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If you are going to install replacement windows, it is important to understand the specifications, especially the difference between U-value and R-value. These same meanings apply to wall insulation or any other products as they relate to heat transfer (loss or gain).
U-value is the measurement of heat transfer through a given building material, glass, etc. R-value depicts the resistance a material has to heat transfer. These terms refer to conductive heat transfer - the way the handle of a pan heats up when you hold the pan over a flame. In number values, R-value is the direct inverse of U-value (R-value=1/U-value). If a material has a U-value of .5, it has an R-value of 2. If it has a U-value of .25, it has an R-value of 4.
This is only part of the story concerning windows. Some manufacturers measure the R-value at the center of the glass giving it a high R-value. It can be several times greater than the R-value near the edge of the glass (frame and spacer nearby). To get a true idea of the overall insulation value of a window, the NFRC (National Fenestration Rating Council) has devised standards for testing windows. Use only manufacturers' specifications that state that they were conducted to NFRC standards when comparing windows.