Energy-efficient glazing: all you need to know

As we’ve said before, one of the most important steps you can take to reduce your carbon footprint is to make your home greener. Energy-efficient glazing can go a long way to helping you do this.

The amount of heat lost through single-glazed windows varies according to which source you check, but it is somewhere in the region of 10 – 40%. But whatever the true amount, choosing energy-efficient glazing will considerably reduce this heat loss – making your home warmer, more efficient and reducing your energy bills.

How energy-efficient glazing works

The heating effect of the sun in a building is known as solar gain. It varies with the strength of the sun, its angle and the effectiveness of the glazing to transmit or reflect its energy. Energy-efficient window design aims to maximise the building’s solar gain in the winter and control it in the summer. Check our blog post on passive solar for more details of how to benefit from solar gain.

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Image via Wikipedia: NcLean/ CC-BY-SA-3.0

The U-value is the measurement of how effective a material is as an insulator. The lower the U-value, the better the insulator. Remember that while a particular glass may have a very low U-value, you need to take into account the window as a whole – if the frame leaks heat then it doesn’t matter how fancy the glass is.

Speaking of glass, there’s more than one type of energy-efficient glazing and various options within each type. Double glazing – or to give it its official title, an insulating glass unit (IGU) – comprises two panes of glass with a gap between them.

The purpose of the gap is to reduce heat transfer, or in other words, to slow down the rate at which heat escapes.

See the image to the left: surface 1 is facing outside, surface 2 is the inside surface of the exterior pane, surface 3 is the outside surface of the interior pane, and surface 4 is the inside surface of interior pane. The window frame is labelled 5, a spacer is indicated as 6, seals are shown in red (7), the internal reveal is on the right hand side (8) and the exterior windowsill on the left (9).

Triple glazing is – as the name suggests – three panes of glass, with the same construction and purpose.

Now, you may assume that three panes are better than two, but this is not always the case. The aim is to take advantage of solar gain so bear in mind that you may get more residual heat from double glazing than triple. It’s something to discuss with your architect or window provider.

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Here’s where it gets complicated: the various options within energy-efficient glazing. As well as just normal air, the gap between panes of glass can be filled with an inert gas sealed between the panes, which further minimises heat conduction.

Argon, which has 34% lower thermal conductivity than air, is the most commonly used and can improve a double-glazed window’s U-value by more than 30%. You can also use xenon and krypton which are more effective, but more expensive and so less frequently used.

More recently, vacuum insulated glazing (VIG) has come on the market. VIC is similar to an IGU but the gap between panes is a vacuum – making it more efficient and much thinner. These windows are perfect for older buildings where you are replacing single glazing or want to use slim frames.

Another innovation in energy-efficient glazing is low emissivity (or low-E) coatings for glass. Emissivity is the ability of a material to radiate energy, therefore low emissivity refers to a surface which emits low levels of radiant thermal energy.

Low-E glass has a thin, transparent coating that reflects infrared energy (or heat). When the interior heat energy tries to escape to the colder outside during the winter, the low-E coating reflects the heat back to the inside, reducing the radiant heat loss through the glass. The reverse happens during the summer.

To borrow an analogy: low-E glass works the same way as a thermos. A thermos has a lining which reflects the temperature of the drink it contains. The temperature is maintained because of the constant reflection that occurs, as well as the insulating benefits that the air space provides between the inner and outer shells of the thermos, similar to an insulating glass unit. Since low-E glass is composed of extremely thin layers of low emissivity materials, the same theory applies. The low-E coating reflects the interior temperatures back inside, keeping the room warm or cold.

What’s more, since UV light is responsible for sun bleaching, some low-E coatings claim to be able to reduce the fading on your fabrics. Bonus!

So there you have it: all you need to know about energy-efficient glazing. If you have any questions or comments, don’t hesitate to get in touch with us – find us in the usual places online @KoruArchitects


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