In equatorial warm humid climates, ventilation has been largely adopted as a major strategy for natural passive cooling. In those climates the use porous elements are common to allow for permanent ventilation as temperature rarely drops below 20°C. Nevertheless, the performance of many building components has not been thoroughly determined, making it difficult to predict buildings performance as ventilation rates, estimated in most simulation codes are often based on apertures typologies from temperate and cold regions. This paper is the result of an experimental assessment carried out at the Universidade Federal de Alagoas, comparing the airflow velocity inside two identical rooms. The rooms were shaped and sized similar to a typical local bedroom (2.80 x 3.50m), having the same window area as inlet and a door as outlet, both placed in the centre of external walls of the test rooms. In one of the rooms the aperture would have a permanently open window, while in the other one there was a window with horizontal slates. Air speed near the windows were measured to obtain the resistance to the airflow produced by horizontal slates (with and without mosquito screens), under different wind speed and directions. Results shows that the resistance to the airflow may vary significantly as a function of wind speed and direction.