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Thermal insulation coefficient

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If you want to reduce your energy consumption, the choice of glazing is essential. To choose the right glazing, you need to take a close look at its thermal insulation coefficient.

Thermal insulation coefficient (U): definition

The U-value measures the thermal conductivity of a glazing unit. It is expressed in W/m²K.


  • W for Watt = energy lost

  • m² for glazing area

  • K for degree Kelvin, the temperature difference between inside and outside.

The U insulation coefficient is differentiated into 3 other coefficients: Ug for the glazing, Uw for the window and Uf for the frame.

Ug insulation coefficient

The Ug coefficient (g = "glass") is used to measure the thermal coefficient of a glazing unit.

This coefficient indicates the glazing's insulating performance by measuring the amount of energy lost (or heat exchanged) as a function of the temperature difference between the interior and exterior glazing. It therefore corresponds to its capacity to retain heat in a room.

The lower the factor, the more insulating the glazing.

Ug-value of single glazing

The Ug thermal coefficient of single glazing is very high, at around 5.7 W/m²K. So energy loss is high.


In the case of renovation projects in particular, and to improve the insulation of a building, it is strongly recommended to change from single glazing to double (or even triple glazing in colder regions).

Ug-value of double glazing

The first double-glazed units on the market had a Ug value of 2.6 W/m²K. Insulation was already much better, but today most glazing is fitted with a low-emissivity layer, which significantly improves the Ug thermal coefficient. This now reaches a value of 1.1 W/m²K. This value is now considered the market standard for glazing.

What is a low-emissivity layer?

This is a film applied to the inside of the glazing. It keeps a room warm while letting light through.

Ug-value of triple glazing

The Ug thermal coefficient of triple glazing reaches very low values, ranging from 0.5 to 0.8 W/m²K. Although this is a very good indicator, it is not suitable for all regions and can be very expensive.

Factors influencing the Ug thermal coefficient

To improve the Ug-value of a glazing unit, the manufacturer can modify :


  • The distance between glasses: optimized between 16 and 20mm.

  • The type of gas in the space between the glasses: Argon is the most widely used.

  • Glass composition: by adding a low-emissivity layer.

  • Warm Edge: a thermal break spacer (where insulation is broken), it acts as an insulating barrier to reduce heat loss.

Uw insulation coefficient

While it's important to choose glazing with a good Ug rating, the Uw (w = "window") insulation coefficient is also important.

Also expressed in W/m²K, it takes into account both the glazing and the joinery. As with the thermal coefficient of glazing, the lower the Uw value, the better the window's insulation performance.

Uf insulation coefficient

Finally, there's also the Uf coefficient (f = "frame"), which gives the thermal performance of the window frame.

Thermal insulation coefficient and solar factor

When choosing glazing, the solar factor must also be taken into account: it measures the ability of a glass pane to transmit solar energy; the higher this coefficient, the more solar energy will enter the room and heat it.

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