James Elston

Published April 9, 2024

Double glazing involves using two panes of glass, separated by a gap, to reduce heat transfer between the inside and the outside of the window or door in which it is installed.

87% of homes in the UK use double glazing, so we have all heard the term. But how exactly does it work?

In this guide, we’ll take a deep dive into how this product prevents thermal transfer. We will start by looking at what double glazing is made up of, then we will look at how these parts work together.

We will then look at the benefits of double glazing, how it achieves its high energy efficiency ratings, why double glazing has such a high cost, and how long double glazed windows and doors last in the average household.

What Is Double Glazing?

Double glazing consists of three main parts: the glass panes, the spacer bar, and the fill gas between the panes. Referred to as a double glazed sealed unit or insulated glass unit (IGU), these insulate your home, as thermal energy cannot easily be transferred from one side to the other.

Let’s take a closer look at the inner workings of this unit.

Glass pane

Double glazing comprises two panes of glass. There is an inner pane facing the interior of a building and an external panel that faces the outside. The thicker the glass, the harder it is to make that glass hot or cold, making it harder for heat to escape or for cold to come in. You can also find triple glazing windows which add an additional layer, further reducing thermal transfer.

Glass panes can have treatments applied to them that make them better insulators. One of the examples of these treatments is called low emissivity glass (low-E glass). During manufacturing, a metallic coating, like tin oxide, is added to the glass to improve the material’s thermal insulation.

The coating comes as a hard coating, which is cheaper and more durable, or a soft coating. The soft coating is more efficient but it must be applied to the internal vacuum-sealed panes and is more expensive to produce.

Spacer bar

The spacer bar is what keeps the double glazing panes of glass separated and sealed airtight.

Spacer bars are most commonly made of plastics thanks to the material’s low heat conductivity, but aluminium and steel spacer bars are also used and are sturdier than uPVC.

Often, the spacer bar is filled with a desiccant which helps ensure that the air-tight space between the double glazing remains completely dry during and after manufacture.

Fill gas

Thegapbetween the two double glazing panes can either be filled with air or low-reactivity gases like xenon, krypton, or, most commonly, Argon. These gases are very poor at heat transfer and are fantastic insulators.

For example, anargon-filled chamber is 67% effective in transferring heat from one source to another compared to air. Argon is the cheapest insulating gas, as it is more common in nature.

All three of these gases are non-toxic, clear, and odourless. They present no health risk to you if they leak out of the double glazing unit.

How Does Double Glazing Work?

Thermal insulation

When cold air hits the exterior window panes of a room, it will cool the glass, cooling the air in your home.

Because double glazing has an argon gas layer, for example, between two panes of glass, heat transfer is much less effective. Triple glazing windows add an additional glass and gas layer, which is even more effective.

If you have a low-E coating on the interior glass panes, it will further inhibit the thermal transfer of your double glazing.

Acoustic insulation

Each time a sound wave moves through a different medium, it loses energy and becomes softer.

Double glazing is an acoustic insulator because it consists of three mediums. The sound waves hit glass, then an air gap, then another layer of glass.

By the time sound waves reach you, they have passed through multiple media and lost a lot of energy. You can increase the amount that double glazing reduces noise by adding a laminate layer to the glass panes, referred to as noise reduction glass.

Standard double glazing can cut noise by 20 to 30 decibels. Noise reduction glass can cut noise by between 40 and 50 decibels. For reference, standard double glazing can make light traffic as loud as a whisper, while noise-reduction glass can make a lawn mower as quiet as a computer fan.

Reduced condensation

Condensation on windows forms whencold air meets a hot glass pane orwarm air meets a cold glass pane. We see the same effect on ourbathroom mirrors when we have a hot shower. Moisture-laden air plus a surface with a temperature differential equals condensation.

Double glazing prevents the cold air from outside from coming in direct contact with the warm air inside, and vice versa.

Additionally, there are desiccating materials in the spacer bars. This is another way in which double glazing reduces condensation.

