When buying windows look for the Energy Star label, which refers to the overall performance of a window based on heat loss and heat gain through all parts of the window.
An ER rating of zero means a window gains as much heat from the sun as it loses. Windows are expected to lose some heat, so an ER above zero is exceptional, as is anything rated between -11 and 0. A poor performer ranges between -35 and -50. ?
An Energy Star label is optional but highly recommended by experts because it means the window has proven its energy efficiency to a host of organizations across North America, including Natural Resources Canada's Office of Energy Efficiency.
Energy Star windows can cost as much as 20 percent more than regular windows, but they can yield nine to 18 percent return on your heating costs.
