I notice that a B rated window has a negative value and an A rated window usually has a positive value what does this mean?
This figure is the Energy Index and it shows how much energy the window will save or lose once it has been installed, this will depend on the building, local climate and indoor temperature. A positive value means the window will enable more energy (free heat) to pass through the glass and into the room than is going to be lost through the window, therefore the window is a ‘free energy’ supplier.
