An algorithm was developed to predict the effectiveness of openings in naturally, ventilated agricultural buildings. Validation was conducted in a wind tunnel using scale models under isothermal conditions. Wind angle of incidence, wind velocity, the ratio of height to width of openings and roof slope were varied. Wind angle of incidence and wind velocity had the greatest influence on opening effectiveness. Changes in the ratio of height to width of openings and roof slope had measurable but smaller effects.413767771