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What Are Double-Glazed Windows? image

What Are Double-Glazed Windows?

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You’ve most likely heard the term ‘double-glazed windows,’ as this product has been around for decades, but you may not yet have a comprehensive understanding of what they are and how they work. For that, we’ve put together the following guide that includes everything you need to know about double-glazed windows.

Proven Track Record

Term familiarity comes from the fact that double-glazed windows have been used in buildings since the 1950s in the United States, and the late 1970s/early 1980s in the United Kingdom. With obvious and measurable benefits, they have only grown in popularity since their introduction.

In 1997, double-glazed windows received an additional boost in usage, as they became an effective tool in fulfilling conditions set forth by the Kyoto Protocol signed that year. This agreement called for reducing carbon dioxide emissions to pre-1990 levels, and a key method for achieving this is to reduce energy requirements. 

By the latest measure, domestic households in the UK were responsible for 38% of electricity consumption in 2021, so any improvements in home insulation can help bring this number down. 

Alignment with Building Regulations

Recognising this, UK building regulations in 2002 mandated the use of double-glazed windows with at least a C rating in Part L of their guidelines, which covers the conservation of fuel and power in buildings. 

The only exceptions to this requirement to be aware of are the following:

  • Designated ‘historic’ buildings – must merely do the ‘best they can’

  • Conservatories – if unheated or separated by doors

  • Broken sealed units – if not replacing a whole window, substitutions can be made with ‘like for like’

A Peek Behind the Curtain

To understand how double-glazed windows can make a home more energy efficient, let’s take a look at their inner workings.

As the name implies, a double-glazed window is made from two panes of glass sealed into a framing material, leaving an air space in between. This air space is filled with inert gas, such as argon, krypton or xenon, creating an insulating layer between the two different temperature zones indoors and outdoors. 

The spacer is typically made from either polymer or metal and is filled with a desiccant (drying agent) that helps draw moisture out of the air, preventing it from getting trapped between the sheets of glass.

“Sandwich Construction” Benefits

This sandwich construction does a much better job of prohibiting thermal transfer than a single pane of glass. The inert gas layer significantly reduces the transfer of heat to the outside. Gas is much worse at conducting heat than air. So when the comfy warm air of your home hits the glass, the transfer of heat to the outside will be slowed down by the gas between the glass panes.

Result: you keep in the warmth and keep out the cold. In this way, both panes of glass are working in your favour.

  • Window frame material – timber is highly insulating, while aluminium is less so

  • Glass thickness – using different thicknesses of glass in the two panes reduces sound transfer

  • Glass coatings – low-emission coatings help reflect more heat back into the home

  • Spacing between glass – anywhere from 6-12 mm is narrow enough to minimise air circulation

  • Pane spacer – often made with little to no metal, called a ‘warm edge’ spacer

  • Type of gas – argon has very low conductivity with a 34% reduced thermal transfer rate

Nowadays, there are also triple-glazed windows on the market, featuring three panes of glass and improved functionality in some areas. If you’re wondering whether triple-glazing is worth it, check out our article here.

Making Sense of it All

How does one evaluate double-glazed windows when so many variables are at play? Luckily, one need not be a building professional to make the right choice regarding high-performance double-glazed windows. 

Most window manufacturers have their products accredited for energy efficiency by the British Fenestration Rating Council (BFRE), which has established a domestic window energy rating scheme based on the amount of heat, sunlight and air passing through a whole window.

A calculation is made of the positive contributions of solar gain minus the negative factors of heat loss and air leakage to arrive at a rating from the lowest at E to the highest at A++. Any rating of 0 or higher is considered energy positive and will have a designation from A to A++.

Another common measure for the thermal efficiency of double-glazed windows is known as the U value, where the inverse is true.


The lower the U value => The more effective a window is at retaining heat

The U factor is a measurement of heat loss in watts per square metre Kelvin (W/m2K), and some baseline comparables are as follows:

Glazing typeU-value
Single-glazing5.6 W/m2K
Double-glazing with air cavity2.8 W/m2K
Double-glazing with argon gas cavity2.6 W/m2K
Double-glazing with low-emissivity glass and air cavity1.8 W/m2K
Klar double-glazed window with low-emissivity glass and argon gas cavity1.3 W/m2K

What’s In It for Me?

Hopefully, by now, it is apparent that everything about a double-glazed window is made to most effectively reduce heat loss from your home. And while this is a significant benefit you will notice right away, there are plenty of additional reasons to start reading up on the different styles of double-glazed windows available here.

  • Reduced carbon footprint

  • Cost savings in energy bills

  • Better soundproofing

  • Improved home security

  • Less condensation

  • Increased property value


  • What is double-glazing of windows?

Double-glazing of windows refers to the two panes of glass used in their construction, separated by an air cavity, which creates an extra layer of thermal insulation. Secondary glazing is when the second layer of glass is added inside your existing windows and is not considered as effective as double-glazing.

  • How can you tell if your windows are double-glazed?

A quick visual check will help you determine whether your windows are double-glazed. You should notice two panes of glass with a small gap for air in between. The air gap typically contains a type of inert gas that does not conduct heat, like argon, to help insulate the home.

  • What are the disadvantages of double-glazing?

There are not many disadvantages to double-glazing. Some might consider the expense of having new windows installed a drawback, but typically the cost savings realised, in the long run, outweigh the initial investment. Sometimes the improved insulation of double-glazed windows can cause condensation inside the home without adequate ventilation, but again, this can be remedied to enjoy the many benefits of double-glazing.