You've just moved into a new house. Or you recently replaced your windows. Or perhaps you're buying a second-hand apartment. In any case, one question is on your mind: "Are the glass panes I have really energy-rated (Low-E) - or did they install plain glass?"
The truth is that, looking at a pane of glass, you can't see anything. The Low-E coating is invisible to the naked eye. However, there's a clever, simple trick - all you need is a lighter or even your phone's flashlight - to learn the truth in 60 seconds. Let's see how.
A double-glazed unit consists of two glass panes. Each pane has two surfaces: one outer and one inner. In total, a double-glazed unit has 4 surfaces. They are usually numbered from outside to inside:
Surface #1: Outer side of the exterior pane (facing the
street).
Surface #2: Inner side of the exterior pane (facing the
cavity).
Surface #3: Inner side of the interior pane (facing the
cavity).
Surface #4: Outer side of the interior pane (facing your
room).
When you hold a light source (flame, LED, candle) close to the glass, each of these 4 surfaces creates its own reflection. You'll see 4 "little images" (reflections) of the flame: two closer together (from the pane nearest to you) and two further apart (from the pane facing the street).
You'll need a room that is relatively dark. Turn off the lights or wait until nightfall. Light the lighter (or a candle) and hold it close to the glass, at a distance of 3-5 centimetres.
Look at the 4 reflections. If one (or two) of the 4 reflections has a different colour from the rest, your glass has an energy coating!
Typically, the reflection with a Low-E coating appears in pink, purple, green-bluish or lavender, while the other reflections (without coating) show the normal yellow-orange colour.
On most European energy glass, the Low-E coating is placed on Surface #3 (inner side of the interior pane, "facing" the cavity). This means the 3rd reflection (counting from inside) should be the "coloured" one.
If all 4 reflections are the same colour (yellow/orange, like a normal flame), unfortunately there is no Low-E coating. It is plain, non-energy double glazing.
If you can't or don't want to use a flame, there are alternative methods:
Look at the metallic strip (spacer bar) separating the two panes. On this, a reputable manufacturer engraves or prints codes and technical information. Look for markings such as "Low-E", "LE", "4S", "SN", "Solar", or alphanumeric codes.
If it's cold outside (below 5°C), touch the inside surface of the glass. If you feel the glass is almost at room temperature (lukewarm), it most likely has a Low-E coating. If you feel it's clearly cold like metal, it probably doesn't.
There are handheld Low-E detection devices that attach to the glass and instantly identify whether each surface has a coating. They are mainly used by professionals (energy auditors).
This is more difficult.
Four-Season glass will also show a coloured reflection (it is also Low-E, but advanced). However, some Solar Control glass may also show a reflection on Surface #2 (exterior pane, facing the cavity), indicating an additional solar coating.
Check the spacer bar for details. Alternatively, ask the manufacturer or previous owner for the EN 1279 certificate of the glazing unit, which will list in detail the composition, U-values and g-values.
Before paying the final invoice to the manufacturer, spend 60 seconds doing the lighter test. Don't rely on verbal promises - the flame doesn't lie.
Demand a written guarantee with an EN 1279 certification code and CE marking. A reputable manufacturer will provide these without hesitation.
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