To find out whether your window has energy-efficient glass, a simple home test is enough — all you need is a match or a lighter flame.
How to do the test
- Turn off the lights in the room or wait until evening so the reflections in the window are clearly visible
- Hold a lit match or lighter flame at roughly a distance of 10–15 cm from the glass
- Looking at the glass at a slight angle, watch the flame reflections on the glass surfaces
- Count how many flame reflections you can see — a standard single-chamber glass unit should show four reflections
- Look carefully at the colour of each reflection — this is the key indicator
Why four reflections? A single-chamber glass unit has two panes — each pane has two surfaces, four surfaces in total. Each surface reflects the flame, so you see exactly four reflections.
What the colours of the reflections mean
The reflections are numbered as in the image — from the street side inwards: the 1st reflection is the farthest (street side), the 4th reflection is the closest to you (inside):
⚠️ Note — is the glass unit installed the wrong way round? If it is not the third but the second reflection that is a different colour, while the others look the same colour, the glass unit is most likely installed with the inner side facing out. This is an installation fault: the Low-E coating is on the wrong side, the insulation does not work, and it can be one of the reasons why the glass unit fogs up.
What is Low-E glass?
Low-E (“Low Emissivity”) is a thin, almost invisible metallic coating applied to the glass surface. This coating:
- reflects thermal radiation back into the room in winter
- prevents excessive heating from the sun in summer
- significantly improves the window’s insulation (lowers the U-value)
- reduces heating energy use by 20–40%
It is because of this coating that the third reflection in the glass looks a different colour — usually a slight bluish or greenish tint confirms the presence of a selective coating.
What to do if there is no Low-E glass?
If the test shows your windows have no energy-efficient glass, it does not mean the window must be replaced straight away. But replacement is worth considering if:
- there is condensation or fogging between the panes
- you feel a draught or cold at the window in winter
- heating bills are higher than expected
- the window is more than 15–20 years old
Conclusion. The candle test is a quick, simple method that lets any homeowner judge the quality of a glass unit in a few seconds. If the third reflection is a different colour, you have a correctly installed energy-efficient window. If all four flames look the same, the window has no Low-E coating and replacement is worth considering. If the second reflection is a different colour, the glass unit is installed the wrong way round and it is best to consult a specialist.
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