Infil UPVC Porch
UPVC Porches
October 14, 2017
UPVC Patio Doors
UPVC Patio Doors
November 1, 2017
Show all

Double Glazed Windows

double glazed windows

Leaded Coloured French Casement Windows

The comprehensive guide to double glazed windows

Double Glazed French WindowAre you moving to a “fixer-upper”, are your windows getting past their “sell-by-date” or do you just fancy a new look for your home?

If so, it’s highly likely that you could be considering buying some new or replacement double glazed windows in the near future.

You may already know quite a bit about double glazing windows already, but before you go out and spend your hard earned money, it could be worth your while to top up or refresh your knowledge just in case.

So, whether you are new to home improving or you are a veteran of many projects, we think you can find lots of useful information in our article.

To help you find your way around the article, it's split into 4 main sections that deal with the concept of double glazing, the various design options, pricing and a few questions and answers, as you can see in the contents menu below.

About Double Glazed Windows:

  • What are they, what do they consist of?
  • Why they are effective.
  • What are the pro’s & con’s?
  • When you should replace your windows.

Average Prices Guide:

  • How much do “Fully fitted” Double Glazed Windows Cost?
  • How much do “Supply Only” Double Glazed Windows Cost?

Double Glazed Window Style:

  • What materials can be used?
  • What are the design choicesr?
  • Personalising your new windows.
  • How to identify the energy efficiency?

FAQ’s, Building Regulations

  • Summary of some building regulations that apply.
  • A collection of common questions & answers.

What are Double Glazed Windows?

What are they, what do they consist of?

The basics about double glazed windows

divider green

In order to explain what a double glazed window is and how it works, we will look at what the different parts of the window are and what they do.

To begin with, it might be helpful to mention a couple of abbreviations used by the double glazing window industry, so that if you see them mentioned in the article you know what they mean (if you don’t already know).

  • Double Glazed Unit: This refers to an entire sealed double glazed glass section of the window as a whole.
  • Insulating Glass: This refers to the actual panes of glass used in the sealed unit.

In short, a double glazed window consists of a Double Glazed Unit made from Insulating Glass, fitted into a surrounding frame. The window can be fixed (non-opening), or fitted into a sub frame in such a way as to allow it open and close.

What are the different component parts of a double glazed window?

There are around 6 main component parts of a double glazed window and some of those parts can be modified or made out of different materials.

Modifying one or more of these components, such as the glass thickness or primary frame construction material can have an effect on the thermal efficiency, appearance and cost.

  1. The frame: Usually made from timber, aluminium or uPVC.
  2. The glazing: Typically, 4mm thick float glass with a 16mm cavity.
  3. Primary & secondary seals: The seals protecting the window and the seal between the frame and the surrounding aperture.
  4. The void, cavity or gap between the insulating glass panes: Can run from 6mm to 20mm, be a vacuum or gas filled.
  5. Spacer bars: The internal strips that keep the outer & inner panes of the glazing separated.
  6. Desiccant: A moisture absorbing substance within the spacer bars that prevents internal condensation.

Why are double glazed windows effective?

The effectiveness of double glazed windows is 3 fold. Firstly, they reduce the convection (transfer) of heat, cold together with effectively nullifying the penetration of harmful UV rays that are responsible for fading your furnishings and decorations. Secondly, they act as a sound deadening barrier which can reduce the amount of noise you can hear from the outside by as much as 60% more than a single glazed window.

Thirdly,the twin-walled window glass is more secure than a single glazed window because it’s much harder to break.

Typical UPVC Double Glazed Window Profile

Typical UPVC Double Glazed Window Profile

These qualities can be further manipulated up or down by:
  • Widening or narrowing the gap between the panes of glass.
  • Using “warm edge” non-metallic spacer bars.
  • Filling the cavity of the double glazed glass with a gas such as Argon or Krypton.
  • Using low emissivity glass in the glazing.
  • Using "solar-control" glass in the glazing panels.
  • Increasing the thickness of one or both of the glass panes.
  • Using tempered or laminated glass.
  • Using tinted glass.
Positioning of glass for  glazing panels.

How the glass is fitted within the double glazed unit is quite important to making it function properly. Think of it as having four sides or faces, each oriented in a different direction.

