Q: How are you calculating the R value? are you using the center of glass U value only? should you consider the frame?
A: The R-values I measured for the window inserts are taken in the center of the glass or plastic. As you can see from the infrared photographs in Chapter 3 of Zero Carbon Home, the frames are all considerably warmer than the glass or plastic sheet. The frames also have far smaller area than the glass or plastic sheet. Hence I think that the center of the glass measurement is a good approximation of the overall performance of the window insert.