On the website of several distributors and windows manufacturers, energy efficiency or energy efficient are words that are commonly used. We note that the simple use of the words "energy efficiency" does not necessarily attribute the qualifier to the product! There must be an explanation, a procedure, an elaborate protocol. For example, the simple words: "tested at -30C" without further explanation should make you very skeptical. Yes indeed! -For how long? what period? -What is the test protocol? But still. We have found that many provide performance only for glazing. It is not so much a secret that the window is composed and the glazing and its framing! The overall performance of the whole represents the real value of the energy performance! And still it will change according to the size of the window! At VST Euro Technologies, we supply the Uw Value for each window (framing and glazing) and doors delivered! Our products carry the high certification IFT Rosenheim! Contact us to learn more and in the meantime, wherever you go, be curious and ask questions before you buy!