Double edged sword. Heat blocking windows are nice for hot summer days. But they also block the heat on cold days when that heat is beneficial. You will actually notice it in a house where the windows were replaced. The heating system has to run a lot more. The house does not heat up in the daytime just from the heat of the sun. What would great if they could design a window that can switch heat blocking characteristics on and off depending on need.
As one FReeper put it: Turn the window around!..................