Tips on Choosing New Naturally Ventilated Greenhouse Designs By Dr. Ted H. Short Dept. of Food, Agriculture and Environmental Sciences OSU/OARDC, Wooster, OH Ohio State engineers have changed the "guess & try" practice of naturally ventilated greenhouse designs to a science! They now use aerodynamic computer models to test and demonstrate some of the newest double poly, multispan greenhouses. The model has been verified on one full scale commercial greenhouses with some very positive results. Naturally ventilated greenhouses rely primarily on air blowing into a windward side opening and out the open roof vents. Wind can also create a vacuum pressure along aerodynamically designed roof vents to accelerate the outward flow of hot air. A secondary, much smaller effect is air buoyancy. Buoyancy helps move hot, humid air up and out of the greenhouse if there are no internal traps. Any barrier to this process such as gutter vents or wide internal framing can completely negate the effect. In all cases, including open roof designs, it is essential that there is at least one very effective windward side inlet with multiple roof outlets. And, air must move from inlet to outlet through the plants to have good ventilation. The advantages of natural ventilation: low energy requirements all side doors can be open in the summer pleasant internal working and shopping environment unused greenhouse can be easily cooled in summer no ventilation restrictions on length of greenhouse air temperature can be maintained very close to outside air very high ventilation rates are possible low temperature gradients across greenhouse are possible Some disadvantages of naturally ventilated greenhouses: can easily be designed improperly must be incorporated with some form of shade system pad cooling cannot be used may not allow for micro insect screening plants near the side vent can be wind damaged is dependent on wind speed and wind direction vents are subject to wind damage low light plants below open vents can be sun scorched Major design considerations: roof vent opening should be 10-15% of floor area and open leeward to wind windward side vent opening should equal one fourth all the roof vent openings side vent should be prevented from going directly out the first top vent the windward side should be highly aerodynamic a 50% shading system is very important for summer growing internal profiles should curve smoothly upward and out the vents (no traps) computer control is highly desirable for climate control and vent protection