In your average house the windows and doors will have U Values which are 2 to 3 times worse than an uninsulated wall and single glazed windows have a U Value, sometimes as bad as 6.0W/M2/K. If we then go to the trouble of insulating the walls to a U Value of 0.2W/M2/K and don’t address the windows and doors then the heat loss through your typical double glazing will be 10 to 15 times greater per M2 (2.0 to 3.0W/M2/K). The very best window and door U Values that are available today go as low as 0.5W/M2/K but the cost of purchasing them is prohibitively expensive and therefore we set ourselves a standard of only fitting windows and doors which had a U Value less than 1.0W/M2/K but typically we achieved just less than 0.8W/M2/K. Although these products are still more expensive than the average products, they are much more affordable than the very best ones. It is also worth noting at this point that even with U Values as low as 0.8W/M2/K the heat loss would still be nearly 4 times greater than the heat loss through a well insulated wall.