The Autumn-Winter Greenhouse Heating Guide:
Proven Formulas For Maintaining Consistent Warmth At Lower Cost
For greenhouse gardeners who live in areas that experience cold winters, heating is typically the greatest expense. But there are strategies and materials that can significantly reduce the expense of heating any greenhouse.
The minimum temperature you must maintain in your greenhouse depends on the plants you’re growing. The goal, of course, is to create a greenhouse environment that duplicates the natural environment of your plants as closely as possible.
The temperature inside your greenhouse will always be a few degrees warmer than the temperature outside. In a cool or frost-free greenhouse, optimum temperature is typically 40°F to 45°F; in a warm greenhouse/intermediate-temperature greenhouse, 55°F to 60°F; in a hothouse, 65°F to 70°F.
Cool greenhouses are suitable for frost-sensitive plants and rooted cuttings. Warm greenhouses are suitable for a wider variety of actively growing subtropical plants, such as flowering houseplants, annuals, herbs, bulbs, ferns and warm-weather vegetables. Night temperatures can drop as low as 50°F with no noticeable detriment to warm greenhouse plants. Hothouses generally require greater vigilance, since they must be optimized for tropicals and exotic specialty plants.
Unless you live in an area that experiences mild winters, you’ll probably need to provide auxiliary heat.
GREENHOUSE HEATER OPTIONS
It’s best to choose heaters with thermostats that will trigger the heater to turn on and off at your pre-set temperatures, in order to maintain consistent greenhouse temperatures.
The most common types of greenhouse heaters include:
Electric Greenhouse Heaters. Clean, efficient, readily available and easily installed. Electric heaters are designed to simply plug into either 120-volt or 240-volt circuits, and are available in both wall-mounted and floor-standing models. Many electric heaters incorporate a fan and user-programmable thermostatic control. The only drawbacks of electric heaters are the cost of electricity in some locales, and potential power outages.
Gas Greenhouse Heaters (Natural and Propane). Gas heaters are available in both freestanding and wall-mounted models. The carbon dioxide they produce is beneficial to plants, though not to humans. Gas heaters provide excellent warmth, but they must be vented outside the greenhouse because they emit human-toxic fumes such as ethylene and carbon monoxide. Some come with an electric blower option.
Wood-Burning Stoves. Wood-burning stoves will keep a greenhouse toasty, but they demand frequent attention and must be vented outside. They’re not usually the best solution.
Kerosene Stoves. Kerosene stoves heat well, but require adequate fresh air for complete combustion and are prohibited in greenhouses which are attached to a dwelling by some local and state codes. Kerosene stoves are not ideal for greenhouse heating, except as an emergency backup heat source.
OTHER HEAT SOURCES
Heating Cables. Cables supply bottom heat to propagation mats and benches. It’s considerably cheaper to heat a propagation mat to 70°F than to heat an entire greenhouse.
Radiant Heat. Radiant heat can be installed in some greenhouse floors during construction.
Shared House Heat. Lean-to greenhouses and conservatories can often tap into the home’s heating system.
Solar Heat. The sun can provide up to 25% of the heat required by most greenhouses; on warmer winter days, an even higher percentage. Some greenhouses can be outfitted with solar panels which allow them to be heated solely with solar energy.
HOW TO CALCULATE YOUR GREENHOUSE’S HEATER REQUIREMENTS
Heat is measured in British thermal units (BTU). Your heater’s BTU output per hour must be high enough to replace the greenhouse’s heat loss.
Use the following formula to determine which heater(s) to buy:
A x T x U = BTUs/Hour
A = Total surface area (walls and roof) of the greenhouse in feet.
T = The difference between the lowest outdoor temperature and the lowest greenhouse temperature you desire.
U = The U-value of your greenhouse glazing. Your supplier should be able to provide your greenhouse’s exact U-value. Most polycarbonate greenhouses, for example, have a U-value of 1.1.
Purchase a heater that has a BTU output approximately equal to the BTUs/Hour total you calculate. If your greenhouse shares a heated house wall, subtract 30% from the BTUs/Hour total.
If your greenhouse is well-insulated, you’ll be able to subtract another 30% from the BTUs/Hour total. Because insulation is inexpensive and reusable year after year, it’s a great way to reduce heating costs.
INSULATION
Attaching insulation to your greenhouse walls and ceiling reduces heating costs by 30% or more and helps retain moisture inside the greenhouse. Insulation can be attached with Velco strips, taped to splice joints, or stapled to wooden framing. Vents should also be insulated, but in a manner that allows them to be opened on warm days.
Greenhouse bubble insulation transmits evenly diffused light and works with all types of greenhouse glazing. At a cost of only $100 per 500-square foot roll, it’s an excellent investment that will pay dividends for years to come.
You may also want to consider a rigid bubble and silver foil combination insulation such as Reflectix, which both insulates and reflects light. Reflectix reduces four kinds of heat loss: radiant, conductive, convective, and infiltration. Reflectix, 5/16" thick, costs around $1.50 per linear foot for 2′-wide panels; $3 per linear foot for 4′-wide panels.
