Combat energy costs
Double-bubble insulation between the frame and wall poly keeps the heat inside the greenhouse.
Photo courtesy of John Bartok

Combat energy costs

Departments - Tech Solutions

Implement these conservation measures to combat the soaring price of heating the greenhouse.

August 22, 2022

Fossil fuel prices have risen as much as 100% over the past year with daily changes as much as 20%. The first step to compensate for this is to implement energy conservation measures. The following tips are four low-cost measures with a short payback based on data from 20 years of greenhouse energy audits.

Service the heating system. With fuel prices more than doubling over the past two years, maintaining the heating system at peak efficiency is important. The increase in efficiency from a service call usually has a payback of only a couple of months.

I have observed efficiency in some older systems as low as 60%. For furnaces and boilers that test less than 75% efficiency, consider replacement. New oil-fired equipment can have as high as 85% efficiency, and gas equipment, especially condensing units, will operate at more than 90% efficiency. There is also a significant reduction in pollution with higher efficiency. The Natural Resource Conservation Service (NRCS) in most states has incentive payments to help cover the cost of upgrading.

Energy screens. A single screen can save 30% or more in fuel use. Although initially expensive to install, payback is usually four years or less depending on your location and when the greenhouse is heated.

Screen systems do need to be maintained. Regular maintenance is needed to keep proper tension in the cable system. Pulleys and gear motors should be lubricated once or twice a year. Screen materials tend to wear at the rub points or where they are supported by hooks. The NRCS EQIP program in some states has been funding replacement of material that exceeds its normal useful life of eight to 10 years.

Be sure to check your screen seals. Gaps at the truss or around the perimeter allow heat to escape due to the chimney effect. I have been in greenhouses where it was warmer above the screen than below showing that heat was escaping through holes, or gaps around the edges. Closed weave energy screens need to be installed to provide a tight seal all the way around the edges. The most common method is to install a narrow ledge along the sidewall made from polycarbonate sheets or fire-resistant screen material. The side edges of the screen slide along the top of this ledge. The back edge is attached to the truss and the leading edge is attached to rigid tubing that seals the screen against the truss. Adhesive backed foam can be used to seal gaps between the glazing and the ledge.

Add insulation. Glazing has high heat transmission properties that let the expensive heat inside the greenhouse escape. Adding insulation reduces the heat loss. Double wall poly or polycarbonate have an R-value of about 1.67. Adding an inch of foil-faced foam board with an R-value of 6.0 will save about 78% of the heat escaping through the wall area. Installing it to bench height along the inside perimeter walls will have a short payback without reducing light on the plants.

For hoophouses, a low-cost material is aluminum foil faced double bubble insulation. I found a 40° F decrease in greenhouse knee wall temperature by adding this behind heat pipes. The insulation can be attached to a kneewall, clipped to the frame or woven between the greenhouse frame and the plastic. If you have rollup sides, the material can be installed for the winter and then removed in the spring when ventilation is needed. It should last several years.

Lubricate fan shutters frequently so that they close tight. A partially open louver may allow several air changes per hour. For example, a 48” fan louver that fails to close properly leaving 1” gaps, allows 23,000 Btu/hr of heat to escape costing $0.92 if you are burning $4 fuel oil. Shut off some fans during the winter and cover openings with insulation or plastic to reduce infiltration of cold air.

The high price of energy has shortened the payback for conservation measures. By installing these measures, you are helping the country meet its environmental goals.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com