Don,
How have you taken a 1.8 centre pane dgu and turned it in to a 0.8 centre pane dgu?
An no, windows aren’t the most energy efficient way to create an external part to your home. A highly insulated wall would keep more heat in in the winter, more heat out in the summer, but would be a little dark.
There aren’t many buildings that don’t have some form of glazing in them, so i’d hardly say that I was trying to say windows/glazing are most energy efficient simply because i’m in the industry.
On a side note, draughts take a hell of a lot of heat out of houses. If you have trickle vents on the windows (the small vents above the glass) then these usually lose a lot of heat. Sealing these up can often help keep heat in