Double Glazed Units

How is a double glazed unit made?

With all this talk of double glazing and its benefits, how many of us actually know what is involved in making a double glazed window? Ok, we know some of you are now saying, yes that’s easy, but for those shaking their heads and thinking that’s an interesting question, here goes!

Let’s start with a little bit of window history. The double glazed window was invented in the 1930s, and was commonly available in America in the 1950s under the Thermopane brand name, and registered in 1941 by Libbey-Owens-Ford Glass Company. Now many decades later, the manufacturing process is well established.

Not to be confused with single paned secondary glazing, a Double Glazed Window comprises of two panes of glass, separated by a spacer bar and seals, to create a hermetically-sealed environment. An adhesive sealant is applied to the face of the spacer on each side and the panes pressed against the spacer bar. The cavity is filled under vacuum conditions with either air or an inert (unreactive) gas like argon to improve insulation and prevent condensation within the unit. As the air or inert gas are both poor conductors of heat, this results in a great insulation effect. The filled gap between the two sheets of glass reduces the heat conduction between the panes, keeping the warm air in and the cold air out! This makes the home more energy efficient and helps to reduce energy bills. Argon gas has a greater density than air, so it more effectively reduces the heat loss from the property. Argon is the gas most commonly used in double-glazed windows, as it is extremely cost-effective, colourless, is non-flammable and does not react to other gases.

If the double glazed unit is gas filled, two holes are drilled into the spacer of the assembled unit, lines are attached to draw out the air out of the space and replacing it with the desired gas. The lines are then removed and holes sealed to contain the gas. The more modern technique is to use an online gas filler, which eliminates the need to drill holes in the spacer. The units are then sealed on the edge side using either polysulphide or silicone sealant to prevent humid outside air from entering the unit. This will make sure that no condensation of the glass panes facing the air space during cold weather will occur.

The humble double glazed window has come a long way, from the original double glazed sealed units of two sheets of float glass with a very small spacer bar, to today’s high specification and more energy efficient double glazed sealed units, with toughened safety glass, wide spacer bars and sometimes a film on the glass which allows the double glazed unit to clean itself. An average home loses 10% of its heat through windows and doors. Good, energy efficient glazing reduces this heat loss, keeps the home warmer and reduces heating bills. It can also make the home quieter.

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