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How Double Glazing Works

The science behind this type of window is actually quite easy to understand: It depends on the fact that air or gas is a much poorer heat conductor than glass.

The layer of air or inert gas between the two glass panes of a double glazed window does not conduct heat very well, which means that much less heat will escape through a double glazed window in winter than would escape through a single-glazed window.

A scientist examines a molecular model

Heat transfer occurs at the molecular level

Heat is simply the movement of molecules. In a warm substance, molecules are moving more quickly than in a cold one. When one quickly moving molecule bumps into the molecule next to it, that molecule will also start moving more quickly, thus heat is conducted.

Materials that contain molecules that are close together such as glass or metal spread heat quite quickly, as the molecules bump into each other more easily. On the other hand, materials like air, in which molecules are spread further apart tend to spread heat less quickly.


Double glazing as a woollen jumper for your home

Remember your mum telling you to put on a woollen jumper on a chilly day? She was actually relying on the same scientific principle that makes double glazing work.

Here’s how it works: the woollen fibres of your jumper trap air between them. When the quickly-moving molecules of your warm body bump into those air molecules, they transfer their heat energy to the air trapped in your jumper.

However, the quickly moving warm air molecules in your jumper can’t easily bump into the more slowly moving molecules in the air around you. That air forms an insulating layer between your body and the colder air around you, helping you keep your body heat close and keeping you from feeling chilly.

Double glazing does the same thing: the air trapped between the panes of glass forms an insulating layer between your home and the air around it. In a way, when you install double glazing, you are essentially wrapping a woollen jumper around your home. Yes, that’s a bit of a funny image, but technically speaking you are drawing on the same scientific principles.


Argon gas versus air:

Double glazed windows can be filled either with air or with an inert gas like argon. ‘Inert’ simply means that the gas does not easily react chemically with other materials.

Argon gas has a slight advantage over air in that it is an even poorer heat conductor: Argon gas conducts about 33% less heat than air does. Whether your replacement windows use argon gas or air for the insulating layer, the basic principle is the same: the layer of air or gas between the glass panes prevents heat from escaping through the window.


Spaces between glass panes:

One key factor that determines the efficiency of replacement windows is the gap between the panes of glass. If the panes are too close together, heat can more easily transfer from one side of the window to the other, reducing the window’s insulating efficiency.

If the panes are too far apart, temperature differences between inside and outside can cause the air between the panes to move around. This also reduces the window’s insulating efficiency. Most sealed units achieve maximum insulating efficiency by placing the two panes of glass between 16 to 19 millimetres apart.

molecular model of PVC on plastic pellets raw material

Molecular model of PVC


Double glazing frames:

Window panes, of course, must be contained in some sort of frame. The frame you use to contain your windows affects their ability to insulate your home.

Window frames in the UK are usually made of aluminium, wood, or uPVC. While aluminium and wood are self-explanatory, uPVC needs a brief note of explanation. The abbreviation stands for ‘unplasticized polyvinyl chloride’ and it is a type of plastic invented in the early 20th century.

When plasticising agents are added to PVC, the material becomes flexible and can be used to make plastic products like raincoats. Unplasticized PVC or uPCV, on the other hand, is a firm, durable material often used in construction.

All three common frame materials – aluminium, wood and uPVC – have strong and weak points:

Aluminium is light, strong, durable and requires little maintenance, but it is unfortunately an efficient heat conductor. In order to counter aluminium’s heat-conducting properties, most aluminium window frames contain an insulating material between the inside and outside of the aluminium frame.

Wood, on the other hand, is a very good insulator. Wooden double glazing frames allow very little heat to pass from one side of the window to another. However, wood, as a natural material, is not as durable as manmade frame materials such as aluminium or uPVC and it can swell or shrink if exposed to rain or sunshine.

If you choose wooden-framed windows, you will need to ensure the wood has been treated and sealed against the elements. This weather treatment must be maintained regularly.

Finally, uPVC double glazing frames have the advantage of being good insulators, durable, low-maintenance and relatively cheap to install. Double glazing frames made of uPVC are therefore quickly becoming a popular choice among UK homeowners.

However, in recent years concerns have been raised over the environmental cost of manufacturing uPVC for double glazing frames. The manufacturing process does involve chemicals and creates hazardous waste.

It may seem that there is a lot to consider when choosing which type of windows to install in your home. While there are important factors to consider when deciding on which type of insulating layer (argon gas or air) or window frame (aluminium, wood or uPVC) to use in your installation, remember that the basic science of double glazing is the same no matter what option you choose.

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