What is an R-5 Window?
R-5 windows are highly insulating windows with a whole-window R-value of 5 (or a U-factor of around 0.2) and are the top tier of energy-efficient windows for cold and mixed climates available today.

What is an R-value and U-factor?
An R-value indicates an insulation's resistance to heat flow. So, the higher the R-value, the greater the insulating effectiveness. U-factor is the inverse of R-value. So, the lower the U-factor, the better the thermal performance of the insulation..

Research indicates that R-5 windows can reduce the average heat loss through windows by as much as 40 percent.

In accordance with October 27, 2009 draft specification requirements provided by the U.S. DOE, to be considered for the R-5 Volume Purchase Program, all units must achieve a maximum 0.22 U-value for operable units and 0.20 U-factor for fixed, which will likely mandate triple-glazed insulating glass.  Additionally, they must be certified to ASTM E2190 for insulating glass durability, NFRC 100/200 for U-factor/Solar Heat Gain and AAMA/WDMA/CSA 101/IS2/A440-05 for air, water and structural performance.  All windows must be offered at a fixed price and provide a 20-year warranty on the glass.

The U.S. Department of Energy (DOE) Building Technologies Program launched the R-5 Windows Volume Purchase Program that gives manufacturers an opportunity to be listed as preferred vendors.