Double Glazing

What is Double Glazing and how does it work?

So, in this article we are going back to basics to revisit the meaning of double glazing, in terms of function, features and benefits.

To create a comprehensive guide to double glazing we have looked at, what we think, are the key points to cover and set them out below.

Double Glazing Windows

UPVC Double Glazed Window Cutaway Section

What we hope you will understand from reading this article is:

  • What is Double Glazing -  how does it work? -  about energy ratings.
  • Components of Double Glazing - Types of frames & glass options -  repair & refurbishment.
  • Pros / Cons of Double Glazing – other options. (triple / secondary).
  • Double glazed windows –  the different types - How much do double glazed windows cost?
  • Double glazed doors – the different types - How much do double glazed doors cost?
  • Double Glazing Manufacturers, Suppliers & Trade Associations.
  • How to get competitive double glazing prices online?

With around 25 million houses in the UK, home improvement is a big market, and with property prices being unreasonably high, it’s understandable that more and more homeowners are choosing to “improve, rather than move”.

Replacing the windows and doors in a property is very often one of the first things that owners look at doing when they start to work on the house and double glazed windows & doors are the product of choice.
double glazed windows

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Double GlazingComponentsPro's and Con'sWindow CostsDoor CostsSuppliersGetting Quotes

The basics of what double glazed windows are.

What is Double Glazing?

How double glazing works?

The name itself is obviously a bit of a “give-away”, so there are no prizes for guessing that double glazing involves 2 panes of glass.

The simplest way we could think of to describe double glazing is to compare it to a sandwich. If you could imagine a sandwich where the bread is glass and the filling is “fresh air”, I’m sure you get the idea.

The modern double glazing product is usually called a sealed unit. The sealed unit comprises of 2 opposing sheets of equally sized 4mm float glass held apart by spacer bars fitted between the panes.

The spacer forms a gap between the panes, which can be a vacuum, partial vacuum or filled with an inert gas such as Argon. The gap between the panes of glass can range from 6mm to 20mm.

These sealed units are now used in all the glass sections of modern new or replacement double glazed windows and doors.

How Does Double Glazing Work?

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The idea behind double glazing is to create a more effective barrier between the inside and outside of a room. The priorities are to stop heat from escaping from the inside and cold penetrating from the outside through the glass.

It is the “air-gap” that provides double glazing with a lot of its’ insulating and energy saving properties by creating a barrier that is a poor conductor to heat or cold (the window or door frame provides the rest of the insular properties).

A useful by-product of the “glass-gap-glass” format is that it also reduces sound transfer. A high quality double glazed window can reduce sound pollution by as much as 60% more than a single glazed window.

As mentioned beforehand, the “air-gap” between the outer and inner panes can be varied, usually from around 6mm to 20mm. With the thickness of the glass taken into accounts (2 x 4mm panes) this would give an overall dimension of from 14mm to 28mm.

You can sometimes see these dimensions represented as: 4-6-4 / 4-16-4 / 4-20-4. Where the “4” is the thickness of the glass and the 6, 16 or 20 is the width of the gap between the panes.

The optimum gap for the average sealed unit is said to be 16mm. If you increase the gap between the panes too much, it can encourage convection currents to circulate the air or gas inside. Convection is the last thing you want in your double glazing, as it will drastically reduce its’ insulating properties – so in this case, bigger is not necessarily better.

What are Window Energy Ratings?

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Typical WER Label - Courtesy of the BFRC

In order that buyers could more easily tell how energy efficient double glazing products are, energy rating labels were introduced some years ago.

Commonly called WER’s, they are similar to the colourful labels you see on things like fridges or washing machines etc.

Ratings can typically be from the British Fenestration Rating Council (BFRC) or the British Standards Institution (BSI issue the “Kitemark”).

The BFRC rating is “rainbow” colour coded and also alphabetical. The alphabetical & colour coded ranking goes as from A++ to E, with A++ being the highest rating. It is also worth bearing in mind that the better the performance, the more the window unit is likely to cost.

  • A++ / VIOLET
  • A+ / INDIGO
  • A / BLUE
  • D / YELLOW
  • E / ORANGE

The window energy rating of C is the lowest usually accepted as compliant by the UK building regulations.

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How does it all fit together?

What are the different components of double glazing?

What are the parts that go together to make a double glazed window?

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When you are looking to buy new or replacement windows or doors, it’s going to be useful to know what the main parts are called and what they do. That way, you’ll have a better understanding of the product, and it can help you make a more informed decision about what the best product is for your situation.

