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Energy Star & Windows 
It's counter productive to have a passive solar home with windows that block up to 83% of the sun!
Sometimes Sun Plans recommends that south windows in passive solar homes NOT meet Energy Star guidelines. We hope this changes soon as we are not one of the only energy professionals that understand the issues. When Sun Plans prepares the Custom Specs that come with Construction Prints or CAD Files, the details of the particular climate are reviewed prior to making recommendations. And for the same location, our south glass recommendations could still vary based on the particular of each house plan. Seldom are the energy-related details identical from house to house. Especially in northern latitudes, but elsewhere as well, the current solar heat gain coefficients may be too low in Energy Star windows for acceptable passive solar standards, even when considered in conjunction with additional cooling benefits of glass that blocks the sun.
Alex Wilson of BuildingGreen could not have summed up our concerns about south glass better in his article:
New Tax Credit Limits Window Choices
What's Happening from Environmental Building News , February 1, 2010
"The SHGC requirement fails to recognize the benefits of windows for passive solar heating."
Sun Plans says "Buyer Beware" when windows have the word "solar" in their ads. In most cases, that indicates that the glass does a great job of blocking OUT the solar energy - not letting it in! That is great for unprotected east and west windows to reduce early morning and later afternoon sun in summer, and it's OK for north windows too, but please, not on the south side of the home!
Window manufacturers' glass specs list the SHGC (solar heat gain coefficient). It is expressed in a number between 0.0 and 1.0 with 1.0 letting in 100% of the sun, which of is not even possible with clear, single pane glass (which has a SHGC of 0.86). Typical ranges for the major glass manufacturers (there are about four, and most window manufacturers' use one of those types of glass) seem to range between 0.17 and 0.78 for double pane or insulated glass. For south glass as a rule, in all but the extremely hot regions, begin looking for SHGC numbers above 0.45 for south glass. For double pane, the coating should usually be on the #3 surface, and for triple pane, the coating should be on #5 surface.
The Sun Plans Custom Specs cover this more with each order, but if you decide to start looking for windows before you have your plans, ask your window rep. about the options. Each major window manufacturer should have at least one window line available with high solar heat gain glass.  Visit our favorite window manufacturer if you are serious about windows with appropriate south glass.