A currently unresolved problem in building design is the paradox between increasing demandfor good thermal insulation, and the requirement for ample levels of ventilation, to maintain ahealthy indoor environment. A possible solution to this problem is a supply air ventilatedwindow. This utilises an airflow between panes to pre-heat ventilation air to the building, andto reduce thermal convection losses thus reducing the window U-Value. At the base of thewindow is a vent to the external environment, allowing air inflow. This relatively cold air isheated by convection/conduction from the warmer inner pane and will subsequently rise, orbe drawn up, entering the room through venting at the top of the glazing.This paper describes an experiment to determine the thermal performance of two Supply AirVentilated windows of different aspect ratios. The experiments are carried out in advancedtest cell facilities, and are intended to measure changes in thermal performance due to solarirradiation and aspect ratio. The windows show a significantly lower U-Value thanconventional windows, and also achieve significant ventilation preheat especially in thepresence of high solar irradiation. Aspect ratio appears to have a stronger influence on solarefficiency than it does on the window U-Value.