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Mind the Gap!

What you need to know about the gap.

If you have decided to explore replacing the windows in your home, you will soon discover that not all windows are created equal. Many different factors affect quality, performance, and price.

As you might expect, the quality of the glass itself can make a difference in the effectiveness of a double glazing unit, as can the type of frame holding the unit together. But you may be surprised to learn that the double glazing gap – the pocket of gas between the panes – is the most important element of all.

A section through a double glazed window showing two panes of glass with gap between them.

The gap between panes of glass


What makes the gap so important in double glazing?

The gap that lies between two panes of a double glazed window forms part of a highly effective insulating layer that retains heat within the home and blocks the cold outside from penetrating. The combination of a pane of glass, a gap, and a second pane allows light (short-wave radiation) to pass through the window, but prevents heat (long-wave radiation) from escaping from the home.

The effectiveness of a double or triple-glazed window will rely on the size of the gap and the type of gas that this gap is filled with. In order to maximise the insulating properties of the structure, the gap must be carefully calibrated.

If the space between the panes is too narrow, the outer pane will be able to transmit cold temperatures to the inner pane. If the space is too wide, the gas trapped between the two panes will be able to circulate freely and conduct heat from the inner pane to the outer pane, resulting in the loss of heat from inside the home.

Maintaining the optimal width between the panes will help retain heat and create a comfort zone of warmth around the window area.


What is inside the gap?

The gap between the panes of a double glazing unit is always filled with an inert, nontoxic gas. In less costly units, the gap will be filled with air. In pricier models, Argon gas will be used. Because Argon has a higher viscosity level than air, it possesses better thermal properties: in fact, Argon’s conductivity is just 67 per cent that of air, which is why Argon-filled double glazing units offer superior heat retention.

In very rare cases, Krypton, an even more viscous gas than Argon, may also be used to fill the double glazing gap. However, because of the high costs associated with this gas, it is seldom used except in cases where very thin double or even triple glazed units are required.

In better-quality units, the gap is created using a “warm edge technology” spacer, which is made up of foam, acrylic, and a multi-layer vapour barrier. Cheaper units will use spacers made of aluminium, a conductive metal that tends to transmit temperatures between the panes of glass, thereby lessening its effectiveness as an insulator.

Once the unit is filled with gas, it is sealed with a polysulphide or silicone sealant that prevents moisture from seeping in. These materials act as “desiccants” attracting any moisture and neutralizing the effects of condensation within the panes.

Several uPVC windows cut through to show structure of frame and gap between panes of glass.

uPVC Windows clearly showing the gap


What different sizes of gap are available?

Generally, double glazing window units are designed to include a gap of 12, 16, or 20mm between the panes of glass. The effectiveness of the window unit will rely on a combination of the size of the gap, the gas with which the gap is filled, and the type of glass used.


What size of gap is best for my home?

In general, the optimal width for the gap will be around 16mm. Double glazed windows with a 16 mm gap have better insulating properties, a quality that is measured in U values. U values are a measurement of the amount of heat that is lost through a particular building material. The lower the U value of the material, the better it will insulate your home.

For instance, air-filled double glazing with a 12mm gap will have a U value of around 3. If the gap is increased to 16mm, the U value will decrease to approximately 2.7. However, adding another 4mm may have little benefit, and could even reduce the insulating properties of the window, resulting in a U value of 2.8.

The heat-retention of a double glazed window with a 16mm gap can be reduced further to 2.6 when Argon instead of air is used to fill the gap. By using Low-E glass instead of regular glass, the U value can be reduced even further – to just 1.6.

(U value figures are necessarily approximate, as other factors such as the thickness of the glass itself, and the material, quality of design and construction of the frame will also have an effect).

However, before making a decision about which type of double glazing unit to purchase, it’s best to consult with an installation professional who can determine the best fit for your budget and insulation needs.

The slim space between double glazed window panes has an important role to play in conserving heat and energy in your home. By “minding the gap” and understanding the science behind its effectiveness, homeowners can make better, more informed choices about the double glazing solution that’s right for their home.

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