Naturally ventilated buildings depend upon the natural forces of wind and temperature difference (chimney effect) to move air. In naturally ventilated barns, the ventilation rate depends on the size of the wall opening and the speed and direction of the wind. Temperature within the barn can be varied by using thermostats to automatically adjust the size of openings in the sidewalls. The advantages and disadvantages of natural ventilation is outlined. For buildings less than 24m (79ft) long, it may be more economical to use mechanical instead of natural ventilation. Vertical panels with added windbreaks tend to minimize temperature fluctuations and provide good air distribution and mixing patterns within the animal room. The use of chimneys presents many advantages over the continuous ridge opening. The chimneys should be fully open during warm weather and closed when average daily temperatures are below the 8 to 10C range. An adjustable baffle can be installed in the chimney to reduce the opening area. End wall openings improve air circulation at the end of the barn where stagnant air tends to develop.