What Window Energy Ratings Actually Mean

The letter band

The band runs from A++ down through A+, A, B and below, and it is calculated from the whole window rather than the glass alone. It combines heat lost through the frame and glass, heat gained from sunlight and air leakage around the seals. Because the calculation rewards solar gain, a large south facing window can band higher than a small north facing one built to the same specification, which is one reason the letter alone is a blunt instrument.

U value, the number to compare

The U value measures how fast heat escapes, in watts per square metre per degree. Lower is better. Building regulations for replacement windows in Wales sit at a maximum of 1.4, a good modern double glazed window lands nearer 1.2, and a properly specified triple glazed unit gets down toward 0.8. When you compare two quotes, compare the whole window U value, not the centre pane figure, because the centre pane number always looks better and never includes the frame.

The other numbers on the label

Two more figures matter in specific situations.

• g value, the proportion of solar heat let through, where higher warms the room and lower controls summer overheating
• Light transmittance, which matters in the deep shaded valleys where daylight is scarce
• Air leakage, expressed as L, which is where cheap frames quietly lose their rating in real use
• Acoustic rating in decibels, which is separate from the energy label entirely

What we fit as standard and why

Our standard uPVC window is an A rated 70mm multi chamber profile with a 28mm argon filled unit, a warm edge spacer and a low emissivity soft coat on the inner pane. That specification is chosen because it comfortably clears the regulation, holds its performance as the seals age, and does not rely on solar gain to reach its band.