What is double glazing?

Double glazing refers to a window that has two panes of glass. The main component of the window is the double glazed sealed unit, also known as an insulated glass unit (IGU) which sits in a frame of uPVC, aluminium or timber.

The IGU comprises of two sheets of glass separated by a spacer bar to create an air gap which is filled with an insulating gas. A bit like a sandwich of glass bread with a filling of air.

The primary benefit of double glazing is thermal insulation – to retain heat in a room whilst stopping cold air entering from outside to make the room more energy-efficient.

Double glazing makes such a difference to energy-efficiency that building regulations were set in 2002, to say that all replacement windows must be a minimum double glazed C rated – unless the property is a listed building or in a conservation area.

Who invented double glazing?

It might be difficult to remember a time before double glazing as most houses today have upgraded their windows to be more energy efficient. But, once upon a time, we all lived in houses with cold rattling windows.

There's a variety of claims on who was the first to experiment with double glazing but it's generally considered that the Scottish were the first in Victorian times. Adding a second pane of glass to their existing windows to try and keep out the harsh Scottish elements. Although, there is mention that the Romans also had their own heat retention system.

Commercially, double glazing was 'invented' in the US in 1930 by a CD Haven and this was later patented as 'Thermopane' by an Ohio glass company. After this, in the 50s, double glazing began to filter into the homes of Americans.

In the UK, in 1965, Everest began life selling a version of aluminium secondary glazing, until 1978 when we developed our first aluminium double glazing product. In 1984, our uPVC double glazing was created and this quickly became the window of choice for the UK and the rest is history!

How does double glazing work?

Double glazing works by creating an air gap that insulates against heat transfer between two different temperature zones (inside and outside).

Two panes of glass are sealed in a unit (IGU) separated by a spacer bar.

The space between the panes (usually 6-12mm) creates an air gap that slows down thermal transfer (hot or cold) by conduction – it doesn't 'trap heat' but it slows down the movement of heat to reduce heat loss.

Because air can't circulate in the narrow gap between the panes, air convection is slowed down and this also reduces heat transfer.

How does double glazing reduce heat loss?

Double glazing reduces heat loss through the thermal cushion and insulation of the air contained in the sealed unit.

As cold air hits the external pane of glass, the pane of glass transfers the temperature to the Argon gas layer.

Argon gas has a 34% reduced thermal transfer rate to air and slows down the conduction of heat between two differing temperatures.

So, the Argon layer transfers less of the temperature difference to the internal pane of glass which remains at a median temperature between the Argon glass and the room.

In the same way, when the warm air from the room meets the internal pane of glass the Argon gas slows the transfer of the warmth from the inside out. And heat is retained in the room.

In a double glazed unit, the outside pane of glass and the internal will be different temperatures.

How does double glazing reduce noise?

Sound is an energy that moves in waves, like waves on the sea, by vibrating the molecules of the air and objects it comes in contact with. When sound waves hit glass, the glass vibrates at the sound wave frequency and transmits the vibration to the air molecules on the other side of the glass.

As the sound wave travels, dense objects absorb the energy which reduces the frequency of the waves as it passes through. This is why, if you put your ear to a wall you can hear people talking on the other side but if you step away from the wall you can’t hear it.

Sound has a longer wave vibration than light which means it can bend (diffract) around corners making it far more difficult to contain than light. If you have a trickle vent, you can still hear a lot of noise even if your window is closed. It also means, if the seals are failing in the double glazed unit you will hear an increase of noise leaking through the window.

Double glazing reduces noise by absorbing some of the energy and slowing down the vibration frequency as it transfers through the glass and the layer of Argon gas. When the sound wave passes through the other side of the window, the sound wave is slightly diminished and the noise a little quieter.

However, the key to reducing noise as much as possible is to disrupt the sound waves and frequencies as they pass through a material. This can be done by combining different thicknesses of glass close together.

Everest Acoustic glass (in double or triple glazing) uses panes of 6mm and 6.8mm. An additional special laminate coating absorbs more energy of the waves and reduces the frequency as they pass through. Because of these special properties, acoustic glass can reduce noise by up to 40db (in triple glazing).

How does double glazing reduce condensation?

The air around us is full of tiny molecular droplets of water that are invisible to the eye. In warm air, the molecules are spaced far apart which holds the moisture but as the air cools the molecules come together. When the molecules are so close together they merge into visible liquid this is known as the 'dew point'.

