How to Heat a Greenhouse, By Mark Seibert, Owner of Sturdi Built Greenhouse

Whether your goal is to grow fresh vegetables year-round, protect sensitive plants from winter weather, or create a comfortable garden retreat, maintaining the right temperature inside your greenhouse is essential.  When temperatures drop outside, how can the greenhouse be kept at a target temperature inside?  Let’s dive in!

Heating sources can be active or passive, low to high cost. Selecting one solution that fits your budget and provides the best solution can work, however the best approach is a multi-faceted, layered solution.

Understanding Winter Heat Loss.  When the weather outside is frightful the typical culprits are cold wind, low temperatures and latent snow and ice; each will chill a greenhouse fast.  Some passive heating options will continue to work in the summer, which then becomes a cooling challenge.

The Greenhouse Structure Matters. The best first defense is the glazing and framing of your greenhouse.  Each material can be rated with its thermal resistance, or R-Value which permits us to judge which is the better insulator.  Better insulation means lower heating costs.

• The exposed framing of your greenhouse is commonly metal, plastic, or wood. Of the three framing choices, wood has the better insulative value, followed by plastic and metal.  Recall if you can, why we replaced all our aluminum framed windows in older homes.
• The exposed glazing can be glass, acrylic, and twin wall products.  Like framing, each has different insulative values.  Twin Wall products are generally superior over glass and acrylic.  A greenhouse with double layers of glazing creates the best insulative values such as Sturdi Built’s Thermal Option®

Positioning of Your Greenhouse can Help.  Positioning your greenhouse with its longest wall facing south provides the greatest solar heat gain during winter months. This orientation allows sunlight to penetrate deeper into the greenhouse, increasing warmth and available light for plant growth during the seasons when both are most needed. A southern exposure, in the winter, can reduce heating costs, improve growing conditions, and enhance the effectiveness of passive solar features such as heat sinks and thermal mass. In the summer, shade cloth may be needed to prevent sunburn. 

When selecting a site, it is also important to consider shading from trees, buildings, and other structures, as even partial shading can significantly reduce the greenhouse's solar gain and overall performance.

Comparison Table of Heating Options

Heating Options	Install Cost	Operating Cost	Effectiveness
Geothermal Systems	$$$$$	$	High
Heat Sinks	$$ to $$$$	None	Medium
Heat Pumps and Mini Splits	$$$ to $$$$	$$$	High
Wood Stoves	$$$$	$$$$	Medium
Pellet Stoves	$$$$	$$$	High
Gas Heaters	$$$$	$$	High
Radiant Floors	$$$$ to $$$$$	$$	Medium
Electric Heaters	$$$ to $$$$	$$	High

 

Electric Heaters are easy to use.  Some can be floor mount or hanging, can be simple 120-volt units or more powerful 20-amp units.  Greenhouses are moist environments, so look for a heater that is rated for garages and/or greenhouses.  The costs for an electrical based solution include the electrical circuit and breaker, for the heater itself and then the ongoing draw from your electrical panel.  Maintenance typically includes cleaning the heater, and checking connections for wear.

Radiant Floors use the same technology as found in bathrooms and driveways.  Similar to electric heaters, electric or water based radiant floors can easily heat a greenhouse, requires installation beneath a floor surface and may require additional electrical or plumbing infrastructure.  A properly installed hot water-based radiant-floor heating system requires relatively little maintenance. For hydronic systems, the water quality or antifreeze solution should be tested every few years and replenished if needed. Because the tubing is embedded in the floor and protected from physical damage, it commonly lasts 30–50 years or more with minimal attention, making radiant floor heat one of the lowest-maintenance heating systems available.

Gas Heaters typically use natural gas or propane.  Gas heaters are effective for larger greenhouses.  Most gas heaters need to be vented.  Cost wise, the gas heater and installation once done is just the cost of the fuel.

Pellet Stoves, when coupled with a circulation fan and thermostat, provide an effective and relatively simple way to heat a greenhouse or garden room. They do use up valuable space inside.  Modern pellet stoves can automatically regulate heat output to maintain a comfortable temperature while using a renewable fuel source. Of course, pellet stoves must be properly vented outdoors and installed with appropriate clearances from combustible materials. They also require periodic maintenance, including adding pellets, emptying ash, and cleaning the burn pot and venting system. For many greenhouse owners, pellet stoves offer an attractive balance of heating performance, operating cost, and the cozy ambiance of a visible flame.

Wood Stoves are more labor-intensive than other heating options, requiring regular loading of firewood, ash removal, and routine cleaning and inspection of the chimney or venting system. Most are not thermostatically controlled, and some offer fan options. They must be properly vented outdoors and installed with adequate clearances from combustible materials. Wood stoves also consume valuable floor space inside a greenhouse, which can reduce growing or seating areas. However, they can provide substantial heat output, operate independently of electricity in many cases, and create a warm, inviting atmosphere that many greenhouse owners enjoy, particularly during colder months.

Heat Pumps and Mini Splits are an excellent way to heat and cool a greenhouse while using significantly less energy than electric heaters. In many climates, heat pumps provide the lowest operating cost of any active greenhouse heating system.  Most greenhouse applications use a ductless mini-split heat pump, with an outdoor unit connected to an indoor wall-mounted air handler. During cold weather, the heat pump extracts heat from the outside air and transfers it into the greenhouse; during warm weather, it can reverse operation to provide cooling. In addition to temperature control, many heat pumps offer a dehumidification mode, which can help reduce excess humidity and condensation. Heat pumps work best in well-insulated greenhouses, making them particularly effective when paired with insulated glazing, thermal curtains, or a thermal greenhouse option. While the initial installation cost is typically higher than electric heaters, the reduced operating costs and year-round climate control often make heat pumps an attractive long-term solution for greenhouse owners.

Heat Sinks use solar energy to capture and store heat during the day in dense materials such as water barrels, masonry walls, house walls, stone floors, or concrete. As temperatures drop in the evening, these materials gradually release their stored heat back into the greenhouse, helping to moderate temperature swings and reduce heating requirements. While heat sinks typically do not provide enough heat to maintain ideal growing temperatures during prolonged cold weather and they cannot be easily controlled, they can be an effective passive heating strategy that improves temperature stability, reduces energy consumption, and helps protect plants from light frost conditions.

Geothermal Systems use the relatively constant temperature of the earth to help regulate greenhouse temperatures throughout the year. In winter, air can be circulated through underground pipes or a ground-source heat exchange system, warming the incoming air to approximately the earth's temperature—often around 50–60°F depending on location.

When coupled with an electric or gas heater, geothermal reduces the heating load because the supplemental heater only needs to raise the temperature from the geothermal baseline rather than from freezing outdoor conditions. This can lower energy consumption, improve temperature stability, and create a more efficient year-round growing environment.

The Best Greenhouse Heating Strategy

The best greenhouse heating solution is rarely a single solution. The most successful greenhouses combine thoughtful design, quality insulation, passive solar features, and a heating system sized for the local climate. Every growing environment is different, and the ideal balance depends on your location, budget, and gardening goals.

At Sturdi Built Greenhouse, we encourage customers to think about heating from the beginning of the design process. Features such as redwood framing, insulated glazing options, thermal mass, proper orientation, and effective ventilation can dramatically reduce energy consumption while improving plant health and comfort. A well-designed greenhouse makes every heating dollar work harder and helps you enjoy gardening in every season of the year.