A u-value is the rate of transfer of heat through a structure, divided by the difference in temperature across that very structure. The u-value is a number that gives you an insight into the given product’s thermal transmittance. The units of measurements are W/m2K and the better insulated the window is the lower the u-value will be. The double or triple glazed glass unit will have u-value and the glass unit fitted into a window frame will also have an overall u-value. You should be more interested in the overall u-value as no glass unit will ever be installed without some sort of frame.