I have been asked by many customers if a benefit exists to having vinyl window frames filled with foam. There are some companies that charge a substantial premium for this feature. A few years ago, I came across an article that dealt with this issue. The article has been repeated in many forum postings such as this post at https://www.vinyl-replacement-windows.com/forum/viewtopic.php?f=2&t=714. The testing and evaluation has been performed by Enermodal Engineering of Waterloo, Ontario. The result is that the net effect on the overall window is an increase of 0.1 improvement in total R-Value of the window. In comparison, selecting triple glazing over double glazing can improve the R-Value of the window by about 0.7; 7 times greater than the foam-filled frame feature. The foam filling is very visible and impressive in crosssection at trade shows, but performance should be the primary goal. As I tell my customers, common sense should also prevail. The largest area of the window is the glass, not the frame. Choosing your glass wisely will yield a better return on your investment than choosing the frame.