U-values measure how effective a material is as a heat insulator. The lower the u-value, the more insulating the material is. A standard double glazed unit, which includes a low emissivity coating (Low E) and argon gas in the cavity, will have a u-value of 1.5W/m2k. But by using the latest in nanotechnology, you can reduce the u-value of the glass to as little as 0.32W/m2k.