Insulation? In a window?
Walls and roofs have an R-value. It simply measures their ability to hold in heat during the heating season and keep it out during the cooling season. In windows, this insulation value is usually referred to as the U-factor. (It’s simply the reciprocal of the R-value.)
The lower the U-factor, the higher the energy efficiency of your new windows.
Hurd’s Ultra-R window is the best-insulated window on the planet. It beats all other windows and most walls for energy efficiency.
Click here for more information on Ultra-R windows.