The increasing demand for thermally improved windows has created a need for a standardized procedure to evaluate and compare their energy performance.

A recent Canadian study has proposed a new concept for indicating the relative energy efficiency of windows, which can be used to indicate the window’s effect on either heating or cooling loads. The recommended calculation combines three energy-related characteristics (U-value, shading coefficient, and air leakage) into a single number. Average climatic conditions have been determined to allow weighting of the three characteristics to reflect their relative importance in heatingload- dominated climates. Work continues to extend the concepto cooling-load- dominated climates.

A voluntary window energy labeling program based on this procedure is being considered by the Canadian government