Thermal insulation coefficient

To measure glazing’s thermal coefficient, we refer to Ug ( g = “glass” ).

This coefficient describes the glass’s insulation performance by measuring energy loss (or heat exchange) as a function of the temperature difference between the inner and outer panes. It therefore represents its ability to retain heat in a room.

Single glazing has a very high Ug thermal coefficient, around 5,7 W/m²K . Energy loss is therefore substantial.

Particularly during renovation, improving building insulation is a strong reason to replace single glazing with double glazing, or even triple glazing in the coldest regions.

The first double glazing on the market had Ug of 2,6 W/m²K. Insulation was already much better, but today most glazing has a low-emission (or low-emissivity) coating, significantly improving Ug. It now reaches 1,1 W/m²K . This value is now considered the market standard for glazing.

Triple glazing’s Ug thermal coefficient reaches very low values ranging from 0,5 to 0,8 W/m²K . Although this indicator is very good, it will not suit every region and can be very expensive.

To improve glazing’s Ug coefficient, a manufacturer can modify:

• The distance between the panes: optimised between 16 and 20 mm.

• The type of gas between the panes: argon is most commonly used.

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

• Warm edge: an interlayer with thermal breaks (where insulation is interrupted) acts as an insulating barrier to reduce heat loss.

Choosing glazing with a good Ug is certainly important, but the Uw insulation coefficient (w = “window”) must also be considered.

Also expressed in W/m²K, it accounts for the glazing and frame . As with glazing’s thermal coefficient, the lower Uw, the better the window insulates.

Finally, Uf (f = “frame”) describes the window frame’s thermal performance.

When choosing glass, also consider the solar factor : it measures the glass’s ability to transmit solar energy; the higher this coefficient, the more solar energy enters and heats the room.