You don't have good quality curtains in your house.
Curtains are the unsung heroes of modern, warm, energy-efficient homes. By “good” we mean at least two layers, full length to the floor, and/or closed at the top (by a pelmet, the ceiling, or tight-fitting track). The thickness of the material is not as important as the number of layers, as it’s the air that’s trapped between the layers that gives the curtains the insulation ability.
Good curtains trap cold air behind them meaning less likelihood of warm air escaping, hitting the cold glass, and dropping back into the room as a cold draught (a "reverse chimney" effect).
For those areas where full-length curtains are inconvenient or just not possible (above heaters or kitchen benches for instance), we recommend Roman or Pleated (sometimes called cellular) blinds. These should still be tight fitting on the side and against the sill and have at least two layers of blind material. Venetian blinds or shutters are ineffective at keeping in warmth (and therefore don’t pass HomeFit®), but have their uses in managing overheating (in summer), glare and privacy.
For more information, see the Eco Design Advisor Curtains factsheet.