Double Glazing Guide

Everything you need to know about double glazing and double-glazed products, from start to finish.

Guide to double glazing

It’s a rare homeowner who hasn’t heard of double glazing – whether that’s through a home refurbishment project or the insistence of experienced friends and visitors that the investment is well worth it.

Still, if you’re in the process of redecorating and replacing your doors or windows, or in the middle of considering a move to double glazing from antiquated old single panes, it’s good to know all of the potential benefits.

Here to help with your decision-making process is our complete guide to double glazing:

What is double glazing and how does it work?

As you may have guessed from the name, double glazing’s central feature is the use of two panes of glass as opposed to just one. At a basic level, double glazing in the home works a bit like woollen clothing – it is designed to trap air between its layers. Whether double glazing is utilised in windows or doors, the desired outcome is the same.

This trapped air is especially useful in double glazing because air is actually a poor conductor of heat, transferring warmth very badly. Consequently, the air between the two panes of glass provides an invisible wall of protection between heat in the home and cold from outdoors. With the application of special low-emissivity coatings, even less heat is lost from the home to the outdoors through your glazing as it is reflected back into the room.

How is double glazing made?

The two panes of glass making up the double glazing are separated by a spacer bar, attached via powerful adhesive sealant. The amount of distance separating the panes – also known as the cavity width – in addition to the type of spacer bar used are key influences on the glazing’s final capacity for thermal and acoustic insulation.

The expected cavity width generally reaches between 4mm to 20mm. This gap between the two panes is usually filled with dehydrated air (which will act as the protective insulating layer) while under vacuum conditions. It can also be packed with an inert gas filling, such as argon gas, to further boost its shielding properties and prevent issues with condensation.

If gas-filled, the glazed unit may have holes drilled into the spacer bar in order to remove the air already within the space, before replacing that air with the argon (or desired gas in question) and resealing the gaps to ensure containment. More modern methods sometimes involve use of a gas filling press facility, removing the need to drill holes in the unit.

After filling, the unit is sealed at the edges using either polysulphide or silicone sealant to ensure airtightness and further prevent condensation.

The recommended minimum cavity width to ensure performance when manufacturing double glazing with dehydrated air is 16mm. This figure goes down to 14mm when using argon gas. The necessary cavity width to ensure thorough acoustic insulation, however, is quite dependent on the specifics of the case in hand.

When was double glazing invented?

As is often the case with inventions which eventually became commonplace, there is some dispute over who originated the idea of double glazing first. Some research shows that certain homes in Germany, Scotland and Switzerland were using some rudimentary form of double-glazed window during the 1870s. However, unlike modern day glazed windows, these seem to have involved the puttying of a second sheet of glass to the existing units.

The invention of the contemporary double glazing we’re all familiar with – utilising two panes bonded together in one frame – is said to have happened in 1930s America.

Who invented double glazing?

The invention of double glazing in the 1930s is credited to an American by the name of Mr. C. D. Haven. Unfortunately the concept was harder to execute than it was to imagine, back then. Requiring glass which was absolutely flat and of uniform thickness meant it was generally far too costly to produce.

The first willing manufacturer was discovered in 1941 and the concept of double glazing was subsequently patented by the Libbey-Owens-Ford Glass Company of Toledo, Ohio, under the trade name of ‘Thermopane’. Yet, because the USA soon thereafter entered World War II, the product would not enter the market at large until 1952.

When was double glazing introduced in the UK?

Although two of the UK’s major suppliers were founded in 1965, it wasn’t until the 1970s that double glazing began to be sold across Britain. This is largely due to the consequences of the 1973 Oil Crisis which prompted new regulatory restrictions on average U-values in domestic walls and windows.

However, the 1970s market was dominated more prominently by secondary glazing, involving a layer of plastic fitted next to the single pane of glass.

When did double glazing become popular?

Specialist double glazing companies really began to flourish during the 1980s, following further changes to building regulations enforcing greater energy efficiency, making double glazed windows and doors the generally accepted standard on new build homes.

Benefits of double-glazed windows vs. single-glazed

Glass by itself is a very effective conductor. Take a single-glazed home for example. During the summer, direct sunlight shines through the window and heats the house. During the winter, valuable warmth passes from inside the home to the outside.

This is all because a single-paned window transfers heat very easily. As a result, homes which lack double-glazing often have to spend more money on energy bills as they attempt to maintain comfortable indoor temperatures.

