Green Builder, June 2014, by Suchi Rudra. (Scroll or jump to page 49.)
http://content.yudu.com/A2vekc/GreenBuilderJune2014/resources/index.htm

"When it comes to energy efficiency, windows are problematic; they're essentially holes in an otherwise insulated wall. Glass makes up about 15 percent of the wall space in an average home, and inefficient windows can cost over $700 a year in wasted heating and cooling costs. This loss accounts for approximately 2 percent of all energy consumption in the U.S. Upgrading window products, whether you retrofit or replace, means upgrading building performance and cutting back on hundreds of dollars in energy bills."