Wisdom on Wind
How you orient your barn may have little impact on cooling effect
For building a naturally ventilated dairy barn in Ontario, the recommendation has always been to orient it north-south, where the site permits. That lets prevailing westerly winds blow in through the open side walls and across the barn 's shortest dimension. Recently, however, some barn builders have been questioning that conventional wisdom.
Prompting the questions was a study done a year ago by two Kansas State University researchers. They reported on heat stress in four-row free-stall barns in Tulare, Calif. Three barns were oriented north-south, with side walls open to the east and west. Three were oriented east-west. Temperature and humidity were measured inside and outside all six barns on three hot days in August. Respiration rates of 50 cows per barn, 25 on each side of the drive-through, were taken at 6 a.m. and 4 p.m.
This study showed north-south oriented barns were 0.24 degrees Celsius cooler and 0.44 per cent more humid than barns oriented east-west. The temperature-humidity index (THI), a widely accepted heat-stress measure, was 0.26 units lower in the north-south barns. Despite the lower "theoretical" heat stress in the north-south barns, respiration rates were 56.4 breaths per minute in the morning and 77.4 in the afternoon versus 52.2 and 60.5 in the east-west barns. More breaths per minute suggest actual heat stress was greater in the north-south barns. The authors wrote that "other factors, including solar radiation, air flow and animal stress may have contributed to the differences in respiration rates.
Ontario builders have pointed out that north-south oriented barns, open to the east and west, allow greater penetration of the morning and evening sun, which is low in the sky. The noon sun, which is high in the sky, doesn't penetrate, regardless of orientation, so east-west barns provide more shade. Some people have also suggested that on hot days, winds were more likely to be from the south than west as well.
More recently, the California study's authors were asked about preferred barn orientation in New York. Their reply used the morning and evening sun argument, and quoted only the respiration information. They stated that "when possible, freestall barns should be oriented east- west to minimize heat stress. " This is surprising, since their report showed north-south to be cooler, and suggested unmeasured factors may have had a role. Solar radiation should have made north-south oriented barns hotter, but they were cooler. Greater air flow from west winds should have made north-south oriented barns less humid, but they were more humid. This factor goes hand in hand with lower temperatures since humidity is relative and cooler air has less moisture-holding capacity.
When a very small research study disagrees with what common sense predicts, the results are best viewed as inconclusive. Making firm recommendations for New York based on inconclusive results from six barns in California on three hot days is not good science. Unfortunately, this is the only information available.
Though a final answer on barn orientation seems to elude us for now, we can easily determine the direction of prevailing winds on hot summer days. Cows are described as exposed to moderate heat stress when the THI is above 79. We reviewed hourly weather records for June through September, 1991 to 2000 for stations in London and Ottawa, and 1994 to 2000 for Mount Forest. Moderate heat stress conditions existed at these weather stations for an average of 35.3 hours per summer in London, 28.1 hours in Ottawa and 16.3 hours in Mount Forest. Average heat stress hours per month were: June, 8.8; July, 12; August, 5.1; September, 0.5. At the weather stations, there were no cows, contributing heat and moisture, or barns contributing shade, so the actual occurrence of heat stress in individual barns would vary.
We matched direction and wind data with the hours for which the THI indicated moderate or greater heat stress. As shown in Graph 1, the predominant wind directions in hot weather are from the west and south-west. It can be argued that wind direction is most important to ventilation when winds are lower speed. Wind direction with wind speeds less than 10 kilometres per hour are illustrated in Graph 2. Although west and south-west still dominate, winds from the south in Ottawa and from the north in London and Mount Forest occur quite frequently during these highest risk heat-stress periods.
At first glance, this data would appear to support the longstanding Ontario recommendation to orient barns north-south to catch the prevailing west wind. But natural ventilation involves two open sides, and perpendicular winds from either side result in good air flow. It follows that the sum of west plus east winds versus the sum of north plus south winds determines which barn orientation is favoured.
Graphs 3 to 5 suggest the most favourable barn orientation in all locations would appear to be south-east to north-west. This orientation benefits from the very common southwest wind, and picks up the dominant west wind and south winds on the diagonal. Since the sun sets slightly north of west in midsummer, afternoon shade in this barn would also be adequate. While this would be diagonal to most roadways, we know of no law, natural or otherwise, that requires buildings to be square to the property lines.
We can make several points about natural ventilation using these results:
Graph 3 London Area. North-south barns catch more wind during the 35 hours of moderate heat stress when the winds are normal. But they are less favourable than east-west barns during the six hours when winds are at their lowest speed.
Graph 4 Ottawa area. East-west orientation is equal to north-south under windy conditions and is preferred during low wind speed, making east-west the favourite.
Graph 5. Mount Forest. Heat stress is less frequent, but when it does occur, both strong winds and light winds favour barns oriented north-south.
California study conducted by J.F. Smith, MJ. Brouk, and J.P. Hamer a. Dairy Sc. Vol. 84, Suppl. 1 pg. 75).
This article first appeared in the July 2002 Ruminations column of the Ontario Milk Producer.
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