Growing organically involves stewardship of natural resources like soil, water and ecosystem health. That stewardship also extends to using energy as wisely as possible. With organic greenhouse production, which is often energy-intensive in northern climates, it makes sense to invest in conservation and efficiency to reduce fuel consumption used for heating. Such improvements will typically pay back in a relatively short time—especially if energy prices rise again. The following tips for reducing greenhouse fuel use come from John Bartok Jr., professor emeritus of greenhouse engineering at the University of Connecticut.
Reduce air leaks
When I visit farms, organic and otherwise, I consistently see “leaky” greenhouses: doors that don’t shut; cracks in end walls you can see through; ventilation louvers that don’t close all the way; gaps in the plastic covering between roll-up sides and end walls. Addressing these issues are some of the lowest-cost steps you can take to save on heating costs. Use door closers or springs to make sure doors shut tight. Apply weather stripping around doors, vents and fan openings. Lubricate ventilation louvers frequently so they close tight. Shut off some of the fans during the winter and cover the openings with insulation or plastic to reduce infiltration of air. And, of course, repair holes in greenhouse coverings.
Insulate the end walls
Greenhouse end walls are not a source of a lot of light, but they can be where a lot of heat is lost. Insulate these areas with poly film or bubble wrap, and consider installing double-wall polycarbonate structured sheets, which will only reduce light transmission a little bit, but they will save heat.
Insulate the foundation
Place 1 or 2-inch polyurethane or polystyrene board to 18 inches belowground, which can increase the soil temperature inside the greenhouse near the sidewall by as much as 10 degrees during the winter. Much of that heat is then released at night back into the air of the greenhouse.
Insulate the knee walls or sidewalls
Use 1 or 2-inch insulation board up to the height of the benches. Applying 2 inches of foam insulation to a 3-foot-high knee wall on a 28-by-100-foot greenhouse will save about 400 gallons of fuel oil per year in a Typical Situation. If Your Greenhouse has Sidewall Heat Pipes, Insulate Behind Them. use Aluminum-faced Building Paper or Insulation Board Behind the Pipes to Radiate Heat Back Into the Growing Area. Leave Some air Space Next to the Wall to Prevent Frost Damage.
Install a thermal blanket
Energy-conserving blankets inside the greenhouse are costly, but can save a lot of fuel—20 to 50 percent in most situations. Of course, not all greenhouse structures are designed to accommodate them. Tight closures should be maintained where the curtains meet sidewalls, framing or gutters. Use a U-shaped trap to prevent heat from escaping overhead. Heat and water lines should be insulated if they are not located below the blanket.
Optimize your heating system efficiency
Before the heating season starts, have a technician check your boiler, burner and backup systems to make sure they are operating at peak efficiency. Have furnaces cleaned and adjusted, and run an efficiency test. A 2 percent increase in efficiency for a 30-by-150-foot greenhouse will save about 200 gallons of fuel oil annually. Clean your heating pipes or other heat-radiating surfaces frequently, and check and repair leaks in valves, steam traps and pipes.
Distribute heat efficiently
Installing a floor or under-bench heat distribution system will allow the greenhouse air temperature to be set 5 to 10 degrees lower in most cases. Some growers connect plastic ventilation tubing to their heater’s plenum and run the tubes on the ground, under the benches or along the base of their tomato plants. Horizontal air flow fans help keep air moving and mixing for a more uniform temperature across the growing area. If heat distribution pipes go through areas where heat is not required, insulate them.
Don’t forget about thermostats
Check them for accuracy every year. Correcting a reading that is 2 degrees too high will save about $100 to $200 annually. Install electronic thermostats or controllers with a 1-degree accuracy. There is potential for yearly savings of 500 gallons of fuel oil in a 30-by-100-foot greenhouse when changing from a mechanical to electronic thermostat or controller. Aspirate your thermostats or sensors, as the differential between on and off can be reduced as much as 6 degrees by doing this.
Manage your crops for energy efficiency
Lower the night temperature, as fuel consumption is reduced by 3 percent for each 1 degree the temperature is lowered. Delay starting the greenhouse by a week or more. Build a germination chamber to start your seedlings rather than heating some or all of the greenhouse. Once the greenhouse is up and running, keep the growing areas as full as possible to reduce your per-plant fuel usage.
Make the best use of your growing space
Movable benches can dramatically increase space utilization compared to multiple walkways between benches. For crops that don’t need high light levels, multilevel racks can make better use of space. Grow a crop of hanging baskets on overhead rails or truss-mounted conveyor system, but try to avoid dripping onto the crops below, which leads to disease.
Optimize your greenhouse site
Install windbreaks on the north and northwest sides of the greenhouse. These can be a double row of conifer trees or plastic snow fence. If you are building a new greenhouse, locate it in a sheltered area to reduce wind-induced heat loss, so long as it does not reduce light.
The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office.