This paper discusses energy efficiency criteria for selecting windows and the limitations imposed by the necessity of a factory rating number as opposed to using specific design criteria for each house window.

The Canadian annual energy rating (ER) system for residential windows that was derived for use in a cold climate where passive solar gains are significant is described. Corrections are noted to account for specific house differences and for cooling situations.

The conclusion is that a rating system must account accurately for solar gain since passive solar is important in this climate. Adoption of the rating system has led to improvements in window design and fabrication beyond simply incorporating low-e glass and argon in sealed double units. Examples are given to illustrate substantially higher ER numbers obtained through improvements that impact on both overall U-factor and solar heat gain coefficient (SHGC).

Units: SI