Converting Single windows to Double Pane?

I've had good luck with inserting 1 1/2" extruded foam board inside the window cavities. The frame will still ice but the foam board cuts down a lot of the heat transfer. I now have residential, Low-E sliders on two windows and a static Low-E window in one location. I'm real happy about how they perform. Still, at -0, I would insert the foam board at night, it just helps so much.