energy-efficient type for double glazing is low emissivity (Low-E) glass. This often has an invisible coating of metal oxide, normally on one of the internal panes. This lets in light and heat but cuts the amount of heat that can get out.

Inert Gases

Energy efficient windows use gases such as argon, xenon or krypton in the gap between the sheets of glass. Argon gas filling can improve the energy index for a given window by about 11kWh/m²/year compared to an air-filled unit.

Pane Spacers

These are set around the inside edges to keep the two panes of glass apart. Efficient pane spacers contain little or no metal – often known as ‘warm edge’ spacers.

How Much Can You Save?

By installing double-glazing in an entirely single-glazed house you could save the following each year:

England, Scotland & Wales

Energy Rating A Energy Rating B Energy Rating C
Detached House
£120 - 155
£110 -140
£105 -135
Semi-Detached House
£80 - 110
£75 - 100
£75 - 95
Mid-Terrace House
£65 - 85
£60 - 80
£60 - 75
Bungalow House
£55 - 75
£50 - 70
£50 - 65
£40 - 55
£35 - 55
£35 - 50

Northern Ireland

Energy Rating A Energy Rating B Energy Rating C
Detached House
£110 - 135
£95 - 125
£90 - 115
Semi-Detached House
£70 - 95
£65 - 90
£65 - 80
Mid-Terrace House
£55 - 75
£50 - 70
£50 - 65
Bungalow House
£45 - 65
£45 - 60
£40 - 55
£35 - 50
£30 - 45
£30 - 45

Frame Materials

U-values can be reduced with advanced frame materials and design.

uPVC frames last a long time and may be recycled. Wooden frames can have a lower environmental impact, but require maintenance. They are often used in conservation areas where the original windows had timber frames.

Aluminium or steel frames are slim and long-lasting, and may be recycled. Composite frames have an inner timber frame covered with aluminium or plastic. This reduces the need for maintenance and keeps the frame weatherproof.

Cost Comparison

Compare the costs of different materials with low and high U-value.

Low U-Value High U-Value
Soft Wood
Hard Wood

Energy Ratings

Window Energy Ratings were launched in early 2004 by the British Fenestration Rating Council (BFRC) , an independent organisation dedicated to improving the energy efficiency of fenestration products. The whole window (the frame and the glasS) is rated on its efficiency at retaining heat.

  • The unit of the rating is kilowatt-hours per square meter per year.
  • 10kWh/m²/year represents a heat loss of about 1 litre oil or 10m³ gas per square metre window per year.
  • The rating combines the three key factors, which affects the window energy performance;
    • Solar Gain (g-Value)
    • Thermal transmittance (Uw-Value)
    • Air Leakage (L50-Value)
  • The three factors are linked by the following equation; WERs = 218.6 x g-Value – 68.5 x (Uw-Value + L50-Value)


Most window manufacturers show the energy efficiency of their products using an energy-rating scale from A to G.

BFRC Energy Rating Scale BFRC Rating kWh/m²/year
> 0
-10 to < 0
-20 to < -10
-30 to < -20
-50 to < -30
-70 to < -50
< -70

A u-value is a measure of how easily heat can pass through a material. Usually materials that let out more heat have a higher u-value whereas materials that let less pass through them have lower u-values.

Consider How Windows Operate

Some operating window types have lower air leakage traits than others which improves their energy efficiency.

Awing Windows

Hinged at the top and open outwards, because the sash closes by pressing against the frame, awning windows generally have lower air leakage rates than sliding windows.

Casement Windows

Hinged at the top and open outwards, because the sash closes by pressing against the frame, awning windows generally have lower air leakage rates than sliding windows.

Fixed Windows

Fixed panes that don’t open. When installed properly they’re airtight and are not suitable in places where window ventilation is required.

Hopper Windows

Hinged at the bottom and open inwards, generally have lower air leakage rates because the sash closes by pressing against the frame.

Single & Double Hung Windows

Both sashes slide vertically in a double-hung window while the bottom sash slides upwards in a single -hung window. Generally have higher air leakage rates.

Single & Double Sliding Windows

Both sashes slide horizontally in a double-sliding window while only one sash slides in a single-sliding window. Generally have higher air leakage rates.

Benefits of Energy-Efficient Windows

  • Smaller energy bills
  • Smaller carbon footprint
  • More comfortable home: energy-efficient glazing reduces heat loss through windows and means fewer draughts and cold spots
  • Peace and quiet: as well as keeping heat in, energy efficient-windows insulate your home against external noise
  • Reduced condensation: energy-efficient glazing reduces condensation build-up on the inside of windows
  • Long-Lasting: Double glazing should last for 20 years or more.
  • Heightens security: Double-glazed glass is more difficult to break than single glazing.
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