For the purposes of this section, we are looking at 4 elements which are common to virtually all double glazed windows.
UPVC Double Glazed Window

Inside a UPVC Double Glazed Window

1 The Glass

coloured double glazing

Decorative Coloured and Leaded Glass

We mentioned it before, but the standard “float glass” thickness for double glazing is around 4mm. There are a few things that you can do with the glass, either to make it more energy efficient or to personalise it.

Energy wise, there are options for solar control glass, or more often referred to as “low-e”.

The “e” stands for emissivity and Low-e glass has a microscopic metal oxide coating that reflects heat and harmful rays from the sun (it can also prevent sun-fade on your décor or furnishings).

To make the glass stronger, you can choose to fit tempered (toughened) or laminated glass. To change the appearance, you can fit obscure, frosted, patterned, leaded or coloured glass.

2 The gap or cavity

Briefly referred to earlier in the article, the width of the gap is very important to the energy efficiency of the double glazing. The optimum sized gap for the average double glazing sealed unit would be 16mm, with a maximum recommended width of 20mm. These would be called 24mm or 28mm sealed units, because you add the combined total thickness of the glass to the gap size, and are readily available.

The gap is maintained by the spacer bar. The idea is to use a spacer bar that does not conduct heat or cold easily. Much is now made of warm-edge spacers that are made from flexible foam or plastics. You can also get them in different colours.

The space between the panes of glass is kept isolated by airtight seals around the edge of the glass and has a desiccant inside to ensure any moisture is absorbed. It can be filled with an inert gas like Argon, Xenon or Krypton to give a boost to the energy efficiency.

So, if you have a 28mm unit that is Argon gas filled with warm edge spacers, for all intents and purposes, it is going to be the most energy efficient product.

3 The Frames

Window Frame - Internal

Window Frame - Internal with Monkey Tail Handle

For a fixed, non-opening double glazed window, or a door, there is only one frame to consider. For an opening window, there are 2 different frames to consider, the first being the inner frame that holds the glass (in other words, the window frame), the second is the outer frame into which the windows themselves are fitted.

The outer frames are fixed to the opening by screws and then a silicone waterproof seal is used around the edges where it meets the walls. Any moving frames are held into position by hinges.

The inner & frames themselves can be made from wood, aluminium or UPVC. Typically, they would be made of the same material, although you can get aluminium double glazing with hardwood outer frames.

Both aluminium and UPVC frames should be multi-chambered, but aluminium must have a built in thermal break to stop heat transfer.

The double glazing is held into the frame by glazing beads. These are strips that clip into place around the inner edges of the actual window frame, and lock the glass into place. On each strip the edge that meets the glass has a rubber weather seal attached. Glazing beads are recommended to be fitted internally (to prevent removal by burglars).

Each window opening will also have rubber gasket seals and brush-like weather seals around the out edge. All these seals are vital to the weather-proofing integrity of double glazed windows.

4 Ventilation

Typical Trickle Vent

Typical Trickle Vent

It may sound odd, but you need to allow fresh air to come in. The reason being to combat condensation. Double glazing is so water tight and efficient in heat retention, if you don’t allow air to somehow circulate in a controlled fashion, your home will start to have condensation build-up.

An average family at home can produce around 10 to 15 litres of moisture per day, and if it’s not vented it will turn into mould, mildew & damp patches.

Double glazing trickle vents, fitted to the top of the frames, are the solution to this problem.

They provide a controlled method of allowing fresh air to circulate, draught free. 

What are the different types of material used in double glazing frames?

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The three most popular materials to use for double glazing frames are UPVC, aluminium or wood.

Aluminium Clad Timber Window

Cross Section of Aluminium Clad Engineered Timber Window

UPVC: Made from a rigid vinyl polymer, UPVC is exceptionally popular to use in the manufacture of double glazed windows and doors.

The key properties are low price points, long life span, durability, strength and low maintenance. You can also find upvc double glazing made from recycled UPVC, but take great care to buy only the highest quality recycled material products. UPVC comes in a decent selection of colours and also with wood-effect surface finishes.

Aluminium: Although, usually higher priced than UPVC, this is also an excellent material to use. The key properties are similar to that of UPVC in that aluminium is light, long lasting and durable.

A plus point is that it is stronger than UPVC, allowing the double glazing frames to be made thinner or slimmer. A minus point is that aluminium is a natural conductor and so need more inbuilt features to make it appropriately energy-efficient.

Wood: We all know that double glazing frames can be made from wood, either softwood, hardwood or engineered wood. The key properties are that it is a natural insulator and has decent strength. The different woods have a great effect on the performance of the window.

Softwood will be inexpensive but need a lot of looking after otherwise it will last nowhere near as long as UPVC or aluminium. Hardwood performs better, but still needs looking after. However, if you do look after them they will last decades. Engineered woods are more stable, less prone to warping or shrinking and price wise, cheaper than hardwood.

