Air infiltration adds significant cost to greenhouse heating. Depending on the temperature difference between inside and outside and wind speed and direction, warm greenhouse air can be replaced by cold outside air. Except for large gutter-connected greenhouses, air exchange is typically 10 to 20 percent of the total heat needs of the greenhouse. For a 30-foot by 100-foot gothic style hoophouse heated to 60°F on a 0°F night, the infiltration heat loss amounts to 15 percent (or about 36,000 Btu/hr based on one volume change of air per hour).
Air leaks are typically found around doors, shutters, vents, baseboards, broken glass or tears in the plastic glazing. In the hundreds of greenhouses I have audited in the past few years, I have found very few that didn’t have some large leaks that needed attention. Closing these leaks before the winter heating season can more than pay for the materials and labor needed.
Install insulated doors
Doors get a lot of use and abuse. Personnel doors, especially homemade ones, tend to sag and develop large cracks between the frame and the door.
Compared to an average single-glazed, poor-fitting door with a 3/32 inch gap around the perimeter, an insulated steel door with a tight seal, thermal break and insulated sill can save an estimated 60 gallons of fuel oil, 90 gallons of propane or 80 ccf of natural gas over the heating season.
Cost of the insulated door installed is about $350. Payback at today’s fuel price is one to two years.
Repair or cover fan and intake shutters
Shutters receive a lot of use. Blades frequently get bent or broken. Lubrication of hinge points is important. A partially open shutter may allow several air changes per hour. For example, a 48-inch fan shutter that fails to close properly — leaving 1-inch gaps — allows 23,000 Btu/hr of heat to escape. Fans and shutters not needed during the winter should be covered with insulation board or film plastic to reduce infiltration of air. Some growers have installed sliding or hinged shutter covers that can be easily opened when ventilation is needed.
Adjust roof and sidewall vents before winter
Wind gusts and wear on the rack and pinion drive may get vents out of adjustment. A gap between the vent and frame acts like a chimney that pulls heated air out of the greenhouse. A visual check and adjustment of the pinion to close should be done. Adding an endwall seal for roll-up sidewalls reduces infiltration.
Fill in holes under the baseboard
Rainwater running off the plastic in hoophouses frequently washes soil away from the base of the greenhouse leaving large gaps. Besides allowing access for rodents, it allows the entrance of cold air under the benches. Installing insulation board or double wall polycarbonate sheet about 12 inches deep will usually eliminate this problem and also reduce perimeter heat loss. Research has shown that perimeter soil temperature will be increased 20°F or more.
Seal the gap where the flue pipe greenhouse wall
I frequently see large gaps in the wall collar where the flue pipe exits the greenhouse. This can allow considerable infiltration air to enter. This gap should be sealed with a fiberglass stove gasket or high temperature caulking. The use of separated combustion unit heaters or condensing boiler will eliminate the need for makeup air that usually comes in through greenhouse gaps.
Reducing infiltration is a low cost method of reducing heating costs. Usually a few dollars for materials and minimal labor is all that is needed.