Condensation manifests when moisture-laden air comes in contact with a cold surface – such as a windowpane. When you boil a kettle, cook food or have a hot shower, the air becomes filled with more moisture than usual and this is why kitchen windows and bathroom mirrors are most steamy.

We also expire moisture on our breath and this is why you find condensation on a bedroom window when we wake up.

Inside the double glazed sealed unit, the contained air also holds a low level of moisture that could form on the glass if the temperature difference should change.

To address this, every sealed unit is built with crystalline desiccant (like what you find in a new bag or shoes) contained in the spacer bar. The desiccant draws the moisture out of the air gap and keeps the internal space between the panes dry.

Internal condensation happens when the seals on the sealed unit begin to fail and more moisture is drawn into the unit. The desiccant becomes saturated and can no longer hold moisture and as a result, condensation forms inside the unit.

The reason we see so much condensation on single glazed glass is that the glass is susceptible to cold and stimulates the dew point from the water vapour in the air.

Double glazing helps to reduce condensation on the inside of your windows (inside the room) because the glass is not as cold as single glazed. The air gap inside the double glazing sandwich helps to keep the internal pane of glass closer to room temperature. This difference in temperature is enough to stop the water vapour reaching dew point and forming condensation on the glass.

Solar gain through double glazing

Double glazing reduces heat loss and acts as a barrier against the cold but it doesn't stop the warmth from the sun, known as solar gain.

Sunlight produces short wave infrared radiation – this is what warms objects in the sunshine. The short waves are absorbed by glass from the outside and the energy becomes re-radiated from the inside of the glass as long-wave infrared.

Short waves can pass through glass but the long waves can't, so the thermal energy of the infrared radiation becomes trapped bumping against the glass unable to pass through. This process is known as the greenhouse effect.

The rise in temperature is known as solar gain – we measure this by G value.

For windows facing direct sunlight, we recommend low-emissivity glass (Low-E) that help to regulate room temperature because rooms quickly overheat with solar gain. If you've ever sat in a conservatory on a sunny day you will know how oppressive it can get.

Low-e glass applies a microscopic magnetron coating that reflects the shortwave radiation away from the outside of the glass. The window maintains the clarity of daylight but stops the build-up of thermal energy and the greenhouse effect.

Low-e glass can also be combined with a solar control glass coating that further filters the light to reduce the solar glare and reduce damage to furnishings and fabrics.

How is double glazing made?

The sealed unit (IGU) is an integral part of the double glazed window and the part most susceptible to failure. This is where manufacturing quality really counts.

A double glazed unit is created by cutting the two panes of glass to size – all Everest units are made to measure. The spacer bars are prepared to size with the desiccant inside.

Old aluminium spacer bars are brilliant conductors of temperature and act as thermal bridges transferring the cold from the outside pane to the inside. This is why the edge of the glazed unit has always been the weak spot where it's the coldest. Warm edge spacer bars are blended from stainless steel and polypropylene which has a low thermal conduction rate and this reduces heat loss at the edge of the sealed unit.

One pane of glass is laid with the spacer bars in position. This moves through our Argon filled chamber and whilst moving, the second pane of glass is precisely pressed on top, sealing the unit and trapping the Argon.

We construct our units by a robot in an Argon filled chamber to ensure the only thing in our sealed unit is Argon. Other companies drill two holes into the frame and hand pump the Argon which can leave the unit filled with a mixture of air and Argon. By not drilling the frame we maintain the structural integrity and airtight seal of the frame.

The unit is then sealed with a bitumen-based sealant around the edge. We apply the sealant in a molten state so that it flexes if the unit does and keeps the unit airtight.

Lifting glass by hand puts unnecessary stress on the pane that can lead to warping and visual defects. We assemble our units mechanically so that panes are lifted evenly using suction cups to ensure no part of the glass is placed under stress to maintain integrity and quality.

The double glazed unit is then ready for assembly in the chosen frame material.

What are the benefits of double glazing?

Thermal transfer, solar gain, dew point, sound frequency and conduction - what all this information means in reality is that double glazing will:

• Keep more external noise out of your home
• Retain more heat and keep your home warmer
• Reduce condensation on your windows
• Reduce the amount of energy you use to heat your home
• Reduce your energy bills
• Add value to your home

If you’re thinking of upgrading your windows, all our windows are bespoke made from a range of different glass products.

If you have an extra cold room that doesn't get light, a room that gets too much sun, or a bedroom facing a noisy road – we can make the perfect window for you.