Heating Performance of Photochromic and NIR-Shielding Window Retrofits: An Experimental Evaluation in a Continental Climate

Windows play a critical role in driving the heating and cooling energy loads of buildings, especially in older buildings with outdated windows. While replacing existing windows with high-performance products can significantly boost building energy efficiency, the associated costs can be prohibitive. Consequently, retrofit technologies that utilize existing glass panes and frames offer a compelling solution. This paper investigates the energy performance of two innovative window retrofit technologies at the BeTOP outdoor test cell facility in Toronto, Ontario, a continental location with warm summers and cold winters. The evaluated technologies include a near-infrared-absorbing liquid-applied nano-coating and a peal-and-stick photochromic window film with dynamic absorptance to visible radiation. These technologies are applied directly to the interior glazing surface, allowing for swift installation with minimal occupancy interruption. Since the studied retrofit technologies primarily target cooling energy savings, they may contribute to greater heating energy consumption in a heating-dominated climate. Hence, the study focused on evaluating the in-situ winter performance of the retrofit technologies against a clear reference under identical conditions. While the considered retrofit technologies did not significantly amplify the heating consumption, a noticeable increase in the interior glazing surface temperature was driven by the retrofit coatings' solar absorbing property. Hence, the studied retrofit technologies could potentially show enhancements in thermal comfort aspects in the winter without significant heating penalties. This suggests potential for effective year-round energy savings. Further work is planned to evaluate the cooling and annual performances as well as the daylight performances for different sky conditions.