  1. The outside of the outside pane, facing the world.
  2. The inside of the outside pane, facing the air gap.
  3. The inside of the inside pane, facing the air gap.
  4. The outside of the inner pane, facing the inside of the room.
Get Free Quotes - click here
Toughened glass can be fitted to either the inside, outside or both panes. Many installations use toughened glass only on the inner pane.
Laminated glass is best fitted to face #1, but there is nothing to stop you using it for both inner and outer panes.
Low-emissivity glass (low-e) can be fitted to inner or outer panes, but the metal oxide coated surface must be on either on face #2 or #3.
Self-cleaning glazing, if used, should have the treated surface on face #1.
“solar controlling” glass is said to work best on face #2.

What are the pros & cons when compared to single glazed windows?

 divider green
  • Much improved thermal efficiency, saving on your heating bills.
  • Drastic reduction in levels of sound pollution from the outside (up to 60%)
  • Reduces Infra-red (heat) & Ultra-violet (fading) ray penetration.
  • Big improvement in home security.
  • Attractive design options make your home more appealing.
  • Can actually improve the resale value of a property.
  • Low quality double glazed units can actually trap heat in your home, making it uncomfortably hot.
  • Pricier than single glazed windows.
  • Repairs are more expensive than with a single glazed window.
  • Some spare parts may be difficult to find after a few years, making a replacement necessary rather than just a repair – which will cost more to do..

How do you know when to replace your existing double glazing?

It’s a fact of life, things wear out and break. It may just be that your existing windows were fitted a long time ago, have had a hard life or maybe you just moved to a new place that needs some refurbishment. In any event, there are some clear signs that will mean your windows are due to be replaced.

Sealed unit failure is normally signified by the appearance of condensation within the cavity of the glazing panels. This is caused by either the desiccant becoming saturated over time due to a small leak, or a recent larger failure of the seal around the outer edges of the glass.

Damp patches around the inner window reveals could be a sign that the frames have warped and broken the seal between the outer window frames and the walls.

If the window no longer opens and closes properly, this can again signify an element of frame warping from the inner, outer or both frames. This bad fitting can cause draughts and water penetration.

Some information about Argon gas filled double glazing windows.

Argon gas is great at reducing heat transfer and for interfering with UV rays (Ultra Violet light), so it keeps you home cooler and stops your furnishings and décor from fading in direct sunlight. However, it is a very “thick” gas with around 60% to 65% less conductivity than “normal air” and is sometimes put into a double glazing sealed unit under pressure.

The glazing unit panels can be expected to leak around by around 1% per year and it’s estimated that if the proportion of gas to air drops below 80% then the efficiency benefits will be seriously degraded (that still gives a good 20-year lifespan).

If the gas was originally put in under pressure and the double glazed panels develop a faster leak it can lead to the glass “bowing” inwards as the outer air pressure becomes greater than the inner pressure (“thin” air can’t get in fast enough to replace “thick” Argon). This will seriously degrade the performance. It also may not be immediately noticeable but, in a big double glazing window, can lead to the glass cracking.

Back to top

Double Glazed Window Designs

What are your options for appearance and function?

What are the different types of double glazed windows?

divider green

What materials are used to manufacture them? Most double glazing windows in residential properties are made from one of 3 different materials and these are UPVC, wood or aluminium, all of which will have different costs

1  UPVC Windows:

Casement Window

Flush fit Double Glazed Casement Window

Of the three materials, the current market is probably dominated by double glazing windows in uPVC designs. Exceedingly popular due to its’ combination of strength, durability, appearance, long life-span, low maintenance and competitive pricing.

UPVC windows are relatively lower priced than hardwood or aluminium, they last for ages and hardly need any looking after.

2 Wooden windows

Softwood timber windows get a lot of “bad-press”, some of it justified, about how they are not as good as UPVC or aluminium.  Wood is a natural insulator so the frames will be pretty effective in that aspect. If they are fitted with good quality double glazed units they can perform as well as any other material.

With softwood, the issues are more to do with how much it takes to keep the timber in good condition, and it does take a lot. If you neglect a soft wood window, it will warp, distort, peel, crack and rot in a very short space of time. Hardwood windows are more robust, but also need looking after quite a bit.

