Canadian-based Cascadia Windows & Doors has created a free online tool to help analyze the thermal performance of a building envelope using a simple area-weighted average Multi-Assembly R-value calculator. Try the new Multi-Assembly R-Value/U-Value Calculator here.
You're not necessarily going to improve the R-value of the building envelope withnew windows. As Allison Bailles, put it, "a good window is still a poor wall" from an energy-use stand point. And you don't want to make energy promises you can't keep in this business, as some have learned the hard way.
But if you're working with a savvy customer who thinks energy is all-important, you can legitimately show the differences in building performance between one window and another. It'll require area measurements of the walls and the windows, (for more, see "Holes in the Wall: To Improve the Energy Performance of Walls, Look at Total R- Value," JLC 2/14cbtardcfwwbueawrytyd), which doesn't make it the easiest sale. But it makes the conversation about one window or another, not one window installer or the next.