Now that we have had the first real cold spell of the winter how well have your new home windows performed?

Out of 14 windows at least 6 bleed sufficient cold air into the rooms that I have had to resort to putting the curtain bottoms up onto the window sill to prevent flow of cold air.

Please note the (6) windows are not draughty but do seem to allow cold air to seep into the rooms.

As space heating in homes is one of the largest contributors to CO2 emissions am I being unrealistic in my expectations of double glazed windows thermal performance.