Sealed Units
Glass on its own is not a good insulator. A single pane of glass causes condensation on the inside of the window, and would need to incredibly as a single window pane to have much effect.
The better solution is to have two panes of glass with an air or gas space between them. The optimum air or gas space is around 20 mm. The other benefit of sealed units is acoustic insulation.
The level of heat loss through single glazing can be at least halved with sealed units. In addition, condensation can be reduced with double glazed units. As well as fuel bill savings the saleability of the property will be enhanced.
If the sealed unit is filled with argon gas the heat conductance between the panes is reduced. This will keep more heat in during the winter and keep it out in the summer. This will result in more comfortable living and working conditions all year round. Thermal insulation can be further improved using a Low energy saving glass on one side of the sealed unit.