What are the different types of glass?

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We covered the components of double glazing earlier, and explained how the unit is constructed mainly using 2 panes of 4mm thick float glass, but you can make decorative and performance related changes to suit your project.

Glass performance related options

Toughened glass, also known as tempered, is basically standard float glass that has been heat treated. The treatment puts compression and tension stresses into the glass. These stresses cause the window to shatter into very small, relatively harmless pieces if the window gets broken.

To know if window pane is toughened, use a pair of polarised sunglasses to look at it. You will be then be able to see the stress patterns clearly. Some examples of double glazing windows may just have one toughened glass pane on the inside only.

Laminated glass is effectively a “sandwich” of a Poly Vinyl Butyral layer (PVB) between two sheets of glass. During manufacture, they are sealed together using heat and pressure which bonds them permanently. When broken, the glass panes stay within the frame and therefore this type of glazing is ultra-safe and very secure.

Low emissivity glass is designed to reflect or reduce the levels of Ultra-violet light (colour fading) and Infra-red light (heat generating) that pass through, without reducing “safe” natural light levels. It does this via a microscopically thin metal oxide layer that is bonded to the surface. The reason this type of product is great for double glazing is that is reduces cold & heat from coming in and stops them from leaking out.

Decorative glass related options

Clear double glazing is what you want most of the time, but if you want to add some “flavour” to your windows, or need some privacy (such as bathroom or toilet windows) there are many options. Glazing can be obscure, frosted, coloured, patterned or even with leaded decoration. This would also include some elements of tinting, which can be bought tinted or added after installation if necessary.

Patterned Glass Selection

Patterned Glass Selection

What are the advantages & disadvantages of double glazing?

Taking a look at the Pros & Cons


  • Costs more than single glazing, but less than triple glazing.
  • Helps to reduce home heating costs.
  • Reduces sound pollution from the outside
  • Makes your home look more attractive.
  • Can add to the resale value of a property.
  • Improved security & safety.
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The only real alternatives to a new or replacement integrally double glazed window would be to use secondary glazing. This is where an independent glass partition is built to fit within the confines of the inner window aperture. Secondary was very popular when double glazing first made its’ impact in the UK, now not so much.

This option is useful where the window itself is of a type that can’t be changed or matched, maybe in a listed building or similar. The secondary double glazing can be fitted internally and therefore the external appearance remains the same.

What about double glazing repair & refurbishment?

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Double glazing can develop problems over its’ lifespan and the most common would be the breakdown or failure of the sealed unit. The constant hot/cold/hot cycles endured by the window over the years make the sealed units flex. subsequently, the seals eventually give way and let in some air.

For a while the desiccant will deal with the moisture, but even that will have a limit and get saturated. This will cause the double glazing to get condensation inside and create fogging. In this situation the sealed unit should really be fully replaced.

Hinges can also become bent; handles can break or weather strips can perish. In a lot of cases, these problems will be able to be fixed quite simply and at a reasonable cost. But before you call a repair person, check to see if your warranty covers the problem. Most double glazing carries at least a 10-year warranty.

Double Glazed Windows

Popular Double Glazing Window Styles

Double glazed windows: What are the different designs & how much do they cost?

There are really only about three main designs, based on functionality (opening style) such as Casement, Sash or French windows and the others would most likely be variations of these designs.

Here is a selection of the types of double glazed windows you are most likely to come across.

How much do Casement Windows Cost?

Single Opener Casement: 600mm x 900mmWhite uPVCfrom £200
Single Opener Casement: 1200mm x 1200mmWhite uPVCfrom £300
Single Opener Casement:940mm x 1600mmWhite uPVCfrom £450
Full House Casement: 3 bedroom, 12 windowsWhite uPVCfrom £4,500 +

How much do Sash Windows Cost?

SASH: 600mm x 900mmWhite uPVCfrom £500
SASH: 1200mm x 1200mmWhite uPVCfrom £600
Full House Sash:Quality woodfrom £10,000+

How much do Tilt & Turn Windows Cost?

Tilt & Turn: 2 light - 1200mmWoodenfrom £800
Tilt & Turn: 2400mm x 1200mmWhite uPVCfrom £500
Tilt & Turn: 1600mm x 900mmWhite uPVCfrom £240+

How Much do French Windows Cost?

Size of WindowBasic SpecificationAverage Prices uPVC French Window
1200 mm x 1200 mmWhite uPVC, plain glassfrom £320 to  £350
1200 mm x 1200 mmWhite uPVC, Georgian Barsfrom £380 to  £390
1000 mm x 1200 mmWhite uPVC, Plain Glassfrom £310 to  £330
 1000 mm x 1200 mmWhite uPVC, Georgian Barsfrom £360 to  £380
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Double Glazed Doors?

