What's the best way to select windows for heating efficiency? I have an older house in a northern climate
The true answer is that it takes more energy to remove heat from a space than it does to add heat to a space. For every watt of heat you want to remove, you have to expend an additional 2 to 3 watts of heat just to carry that first watt away from the area. So, insulation will always be biased towards preventing heat ingress.