Lower solar gain

Solar gain is the term for when a space or object is heated by the radiation from the sun.

In the same way that double glazed units prevent cold from entering your home in the winter, they also prevent heat from entering your home during the summer.

By applying a solar control film, tint or low-E coating, you can further reduce solar gain from a double glazed window or door.

The Benefits Of Double Glazing

Let’s look at the advantages and what makes double glazing worth it.

Reduced heating bills

Double glazed windows work well to lower energy bills. Due to reduced heat loss, you need less energy to heat your home as it will remain at the desired temperature for longer. In the summer, double glazing reduces heat gain from the outside, helping your house remain cooler for longer.

Installing triple glazing will further increase your savings due to its improved insulation properties.

More even temperatures

Once your home is fitted with properly sealed double glazing, you won’t experience varying temperature zones in your home. Draughts through window frames and heat loss through single glazing will be a thing of the past.

Lowers carbon footprint

Because a home with double glazing has better energy efficiency, it lowers a homeowner’s total carbon footprint. According to the Energy Saving Trust, a home that replaces single glazing with basic A-rated double glazing will save up to 375 kg of carbon dioxide from being added to the atmosphere through less energy usage.

Many companies under the UK’s Glass and Glazing Federation banner use sustainable materials and take part in carbon offset programmes to further reduce the carbon footprint of a double glazed unit.

Double Glazing’s Energy Rating

In the UK, the Window Energy Rating (WER) system is used to categorise a window by its thermal performance. This rating system starts at G on the lowest tier and goes all the way up to A++.

Legislation in the UK demands that a double glazed unit has a minimum WER rating of C.

So, what influences a window’s rating?

  • The use of more thermally insulating frame materials, like uPVC, rather than heat-conducting materials like aluminium.
  • The use of low-E glass and other treatments on the double glazed panes allows them to mitigate heat loss or solar gain.
  • The use of more thermally insulating fill gas between panes of glass, like argon gas and krypton, instead of just air.
  • You can install triple glazing glass, which has one of the highest WER ratings.

U value

A window’s U value has a direct influence on lowering (or increasing)heating bills. The U value is the amount of heat that can pass through a material per square metre. The lower the U value, the better that material’s insulation.

For reference,modern double glazed windows have a U value of 1.2. This means for every degree Celsius difference between one side and the other, 1.2 watts of energy will be transferred. Single glazed windows have a U value of 5.8.

Cost of Double Glazing

As we have seen by discussing the construction of a double glazed window or door, it is quite an involved process. So, it’s no surprise that there is quite a high cost for installing double glazed windows and doors. If you want to learn more about the price of installation, you can find out all you need to know in our article on how much double glazing costs.


How do you know if double glazing has failed?

When double glazing no longer reduces condensation, it’s time for replacement windows. When you see your windows misting, it means that the air-tight seal between the glass panes and the spacer bar has been compromised.

Before things progress that far, you may notice either chips or cracks in your window glass. Other telltale signs include cold spots in your house which may be caused by draughts coming through your double glazing.

What are the disadvantages of double glazing?

The biggest disadvantage of double glazed windows and doors is the fact that you cannot repair them yourself. Double glazing requires an airtight seal to function effectively and retain heat during the winter months.

You would need to replace the compromised window with a brand-new double glazed sealed unit. This is the main reason we always advise you to hire professionals for installation.

How long does double glazing last?

The average lifespan of a double glazed window or door is between 10 and 25 years. This is reflected in most manufacturers offering a warranty on their windows of between 10 and 20 years. Bar a manufacturing fault, the main determining factor of a double glazed window’s lifespan is natural wear and tear.


So, now that you know exactly how double glazed windows and doors work and their thermal insulation properties, you can make a well-informed decision to have them installed in your home.

For the best advice and quotes on your home heating solutions, visit Eco Happy. You can contact us or use our helpful and comprehensive quote comparison service to find the best prices and service providers for all your heating needs.

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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