39 pounds per square foot – That’s the weight of snow I had in my yard in Ashford after the January 2011 snow and rain storms. The Connecticut Building Code calls for a design load of 30 pounds per square foot (psf) and many greenhouses are designed for significantly less as the code assumes that the greenhouse will be heated to a minimum 50ºF during a storm.
Winter storms and cold temperatures have been a challenge for greenhouse businesses in recent years. Storm damage to greenhouses have included racking of the frame, bending of the hoops, broken glass or torn plastic and uplifted foundation posts.
In some greenhouses, cold air infiltration adds considerable to the cost of heating. Cracks around doors, vents and shutters that don't close tight, broken glass and tears in the plastic are typical examples. For example, a 4 foot square shutter that fails to close fully and leaves 1/2 inch gaps will allow about 12,000 cubic feet of cold air to enter each hour. To heat this amount of air over a 24 hour period to 60Â°F when the outside temperature is 0Â°F require almost 4 gallons of fuel oil. Most infiltration leaks can be corrected with minimal cost.
One factor that influences heat loss from a greenhouse is the amount of glazed area. In a 30Â° wide hoophouse, the glazed area from the ground to bench height is about 15% of the total surface area. Insulating this area with an inch or two of polyurethane or polystyrene can reduce total heat loss over 10%. Use a closed cell insulation board and not beadboard as this absorbs moisture reducing its insulating value.
Mechanical thermostats tend to loose accuracy over time. You can easily check the accuracy of a thermostat. Start by checking the accuracy of a good thermometer by inserting it into an ice bath. The reading should be 32Â°F. After allowing it to come back up to room temperature, place it next to the thermostat you want to check.. Slowly move the dial until the heater turns on. The reading should be the same as the thermometer reading. If not, mark the thermostat accordingly. Next time the heating system is serviced, have the service person recalibrate it.
Bare heating system pipes waste a considerable amount of fuel each year in areas such as boiler rooms and headhouses where heat is not needed. This heat loss continues every day the system is operating. Adding 1" thick fiberglass or foam insulation to a ¾" pipe will save about $2.25/linear foot and on a 2" pipe about $5/linear foot over the heating season in northern climates. The payback usually takes less than two years. Installation is simple and can be done by unskilled workers in slack time.
Occasionally growers want to use an unvented heater as a back up heat source. The following was taken from an article that John Bartok, Agricultural Engineer wrote on using unvented heaters. Tina Smith, UMass
We came across this short video (15 minutes long) that might interest some growers. A Minnesota farmer, Steven Schwen explains how his thermal-storage greenhouse works. He uses diagrams and talks a little about costs. This project was funded in part by SARE.
Paul Lopes and Tina Smith University of Massachusetts
Nature seems to be getting more violent in recent years with frequent earthquakes, increased numbers of hurricanes and record breaking snowstorms. Insurance damage claims have increased considerably. The International Building Code has revised upward its wind and snow loading requirements for some areas of the U.S. Each year there are reports of greenhouses that have been damaged by weather and natural events. Greenhouse design is different than conventional farm buildings in that the structural profile has to be small to allow maximum light to reach the plants.
Snow that accumulates on a greenhouse can put significant weight on the structural members. Snow can be light and fluffly with a water equivalent of 12 inches of snow equal to 1 inch of rain. It can also be wet and heavy with 3 inches of snow equal to 1 inch of rain. Snow having a 1 inch of rain water equivalent will load a greenhouse with 5.2 psf. This amounts to 6.5 tons on a 25' X 96' greenhouse.