Nowadays there's a vacuum sealed glass product called Pilkington Spacia that's the same thickness as a single pane, but has about the R-value of a triple pane w

Nowadays there's a vacuum sealed glass product called Pilkington Spacia that's the same thickness as a single pane, but has about the R-value of a triple pane window. You could replace the glass with it yourself if you order the correct sizes.