How to: The winter of conserving heat

The boys of summer are gone and the lean winter months will soon be upon us. For part of the country, that means thick piles of snow and gusting blasts of sub-zero temperatures. Keeping your greenhouse warm during the cold season is crucial to maintain top-level production. John W. Worley is a professor and extension engineer at the University of Georgia and he has some tips on keeping your greenhouse warm during the coming winter.

Worley notes that heat loss can be potentially damaging for both crops and your checkbook.

“Assuming you have enough capacity to replace the heat you are losing, the biggest effect of heat loss is increased fuel bills. However if there is insufficient heat available, the temperature inside the building will drop, slowing plant growth and perhaps damaging or even killing the plants,” Worley says.

Most greenhouses are heated with a natural gas or LP gas space heater (basically blowing air over a hot surface and into the building). More efficient systems use hot water radiators, or root-zone heating where the soil under the plants is heated instead of (or in addition to) the air. Depending on the plant, the air temperature can be reduced considerably with this type of heating as long as the roots are kept warm. The hot water for either a root-zone of radiator system is provided from a boiler, usually located outside the structure. If you have more than one greenhouse, one boiler can heat water for all of the buildings. The boiler can be fueled with gas, oil, biomass including wood, or almost anything else depending on the price and availability of fuel. Changing fuels may involve retrofitting the boiler, but the delivery system (pipes) are already in place and work the same regardless of fuel source.

“Of course, a system like this represents a significant initial investment, and you would need to look at the cost and benefits of these systems before making a choice,” Worley says.

Heat is lost from a greenhouse both through radiation (the heat loss we feel when we stand next to a cold window) and conduction (heat moving from warm to cold through a piece of glass, a sheet of plywood, or a concrete wall). The largest and most obvious source of heat loss in a greenhouse is the glass or plastic surrounding the structure. The surface of a greenhouse must be transparent in order to transmit the sun’s rays, but the flip side of that is that the resistance to heat loss (R-value) of these materials is typically extremely low. Other losses occur through leaks in the surface — possibly due to failure to keep doors, fan louvers, or other openings sealed tightly, or in the case of glass surfaced greenhouses, simply loose fitting glass panes.

Typically a smaller source of heat loss would be through the floor and through solid (plywood or metal) walls around the bottom of the house. For houses surfaced with plastic sheeting, a double layer of plastic with a layer of air maintained between the two layers does provide some insulation, although it is still low. Polycarbonate (ridged plastic) surfaces typically have a small airspace inside that works like a double-paned window to provide some resistance to heat flow. Some growers use a screen or blanket that can be opened during daylight and closed at night to reduce radiant heat loss from the warm surfaces in the house to the cold outside. Perhaps easier and cheaper than any of these is to carefully monitor all openings in the house to make sure there are no air leaks during cold nights.

Repairing an air leak can take several different forms, depending on where the leak is located. A grower could replace a broken glass pane, reglaze the edges of a glass pane, tape up a hole in the plastic sheeting, make sure fan louvers are closing properly, use weather stripping around doors, make sure inlets and outlets are operating properly, or any number of similar measures. The important thing is to stay aware of any potential leaking in your greenhouse.