Testing the energy performance of factory-built windows, doors, and skylights is mandatory in Ontario and in most other provinces of Canada.  Windows, entry doors, and full glass sliding doors in Ontario must meet a minimum efficiency standard.  The values that are most often used to represent the energy performance of windows, doors, and sliding doors are the following:

U-value : Indicates the rate of heat transfer from warm to cold areas in watts per square meter Kelvin (W/m2*K) or British thermal units per hour per square foot Fahrenheit (Btu/h*sq. ft.*°F).  The lower the value, the slower the rate of heat transfer.

R-value : This value is the opposite of the U-factor and is not part of the energy performance standards.  It indicates the resistance to heat transfer in square feet per hour in Fahrenheit degrees of temperature per British thermal unit (sq. ft*h*°F/Btu). The higher the number, the higher the resistance to heat transfer.

Solar heat gain coefficient (SHGC) : A ratio indicating the amount of the sun’s heat that can pass through the product (solar gain). The higher the number, the greater the solar gain.

Energy rating (ER):  For windows and doors only, this unitless number reflects the balance between heat transfer (U-factor or U-value), solar gain and air leakage. Higher numbers indicate a slower heat transfer without significantly reducing the amount of solar gain.

Visible transmittance (VT) : A ratio that indicates the amount of visible light that can pass through the product. The higher the number, the more visible light that can pass through.