To use a greenhouse most effectively, it needs to be heated when the weather dips below 32 degrees Fahrenheit. Before choosing a greenhouse heating system, ensure you have done all you can to insulate the walls, floor and ceiling so that the heat stays inside—and the cold stays outside.
Step 1: Upgrade Insulation and Weatherstripping
Determine which side of your freestanding or attached greenhouse faces the prevailing winter winds. Apply a foil-backed bubblewrap-style insulation to that wall, the north roof and the walls not facing south, up to door height. (You can remove this for spring and summer growing.) Use tape to secure it or wedge it into the wall and window frames. Prevent your greenhouse foundation from allowing cold air seepage with insulation. If it is a simple concrete slab, edge all the way around with 1-inch styrofoam panels dug into the ground to a depth of 12 inches.
If your greenhouse sits directly on the ground, dig a 12-inch deep perimeter trench around the greenhouse walls and insert the styrofoam snugly against the walls. When it's in your budget, upgrade the greenhouse glass to double or triple-glaze. In the meantime, line the glass for winter with clear polyurethane sheets. Ensure every crack, cranny and crevice is sealed effectively with weatherstripping, caulking or foam.
Step 2: Install a Second Set of Doors at the Entry (Airlock)
This is a must if you enter and exit the greenhouse from outdoors often in the winter, to prevent both warm air loss and cold blasts of air striking your plants. Leave enough space between the 2 sets of doors so you can completely close the outer door before opening the inner one.
Step 3: Provide for Heat Storage in the Greenhouse
Place some large metal drums full of water inside the greenhouse along the north wall. These will retain the sun's heat that enters the greenhouse daily. Any dense material will do, but water is the cheapest and most adaptable to various greenhouse designs.
Step 4: Choose Your Greenhouse Heater
Once you have made these heat conservation upgrades, you can manage with a much smaller and more efficient greenhouse heater. A passive solar heater can also provide adequate heat for a greenhouse at low cost. A low intensity infrared tube heater is also cost-efficient and easy to install. This slim tube hangs from the ceiling and can be tilted 30 degrees left and right to direct heat where it is needed.
Consider an eco-friendly alternative to fossil fuels: biofuel pellets. These pellets—made of switchgrass, poultry litter or peanut hulls—are fed from a hopper into a biofuel furnace connected to a standard duct system. Biofuel furnaces are proven to be just as effective as a natural gas furnace in maintaining a greenhouse above 65 degrees F during winter months.
Greenhouse heating can be cost-efficient and effective. Insulate your greenhouse fully, accumulate solar heat, and choose eco-friendly heaters wherever they are available, and when they can be integrated into your greenhouse workspace plan.