This paper aims to characterize the thermal performance of a window system that consists in doubling an existing window, converting it into a ventilated double window. The air coming from the outside circulates upwards through the channel between windows and enters the building through a vent on the top of the window's case. A series of experimental measurements was conducted in a test cell exposed to real outdoor weather conditions located in a mountain region at Centre of Portugal, during heating season in order to determine how this window system can act as a heat exchanger. It was found that such window system act as an efficient heat exchanger using transmission heat losses and solar radiation to preheat ventilation air, thus reducing the building's operational energy costs. An average of about 19 m(3)/h of air flow rate was found with an air temperature increment within the air gap of about 6 degrees C, during night-time, for an indoor/outdoor temperature difference of about 16 degrees C. Air temperature increment reached up to 12 degrees C using a plastic shutter. With solar radiation, the average of that increment was about 10 degrees C. This is a simple and cheap building technology which can be implemented both in new and existing buildings. (C) 2010 Elsevier B.V. All rights reserved.