It can normally be reckoned that around 35% of the heat in a house will escape through the windows. Just how great actual heat losses are depends among other things on the window model, whether there are one, two or three panes in the cassette, on the type of glass used and the nature of the gas between the panes. At the same time the incoming solar heat often contributes to the high cost of cooling a property during the summer months. The balance between warm and cold, summer and winter is helped by having energy-efficient windows.
The g-value is a measure of how much solar heat (infrared radiation) is allowed in through a particular part of a building. A low g-value indicates that a window lets through a low percentage of the solar heat. The g-value can be improved by having the outer glass pane coated with an IR-reflecting surface which reflects some of the radiant heat. The aim of this is partly to reduce the costs of cooling the property, and partly to improve the indoor environment in properties without comfort cooling. Untreated insulating glass has a g-value of approximately 1.3.
The U-value is a measure of how much heat escapes via the windows, walls and roof for example. The U-value is often measured for the whole window structure with the combination of glass, frame and sash. The lower the U-value, the better the insulating capacity of the window. The U-value can be improved by the glass panes being coated with a metal-based surface which reflects long-wave radiation back into the room. A U-value of 1.0 means a heat flow of 1 Watt/square metre surface area for every degree which separates the outdoor temperature and the indoor temperature (W/m² °C, previously termed the k-value). A new insulating glass pane has a U-value ranging from 2.7 down to 0.4.
In an ordinary private house it is estimated that there are 30 m² of window glass. The property owner chooses between insulating glass with a U-value of 1.2 and a glass with a U-value of 1.6. What is the difference in the heating costs per annum?
In theory a difference in U-value of 0.1 units corresponds to c. 9 kWh/m2 glass per annum. The difference between 1.6 and 1.2 is 0.4 units.
Thus the saving is (difference in U-value) x (additional consumption) x (area of glass) = 4 x 9 kWh/m² x 30 m² = 1,080 kWh/annum. With an energy cost of 0.13 €/kWh the difference is 140 €/annum.
A school with 300 m² glass consequently saves approximately 1.400 €/annum, based on a U-value of between 1.6 and 1.2 for the window glass.
Hammerglass Insulate triple glazing has a U-value of 0.66, Hammerglass Insulate double glazing has a U-value of 1.53.