Many people are familiarizing themselves with the Energy Star label on windows these days. U Values, while once reserved for architects, are thrown out there as selling tools now to many home owners. What prompted Suntech to write this article is that Harvey Building Products has the Tribute series of windows, which have extreme U and R values. When I looked at the typical U values for the Tribute window with the High Performance Package, they are not listed as the inverse of the R Value! No, it’s not a type-o. I found this great PDF which explains more about it:
Before you click away, know that R is a complete measure of the insulation of the whole window, while U is typically only measured at the center of the glass. So, when you do things like filling the frame of a window with insulation, you can boost the R Value while keeping the U value the same.