U-value or g-value? This is how it works!

Normally, it can be assumed that about 35 % of the heat in a house escapes through the windows. How high the actual heat losses are depends, among other things, on the window model, whether there are one, two or three panes in the cassette, the type of glass used and the nature of the gas between the panes. At the same time, incoming solar heat often contributes to the high cost of cooling a house in the summer months. The balance between heat and cold, summer and winter is supported by energy-efficient windows.

What is the g-value?

The g-value is a measure of how much solar heat (infrared radiation) can penetrate through a certain part of a building. A low g-value indicates that a window allows a small percentage of the sun's heat to pass through. The g-value can be improved by coating the outer pane of glass with an IR-reflective surface that reflects some of the radiant heat. This should, on the one hand, reduce the cost of cooling the building and, on the other hand, improve the indoor climate in buildings without comfort cooling. Untreated insulating glass has a g-value of about 1.3.

What is the U-value?

The U-value is a measure of how much heat escapes through the windows, walls and roof, for example. The U-value is often measured for the entire window construction with the combination of glass, frame and sash. The lower the U-value, the better the insulating performance of the window. The U-value can be improved by coating the glass panes with a metallic surface that reflects long-wave radiation back into the room. A U-value of 1.0 means a heat flux of 1 watt/square meter of surface area for each degree that separates the outside and inside temperatures (W/m²°C, formerly called the K-value). A new insulating glass pane has a U-value from 2.7 down to 0.4.

What does the U-value mean in practice?

In a normal private house there is an estimated 30 m² of window glass. The homeowner chooses between an insulating glass with a U-value of 1.2 and a glass with a U-value of 1.6. How big is the difference in heating costs per year?

Theoretically, a difference in U-value of 0.1 units corresponds to about 9 kWh/m² of glass per year. The difference between 1.6 and 1.2 is 0.4 units. The saving is therefore (difference in U-value) x (additional consumption) x (glass area) = 4 x 9 kWh/m² x 30 m² = 1,080 kWh/year.

With an energy price of 0.13 €/kWh, the difference is 140 €/year. A school with 300 m² of glass area consequently saves about 1,400 €/year, assuming a U-value between 1.6 and 1.2 for the window glass.

What is the U-value of Hammerglass?

Hammerglass Insulate triple glazing has a U-value of 0.66, Hammerglass Insulate double glazing has a U-value of 1.53.