Engineered woods are becoming popular to use because they are very stable, don’t suffer anywhere near the same problems as softwoods and can be less expensive than hardwood.

3 Aluminium windows

They were in fashion in the 70’s & 80’s, they went out of fashion for years, but now they are once more building their reputation as the window of choice for many homeowners. Whilst they are nowhere near as popular as UPVC for residential installations at the moment, they do share a lot of their characteristics.

Strong, light, long lasting durable and attractive, aluminium windows can offer a very wide choice in design and appearance.There a basically 2 types, all aluminium and aluminium clad. The aluminium clad versions can make use of engineered timber or UPVC as the other material.

Prices for aluminium double glazed windows tend to be higher than UPVC.

Back to top

What are the design choices and how are they different?

What style of window to choose?

divider green

We can identify, or classify, basic double glazing window designs by the way they open and close. This way, there are probably only four really different functionalities with one or two variations on these designs that are the most popular.

Within those basic categories of double glazing windows, there are hundreds of variations of size and overall appearance, which gives a huge amount of choice. Once you come to a decision on the category of the window, you can then think about how you want it to look.

Personalising your new windows.

divider green

The two areas that you can work on with reference to the overall appearance of your new double glazed windows are the frames and the glazing. As for window frame options, as referenced earlier in this article, the popular choices for materials are UPVC, aluminium or wood and clearly the choice here will have a big impact on “the visuals” and the overall double glazing window costs.

LIFESPANLongShort, Med, LongLong
UPVC Wood Grain Window

UPVC Wood Grain Window

The colour is a big element in aesthetics, clearly white is a very popular choice. With both UPVC and, especially, aluminium offering a wide range of non-fading colours, more owners are going for coloured widow frames.

For those who can’t quite get away from wanting the look of wood, but also want the lifespan and low maintenance of modern materials, both UPVC & aluminium offer wood-grain or timber effect double glazed windows.

Your glass also does not necessarily need to be plain and simple, with options for colours, frostings and decorative coloured or leaded glass to add some personalisation.

Glazing bars and mullions look good, especially with French windows and can also often be seen in Sash window designs. Some use glazing bar inserts on their picture windows to “break-up” the view.

The furniture, such as handles can be mixed & matched to contrast or complement your home décor, with options such as white, black, chrome, brass etc.

How to identify the energy efficiency of a Double Glazed Window?

As one of the primary benefits of double glazing, understanding the performance characteristics of the product you are about to buy can help you get the best for your circumstances and budget.

The main factors effecting the thermal performance were touched upon beforehand, but it’s simple to say that frame and double glazed unit design are the most important. Frames have lesser impact on thermal efficiency performance than that of the double glazed unit itself.

Window Energy Ratings or WER labels have been around since 2004 in the UK. WER labels are the first thing you should look at to establish how good (or bad) the product is with reference to energy efficiency.

WER’s are produced and issued by all the following bodies and are similar in that they offer a rainbow colour coded visual reference alongside an alphabetical rating. Ratings go from A++ (best) down to G (worst). The rating of C is the lowest current WER you can install and still meet UK building regulations. Ratings are issued and monitored by the following 3 institutions:

  1. The British Standards Institution (BSI):
  2. The British Fenestration Rating Council (BFRC):
  3. The Thermal Rating Register (TRR):

A WER is really about two aspects of the thermal performance, which can broadly be classified as heat gain (G-value) and heat loss (U-value). The rating is calculated and awarded based on overall performance of the complete window unit, not just the glazing in isolation. To keep it simple, in general, you should be looking to install a window unit with the lowest U-value for your budget.

Side effects of energy efficiency

Typical Trickle Vent

Typical Trickle Vent

With the constant drive to be ever more energy efficient, our homes are in danger of almost becoming the equivalent of hermetically sealed boxes (airtight) and these high levels of insulation bring with them a big problem -  condensation.

Sit inside a stationary car on a cold or wet day, switch everything off and you will very quickly see the inside of the car windows fogging. Very shortly afterwards you will see water droplets forming on the glass.

This is exactly the same as what is happening inside your home if you have no method of controlled ventilation – you will get moisture build-up, leading to damp patches, mould and mildew.

Fitting trickle vents to a double glazed window will provide a solution. They allow draught free air to pass both ways in a controllable fashion. If you don’t have them, then keeping a window open for at least 15 to 20 minutes a day would be the alternative.

