There are more than 100 million homes and approximately 20 billion square feet of clear-glass residential windows in the U.S. Most of those homes are more than 30 years old, and as a result, the market is growing for replacement windows with energy-efficient insulating glass. High-performance options are now readily available in all regions of the country. Windows with U-factors and solar heat-gain coefficients less than 0.3 are now standard. And the cost of upgrading from conventional double-pane windows to double-pane low-E units with argon gas is less than $1 per square foot. Heating and cooling energy savings of 10...