Low-E glass coatings to limit heat loss in homes has become very popular to lower U factors in energy efficient windows to meet Energy Star guidelines. Unfortunately this coating also limits solar gain and we need solar heat gain for our passive solar design. But we also wanted the most energy efficient windows and doors. It was a difficult combination to put together.
This article explains U factor and SHGC (Solar Heat Gain Coefficient) for typical window glass types. The highest SHGC also has the highest U factors which is the heat loss calculation for the window.
More about high performance windows and doors
Cold Climate Design Guide
Where can you find high SHGC and Low U factor for windows? There are definitely glass options available.
Milgard has a plant in Denver but their standard glazing options don’t have very impressive numbers. They use Cardinal Glass and will custom make windows with more efficient glass but they are more expensive. I priced the standard windows and doors for comparison though and the cost was similar to the more energy efficient Alpen windows.
Alpen (Serious 2008-2012 then Alpen reacquired) is among the most energy efficient window made in the USA. They are manufactured in Colorado.
Alpen Windows use the Southwall Heat Mirror films for interior layers.
Alpen only offers sliding glass doors but they will put their glass in ThermaTru Doors. I had issues with the Therma Tru Doors I purchased that were factory assembly and paint issues and I was told Alpen has different door choices now.
Window Installation Guidelines