Back to top

Double Glazing Windows Prices

How much do you need to spend to buy new double glazed windows?

How much do “Fully fitted” Double Glazed Windows Cost?

colour divider

Casement Windows Price Guide

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

Sash Windows Cost Guide

600mm x 900mmWhite uPVCfrom £500
1200mm x 1200mmWhite uPVCfrom £600
Full House 3 bed semi:Quality woodfrom £10,000+

Tilt & Turn Window Prices

2 openers - 1200mm x 1200mmTimberfrom £800
2000mm x 1200mmWhite uPVCfrom £500
1600mm x 900mmWhite uPVCfrom £240+

How much do “Supply Only” Double Glazed Windows Cost?

colour divider

Casement Windows Price Guide

Single Opener 600 W x 800 HWhite uPVCfrom £60
Two Openers 1200 W x 900 HWhite uPVCfrom £130
3 panel, 2 Opener 1800 W x 1200 HWhite uPVCfrom £200
4 panel, 4 Opener 1200 W x 1200 HWhite uPVCfrom £230

Building Regulations and FAQ's

Some guidance and information about points to note.

Short FAQ’s & about UK Building Regulations

divider green

Here is a brief summary of some building regulations that apply for new or replacement windows in the UK.

The regulations generally apply to a new installation or a complete replacement and not to repairs or refurbishment. However, it is best to check with your local authority first yourself if you are unsure.

Bow Windows and Bay WindowsIn the UK the building control has been applied to replacement windows since early 2002 and impact primarily upon:

  • Window safety
  • Air supply
  • Ventilation
  • Means of escape
  • Energy efficiency

With window safety, for example, any glass that comes within 800mm of the floor should be safety glass. For means of escape, you can’t fit a new window that is smaller than the original one. Unless the original one was bigger than what was required as a means of escape, then you have to meet the new minimum means of escape dimensions only.

The “short-cut” to building regulations compliance is to have the work done by a FENSA or CERTASS accredited installer. Both FENSA and CERTASS registered installers are deemed competent to self-certify new windows as compliant with building regulations, saving you the time, effort & cost of doing it yourself.

A Short Q & A

1Why should I use an installer that is a member of a Trade Association?

Trade Bodies that deal with Double Glazed Windows and Doors offer better consumer protection, contactor vetting procedures and complaints processes – well known & trusted Associations are:

  • Glass & Glazing Federation (GGF)
  • Fenestration Self-Assessment (FENSA)
  • Certification & Self-Assessment (CERTASS)
  • Double Glazing & Conservatory Ombudsman Scheme (DGCOS)
  • Fairtrades (Multi-trade Association)
2How long does it take to double glaze a 3 bed-semi house?
A professional team of experienced fitters can remove and replace the windows for a 3 bed semi in 3 to 5 days. Start to finish.
3Do I need planning permission to change my windows?
In the majority of cases you don’t need to get planning permission for installing new doors & windows if they are similar to the ones being replaced. However, adding a Bay or Bow window may not be classified as permitted development and could be classed as an extension, so check about this type of window replacement for sure.Most Skylights (rooflights) may also be fitted without planning permission in a lot of instances, but there are some guidelines than need to be followed for protrusion, height level & using obscured glass where fitted to the side of a roofline.Check with your local authority first – listed buildings, protected buildings or homes in an ANOB (Area of Outstanding Natural Beauty or National Park) will always require consultation regarding improvement works.Building regulations apply. Putting a new window opening into a wall may require permission.
4How much deposit should I pay when I order new windows?
That's going to be down to the supplier. If you are buying direct from the manufacturer on a "supply only" basis, it's likely to have to pay in full up-front.
Many would say that 15-20% would be acceptable,but there are some double glazing companies who don't ask for a deposit and will accept payment in full upon completion. Always check.
National & Localy Based Accredited Installers: Free Quotes

Find Installation Companies in Your Area

  • Greater & Central London
  • South East England
  • Midlands
  • South West England
  • Scotland
  • North East England
  • North West England
  • Wales
Review Date
Reviewed Item
Jasmin J
Author Rating
John Francis James
John has been around the building industry since he was a child. With a degree in civil engineering, John is a highly valued contributor to our website.