In today's market, you would think it would be easy to spot the difference between a good uPVC window and a bad one. For example, in the UK and Ireland, windows now come with a cert. This is an energy rating to tell you how good the window is at keeping the heat in. The rating starts at A, with A being the best, and B being next in line, and so on.
uPVC profiles are multi-chambered sections that are reinforced with steel for strength. These sections form the frame in which the glass panes are fitted. Here is how you can check the quality of a uPVC profile:
All that is needed is to change the reinforcement from steel or aluminum to a warmer reinforcement, like carbon. This will reduce heat loss through the frame of the window. Then you must upgrade the double glazing so it also has a warm edge spacer bar and Low-E glass, plus it must also be gas filled. Argon is the most common gas which is used. But with some window systems, this still is not good enough, and they must upgrade the gas to Krypton gas.