The Energy Saving Trust (EST) estimates that replacing your single-glazed windows with even B-rated double glazing could result in a saving of £170 and 680kgCO2/year.
If the window frames of your single glazing are in good or reasonable condition, you may like to consider having the glass replaced with double-glazed units, as well as having the window over-hauled – this will cost up to £400.00 per m2.
Replacing the whole window is likely to result in a more air-tight and better thermal result, which with the right care could last for over 100 years (see Materials) – however this will cost £500.00 per m2, or up to £1000.00 per m2 if you also need to replace your sash boxes.
If your house is currently uninsulated, and given the costs of replacement glazing, it is worth considering that 20% of the heat in your home is thought to be lost through the windows and doors, compared with 25% through the roof and 35% through the walls (take a look at our energy calculator to find out more). However, replacing old windows is an important aspect of ongoing house maintenance, and if you do it will undoubtedly make your home less draughty and more comfortable, which in itself is worth several degrees of heat.