We're all familiar with R-values used a rating of the ability of insulation products to resist the transfer of heat. The lower the number, the better the performance of the window. A microscopic metallic coating applied to the glass that blocks ultraviolet and infrared solar energy but allows visible light to pass. Low-E glass reduces the amount of the sun's heat entering your home during the summer and helps retain warm indoor heat during the winter. Check the manufacturer's specific specs for their Low-E products.
Performance varies, but all Low-E glass performs better than standard insulated glass.
This is an important rating, since loose windows will be major contributors to your heating bills through heat loss due to air infiltration. Casement, hopper, and awning windows tend to be the tightest, and gliders tend to suffer more air infiltration. But compare ratings for various window manufacturers you are looking considering. One such option is argon gas between the panes of an insulated glass unit. Manufacturers also offer various tints on windows, primarily aimed at markets in areas with very high solar heat gain.