[idiotsecant]

At your latitude and climate zone those windows are almost certainly either costing your more than the equivalent wall would have or are providing a benefit on the order of less than $20 per year. I've done exhaustive studies at my latitude and climate zone (which is quote similar to yours) with a wide variety of parameters and the numbers just don't pencil out in a significant way. My analysis was exhaustive and included actual weather data and solar irradiance data. In slightly warmer climates this is less the case, though.

[swader999]

Maybe, but the thick walls and styrofoam insulation give it a high r value. The furnace doesn't run much when the sun is out.

And the view is priceless :)