The US Department of Energy has recently revised its energy efficiency standards for windows. The windows on this house in Laconia, NH, were made by Southwall Technologies.

Windows either lose or save energy. Most lose.

Despite heavily insulated walls and ceilings, 25-35 percent of the energy used in homes and buildings is wasted due to inefficient glass. That’s why the US Department of Energy's Energy Star® program has revised its window energy conservation performance standards. The first phase of the new standards debut in January 2010 and will require windows eligible to obtain the coveted Energy Star designation to provide increased energy efficiency.

Here's what manufacturers need to know regarding how glass impacts the overall efficiency of a window system:

• Single pane glass may keep out the weather but it does little to insulate against heat loss or reflect the sun’s heat that can cause overheating.

• Insulating glass with a heat-reflective coating that reflects ambient heat both inside and outside is appropriate in homes concerned with staying warm in winter, cool in summer.

• Insulating glass with dual heat reflective coatings that simultaneously reflect heat from the sun and ambient heat both inside and outside is even more effective in saving energy and improving comfort. The air space inside the sealed glass enhances insulation and the coating reflects the sun’s heat to prevent over-heating.

Glass available today that will meet the new Energy Star window performance standards include:

• Triple pane glass consisting of three panes of glass and two heat reflective coatings. The good news is that by using a third pane of coated glass triple pane improves insulating glass performance. The bad news is that triple pane glass is 50% heavier than insulating glass, requiring stronger window framing and increasing cost accordingly.

• Insulating glass containing a transparent heat reflective film suspended inside the air space can dramatically increase insulation performance while reflecting unwanted solar heat. Glass with suspended film also provides superior protection against UV radiation that can cause fading and reduces the impact of outdoor noise.

For more information, visit Southwall Technologies.