A currently unresolved problem in building design in the paradox between increasing demand for good thermal insulation, and the requirement for ample levels of ventilation, to maintain a healthy indoor environment. A possible solution to this problem is a supply air 'ventilated' window. An experimental set-up has been designed to test the performance of the window under various conditions. The behaviour of the window is shown, and the factors affecting it's performance discussed. The transition between laminar and turbulent airflow has a marked affect on the window properties, and a new relationship involving Grashof and Reynolds number is shown to determine the conditions at which this occurs.