The u in uPVC refers to the PVC being unplasticised. PVC is a rather hard and stiff polymer which makes it unsuitabe for many uses, such as bottles, packaging etc. A plasticiser is added to modify the physical properties of the polymer.uPVC is used for window frames, as it does not decompose, is weather-resistant, does not change shape under normal weather conditions, and is recyclable, as it can be reshaped at high temperature. uPVC windows are more energy-efficient than those with wooden or metal frames. It can also be used for door frames and conservatories.