In Arctic and sub-Arctic climates, such as those in Scandinavia, multiple-glazing windows that consist of at least three panes are widely used. Typically, the replacement air for the extracted air, especially in low-cost accommodation with forced extraction, enters the interior space in the form of leakage flow through the window jambs and the walls or through the supply air vents. The temperature for the air entering the room is close to that of the outdoor air, which may cause a sensation of draft. Moreover, the whole supply airflow must be heated up to the indoor temperature by the heating system. In a supply air window, air enters from the outside into the window cavity through holes or vents at the lower edge of the window frame. The air flows upwards in the cavity, while being heated by convection from the surrounding panes. The heated air is then drawn into the room through a vent or an adjustable air valve in the upper edge of the window frame. When compared to the conventional case, the following results can be identified: 1) the supply airflow is pre-heated, which reduces the feeling of draft; 2) less heat flow is required to heat up the supply air, which reduces the energy consumption of the room. Furthermore, the size of the radiators can be reduced. All of these factors contribute to a better indoor climate and savings in the required investments and in energy as well. In this paper, the window is considered to be a heat exchanger in which heat is transferred by radiation and convection between the panes and the supply airflow. Denoting the indoor and outdoor temperature with Ti and To, correspondingly, equations are presented for the determination of the recuperation ratio, e, of the windows. The recuperation ratio can be used for calculating the temperature of the supply air entering the room, Tsa = To + e (Ti - To). The results are compared to measured values for recuperation ratio. The equation for the overall heat transfer coefficient, U, is also presented for the supply air window.