There are a number of potential benefits in return for an investment in double-glazed windows or doors:

  • Save money on bills: By limiting the amount of heat that can be conducted through your windows and doors, your central heating or air conditioning units will have to work less hard to provide optimal temperatures.
  • Reduce condensation: With double glazing you minimise the risk of condensation freezing, which makes glass colder and increases the amount of heat needed to maintain warmth in your home.
  • Reduce noise: You’ll hear much less noise from outside with double glazing, due to its excellent sound insulation properties – less sound from traffic, neighbours and practically every imaginable source, leading to a serene home atmosphere.
  • Enhance house value: Ensuring your house is double-glazed, whether it is an older or newer build, is a great way to increase resale value. Most buyers want some kind of guarantee that the house they’re buying is effectively insulated!
  • Reduce fading of interiors: Unprotected glass can lead to harmful UV rays entering the home and fading your carpets, drapes, furniture and more. Double glazing reduces interior fading by inhibiting the transmission of UV radiation much more efficiently.
  • Increase security: For obvious reasons, a double-glazed pane of glass is twice as hard to break as a traditional single-pane unit. Integrating laminated or toughened glass will discourage potential intruders even further.

Double glazing costs

Double glazing costs can vary quite significantly within the UK’s competitive market. Potential customers can benefit greatly from shopping around for the very best prices and special deals in their local area. The more research you do, the better informed you’ll be about current cost expectations within the sector. Use comparison sites, if necessary, to further bolster your knowledge and make sure the quote you get is the very best available for your budget.

You should know: prices tend to increase according to the type of material used in the double-glazed unit. On a sliding scale, uPVC frames are the most affordable. They are followed by aluminium frames which are slightly more costly. Finally there are wooden frames, which are typically the most expensive option available to buy.

Some key pointers to be aware of:

  • Government grants for double glazing have been discontinued
  • Shopping around and comparing prices is therefore essential
  • The size and style of your house (and how many windows you need) will influence the overall cost
  • To reiterate: get more than one quote!

When does double glazing need replacing?

Although double glazing is generally a wise investment, double-glazed units do not have an unlimited lifespan. For instance: the seal that separates the two panes and the spacer bar can eventually break down, resulting in misting and condensation between the two sheets of glass. However, most experts suggest the average life expectancy before seals falter is 20 years, up to 30 in very high quality installations.

There are a number of tell-tale signs that you might need to consider replacing your existing double glazing:

  • My double glazing is fogged up: As explained above, the presence of mist within your double-glazed unit is a good indication that the seal has worn down. Because the gas within – preventing heat transmission – has dispersed, it is an opportunity to consider like-for-like replacement.
  • My double glazing is out of fashion: While decorating houses with uPVC windows was once all the rage, there’s a chance you now find them to be an eyesore. It’s a nationwide trend, in fact, for homeowners to replace uPVC double glazing with aluminium or wooden frames.
  • My double glazing is falling apart: If your double glazing units are made primarily of wood, then they may need replacing. Wood frames are unfortunately prone to rotting and decay, which can present a nightmare for your house’s aesthetics, energy efficiency and even your security. It is important to seek a long-lasting replacement before the winter months arrive.
  • My double glazing is old and ineffective: If you’ve caught yourself griping about noise outside the home recently, you should ask when you last upgraded your glazing. Units which are just five to ten years old are less effective than double glazing today. The larger the gap between the two panes of glass, the less noise can permeate the unit. Nowadays the typical distance is 16mm, compared to only 6mm in old models, with a host of options available including pricy but effective acoustic glass.

What are triple-glazed windows?

Triple glazing works like double glazing, with the only difference being the addition of one extra layer of glass and inert gas. As expected, this quite simply creates window and door units which are even more resistant to heat transfer. For this reason, triple glazing is generally only recommended to homeowners living in extreme climates where ensuring warmth through winter is more of a struggle.

Most customers in the UK would be better off enhancing the double glazing they already have – through lamination, low-emissivity coatings or increasing the existing glazing gap – than seeking triple glazing.

What is secondary glazing?

Not to be confused with double glazing, secondary glazing adds a layer of plastic fitted next to the single glass pane. It’s a common myth that secondary glazing is just as effective in terms of its insulation properties, but the opposite is true. Double glazing – especially low-emissivity products – offer much greater insulation in comparison. A home decked out in exclusively secondary glazing is likely to receive a very poor score in terms of energy performance.

However, it’s worth noting that – due to the size of the gap between the single pane of glass and secondary glazing layer – that secondary glazing can often be a clear winner in terms of noise reduction qualities.

Double glazing myths: planning permission

Do you need planning permission to replace windows with double glazing? Many seem to think so. Yet that’s not quite the case! Replacing your windows only requires permission from your local authority if the building in question is listed, falls within a conservation area, or is technically defined as a flat/apartment.

Thanks for reading

We hope this guide has been useful, answering any questions or concerns you might have about double glazing including its invention. If there’s something you think we’ve missed, don’t hesitate to contact us and let us know – we’ll update this guide right away!