What to specifically look for in windows based off of geographical areas and climate zones:

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Northern - Look for a better U-value and insulating value of the window. Triple pane may be a worthwhile consideration to draw the U-value down. 

North-Central - Depending on your situation, a good quality double pane glass with a respectable U-value is probably your best investment. Triple pane tends to be an overkill for this geographical climate and adds tint, darkening the view. A solar heat gain coefficient rating of between a .25 and .35 would be most effective in this climate. 

 

South-Central - In this climate, nothing more than double pane would be necessary. A lower solar heat gain rating will begin to be important here to allow less heat in the windows and maintain cold air in the house.  

Southern - The solar heat gain rating is going to be essential for this region. 

For all geographic locationsAir tightness and performance grade are very important. You may want to use different glass packs for certain rooms or sides of your house.