Windows are the weakest link in the thermal envelope of a house. Once you’ve insulated your underfloor and ceiling, it’s likely most of your heat loss will now be through the windows.
Double glazing is one option to improve heat retention, but a cheaper and easier thing worth considering is how effective your curtains are. A well-fitted set of curtains can be as effective as double glazing at reducing heat loss – but obviously only when they’re drawn.

Do your curtains meet the three criteria for good performance?

  1. At the top, you want no gap between the curtain and the wall
  2. At the bottom, you want the curtain to reach the floor
    Best explained by the reverse chimney effect.
    If the curtains don’t reach the floor, the cold air next to the window escapes out the bottom as a cold draught. If there’s a gap at the top of the curtains, this air space will be filled with warmer air from above, as warm air sitting at the ceiling will be drawn towards the window.
    Blocking off this circular air current is the best thing you can do to reduce window heat loss, and it’s far more important than the type of material itself.
  3. You want 2 layers of curtains
    Insulation works by trapping layers of air. As still air is a poor conductor of heat, this slows the rate of heat loss from the house. A double-layered curtain works the same way, by trapping a layer of warm air in between the layers.