Insulating Your Greenhouse

Attention, orchid physics students.

The First Law of Thermodynamics for orchid growers states that in the summer, one insulates the greenhouse to keep the heat out and the cold in.

Conversely, the Second Law of Thermodynamics stipulates that in winter, one insulates the greenhouse to keep the cold out and the heat in.

With winter upon us, Law Number Two is today’s concern.

Growers experienced in orchid physics know how to protect their plants and graciously share what works for them.

First, your professors wish to point out that Santa Claus’s insulation requirements may exceed yours. If you grow in Hawaii, for example, you needn’t worry much about cold weather.

Second, the location of your greenhouse and the nature of its construction will dictate the measures you can take. For example, if your greenhouse is attached to your home, or if it faces south, your insulation needs will be different from those of a free-standing greenhouse.

And third, the type of heating system you have in place will influence the kind of conservation measures most effective for you. Heat rising by convection from hot-water pipes in the foundation calls for different conservation measures from what would be needed if your heat comes from a gas-fueled heater hung from the rafters.

The important decision in choosing insulation will be one of convenience and your ability to manage it when the seasons change.

Sally Karabelnikoff grows in two large greenhouses in Anchorage, Alaska, where insulation is a major concern. Recently, she rebuilt her older greenhouse by installing concrete foundations insulated on the outside with soil. In the concrete floor of the interior, she installed a hot-water heating system that gives her “a great overall air temperature.” Her other greenhouse is made of fiberglass purchased in wide rolls and formed over a quonset-style framework.

Last autumn, Karabelnikoff in-stalled bubble wrap on the inside, leaving an air pocket between the wrap and the roof. “My heat bill went down dramatically that winter. It was much warmer in summer, too, but around here, that is not much of a problem. I put lots of orchids out there and couldn’t believe the amount of blooms that kicked in.”

John Kochner, who grows in Scotch Plains, New Jersey, looked into the 
use of bubble wrap as a way to hold down energy costs. He reports that industrial-grade bubble wrap is avail-able from packaging distributors in most areas. He bought a roll of 5/16-inch × 24-inch × 188-foot (.8-cm × 60-cm × 57-m) bubble wrap that will cover 360 square feet (33 sq m). “I’m told it will reduce heating costs by 50 percent,” he says.

But it must be installed correctly.

Ed Wright, greenhouse expert in San Antonio, Texas, observes that some growers wet the inside of the glass or acrylic paneling of the greenhouse roof and stick the bubble wrap directly to it. Used this way, the product is minimally effective as an insulator. You can trap air more effectively by creating a gap of several inches between the wrap and the roof.

“Entrapped dead air is an efficient insulator,” Wright says. You can trap it by tacking 4- or 6-millimeter sheet plastic to the inner framing of the walls and roof. The thinner sheet allows more sunlight to pass through; the thicker sheet lasts longer.

“It is a pain to apply,” Wright says, “but once in place, it will last as long as three years before algae growth begins to reduce light transmission.”

“Dead air space does the trick,” says Dr. Warren Collmann, a resident of Farmville, Virginia. He covers the interior of his Gothic-style greenhouse with 4-millimeter plastic sheeting, leaving 2 to 3 inches (5 to 7.5 cm) of air space between the plastic sheeting and the external fiberglass panels. In addition, Collmann uses plastic sheeting to seal off the dome structure at the top of the greenhouse. “It cuts growing space way down,” he says, “but it is effective.”

Even better, says Wright, is the double-layered “puff-roof” greenhouse in which the dead air space is significant. Useful where snow or high winds are not a concern, the puff-roof house creates a dome-shaped tent with two layers of polyethylene puffed apart by a small electric air blower. “Use outside air to keep a puff roof inflated,” Wright says. “Puffing moisture-laden air from inside the greenhouse will fill the space with water vapor that will lead to weight and algae-growth problems.”

One of the least expensive methods of warming a greenhouse is to store the heat the sun beams into it every day. Water in containers ranging from garbage cans to milk jugs readily absorbs the heat of ambient air and releases it at night. Some growers use 55-gallon (250 L) drums as bench supports; filled with water, they do double duty as heat sumps.

Collmann saves plastic gallon jugs that he fills with water and places along the southern exposure — and part of his eastern and western exposures, as well. “The water heats up with the sun during the day, and during the night gives a source of solar energy,” he says.

For smaller greenhouses and even windowsills, growers might consider foil-faced Styrofoam insulating material that can be placed to direct the sun’s rays where they are needed. Used mostly in home construction, foil-faced Styrofoam comes in 4- × 8-foot (1.2- × 2.4-m) panels up to 1/2-inch (1.25-cm) thick. Position it so the shiny side faces the greenhouse interior to reflect sunlight onto the plants. The same material can be cut to insulate fan housings and exhaust shutters as well. Styrofoam panels can be put up and taken down for several seasons, but can harbor snails, roaches, and algae that damage plants.

Reprinted from Orchids December 2005 (Vol. 74, No. 12).

Tips for Keeping Plants Warm in a Greenhouse

  • Trap dead air with plastic sheeting placed below the roof and inside the walls.
  • Use sumps (bottles, buckets, barrels filled with water) to capture free heat from the sun to be released after dark.
  • Seal openings around windows, doors and ventilators to prevent cold air from entering and warm air from escaping.
  • Position reflective panels to direct sunlight onto plants and heat sumps.
  • Make sure alarm systems for power and temperature are in good repair; install fresh batteries where needed.
  • Melt or remove snow overhead before its weight collapses or ruptures the roof.
  • Have an emergency backup system, such as a portable kerosene or electric heater, in case of a power failure.
  • If none of the above is feasible, spray plants continuously with tap water, which will keep them from freezing.
  • Move plants into your garage and turn on lights as a heat source.
  • If all fails, bring plants into your home as a temporary measure.