The insulation value is actually closer to double on a glass block window then on a conventional double pane window.

The glass block window provides an insulation value similar to a thermal pane window - but offers a good amount of privacy and different patterns.

If you're worried about windows losing heat or cold from your house, why not consider enhancing the window's energy-efficiency with high-quality window shades. Check out some of these options:

http://www.halcyonshades.com/st-louis/window-shade-benefits/energy-efficiency/