Popular Double Glazed Door Designs

Double Glazed Doors: What are the different designs & how much do they cost?

For all intents and purposes, there are 4 types of door, again based on functionality, for example, the way they open & close.

Those that we consider to be the main ones would be Swing doors (front & back), sliding doors, bifold or French doors.

Here is a selection of the types of double glazed doors you are most likely to come across.

How Much do Front Doors Cost?

Approximate sizeMaterialPrice Bracket
 6ft 6ins / 1.981m x 2ft 6ins / 0.762mWhite uPVC£600 to £800
6ft 6ins / 1.981m x 2ft 6ins / 0.762mSolid  Oak hardwoodfrom £900
6ft 6ins / 1.981m x 2ft 6ins / 0.762mAluminum cladfrom £1,100
6ft 6ins / 1.981m x 2ft 6ins / 0.762mCompositefrom £1,000

How Much do French Doors Cost?

 Door size in millimeters Basic Specification Average Fitted Price Guide
 2 pane 500 wide x 1000 high white, upvc double glazed£450 to £500
 1000 wide x 1000 high white, upvc double glazed£500 to £575
 1200 wide x 2100 high white, upvc double glazed£550 to £600

How Much do Sliding Doors Cost?

Door Style Approximate size (mm)  Average Fitted Price Guide 
White UPVC x 2 section2200 x 2200£700 to £900
Wood effect UPVC x 2 section2200 x 2200£1,000 to £1,350
White UPVC x 3 section3000 x 2200£1,100 to £1,300
White UPVC x 4 section3500 x 2200£1,200 to £1,500

How Much do Bifold Doors Cost?

 3 panel doors  Description - UPVC Bifold Doors  Price Guide including VAT 
up to 2700mm x 2100mmwhite UPVC/ double glazed£1,200 to £1,300
up to 2700mm x 2100mmdual colour white & golden oak / double glazed£1,500 to £1,600
up to 2700mm x 2100mmAnthracite Grey UPVC / double glazed£1,700 to £1,900
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Double Glazing Manufacturers, Suppliers & Trade Associations

Finding a trustworthy & competent double glazing installer

The benefits of using a Trade Association Accredited Installer.

One you have a good idea about the type of double glazed windows or doors that you want to install, it’s time to consider where to buy them from, and who to use to install them.

You could buy the windows from one supplier and have them fitted by an independent contractor or even fit them yourself if you have the skill. But there are both pro’s & con’s to that option.

The potential advantage of doing things in that fashion, is that some feel they may get a better deal buying direct from a “big name” and then getting local contractors to do the installation work. It may or may not work out.

If you have the skills to fit them yourself, then definitely you could save money buying “supply only”.

Putting the Skilled DIY option aside, in our humble opinion, keeping everything in one place has a few distinct advantages.

  1. One point of contact.
  2. One set of prices to negotiate upon.
  3. Experience of the company in question can be verified easily.
  4. All guarantees and warranties from a single source – no confusion over who is responsible for what if there is a problem (or two).
  5. You can use a fully accredited FENSA / CERTASS /DGCOS installer.
  6. Up-front cost – supply only will require full payment for the product upfront. A reputable and trustworthy accredited installer may even do the work without an upfront deposit.
  7. Not something we are experts on, or advocate, but you can sometimes get financing from an “all-in-one” specialist double glazing company.

Free quotesOn the subjects of competence, trust worthiness and peace of mind, dealing with a UK trade body accredited double glazing company can give you these important things.

The major UK double glazing trade associations vet all their members for competence and working practices. They also impose working standards and insist their members provide proper insurance backed deposit guarantees. Bodies like DGCOS even provide access to the Double Glazing Ombudsman to settle disputes if problems arise.

In short, you are likely to get a higher level of consumer protection by using an accredited installer from start to finish.

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How to get competitive double glazing prices online?

Comparing Quotes Can Save You Money

A simple, easy and stress free way to compare double glazing prices online

Last, but by no means least, at some point you are going to want to know exactly how much it’s going to cost you to buy double glazing for your property.

The double glazing prices shown in this article are all very well, but are only there to give you a rough idea of what to expect. The only way to get an accurate price is to have your job “measured up” by a professional and then have them produce a written quotation that covers every aspect of your installation from top to bottom.

Fortunately, it’s very simple.

We have years of experience arranging quotations for all types of home improvements all over the UK and are happy to do it for you.

All you need to do is go to our contact form, send us a few simple details about your job and how to get in touch with you and we will then arrange for at least 3 professional accredited installers to provide you with written quotations for the cost of the work.

Click here to request your quote