Heating a greenhouse in winter is a DIY job for the planning and insulation steps, and usually a pro job only if you add gas, venting, or new electrical circuits. For a small greenhouse, you can usually sort the basics in a few hours, then fine-tune it after one cold night.
A greenhouse heater only works well if the house around it stops leaking heat like a sieve. The half-true part is that you do need a heater; the other half is that insulation, sealing, and thermostat control usually matter more than the brand on the box.
If you want the cheapest heat, start with the structure, then choose the smallest heater that can hold your target night temperature.
What to remember
Measure the greenhouse volume and target night temperature before you buy a heater.
Seal gaps, add bubble wrap or another approved cover, and give the heater a thermostat.
Use electric heat for small spaces and spot heating; use fuel-burning heat only with the right venting and CO alarm.
Keep the heater off extension cords and power strips, and keep 3 feet of clearance around it.
Check running cost from wattage or fuel use before you assume a cheap heater is cheap to run.
How to Heat a Greenhouse Efficiently in Winter
Heating a greenhouse efficiently in winter starts with controlling heat loss, not just cranking up the heater. The key steps are measuring your space, sealing drafts, adding thermal mass or covers, choosing the right heater and controls, and testing overnight to confirm stable temperatures.
Sealing drafts is essential for efficient heating.
A greenhouse’s heating load depends on its size, the type of glazing, and how cold it gets outside. There’s no single BTU number that fits all; you need to estimate based on your specific conditions and then match your heater and insulation accordingly.
1
Measure the Greenhouse
Calculate the floor area and volume, note the glazing type (glass, polycarbonate, or plastic), and record your local lowest winter temperatures.
2
Seal and Insulate
Check for drafts at doors, vents, and frames. Use weatherstripping and plastic sheeting to reduce heat loss.
3
Add Thermal Mass or Row Covers
Place barrels of water or bricks inside to store heat during the day and release it at night. Use row covers over plants for extra protection.
4
Install and Set the Heater
Choose a heater sized for your load, set a thermostat to maintain your target temperature, usually around 45°F–60°F in winter.
5
Test Overnight
Monitor temperature stability during the coldest night, adjust insulation or heater settings as needed.
Heater control and sizing
Air Choice Greenhouse Heater with Digital Thermostat
The Air Choice Greenhouse Heater’s digital thermostat and adjustable 750 W or 1500 W modes make it practical for greenhouses up to 160 sq. ft., letting you fine-tune heat output to your needs.
Oversizing wastes energy, stresses plants, and causes uneven temperatures. Efficient insulation and precise temperature control matter more than heater size alone.
3 feet
Required heater clearance
Keep at least 3 feet of clearance around the heater to prevent fire hazards and ensure proper airflow.
1,500 W
Maximum plug-in heater size on a standard 15 A circuit
A typical household circuit supports up to about 1,500 watts without tripping breakers, so choose heaters accordingly.
Electric heat costs depend on power, use time, and electricity rates. For example, 1,500 W for 8 hours is 12 kWh of energy; at 17 cents per kWh, that runs about $2.04 per night.
Greenhouse Size
Approach
Heater Type
Small (up to 160 sq. ft.)
Seal, add thermal mass, use a 750–1500 W electric heater with thermostat
Air Choice Greenhouse Heater
Large (over 160 sq. ft.)
Improve insulation, consider multiple heaters or gas options, install thermostat controls
Electric, propane, or natural gas heaters with ventilation
How heating methods scale with greenhouse size
Heating a Small vs. Large Greenhouse
Say your small greenhouse is 12 by 14 feet with polycarbonate panels and a 20°F winter low. A 1,500 W heater with good sealing and thermal mass maintains 50°F overnight. A larger 24 by 24 feet greenhouse with single-pane glass and 10°F lows needs better sealing, multiple heat sources, and possibly gas heating to keep stable temperatures.
Small greenhouse12 × 14 ft, 160 sq. ft., polycarbonate,
Large greenhouse24 × 24 ft, 576 sq. ft., single-pane gla
What You’ll Need
Tools, parts, and one heater option that fits this job
Digital thermostat
Air Choice Greenhouse Heater 1500W
This heater offers a waterproof temperature probe and digital thermostat for exact temperature settings from 40℉ to 108℉, making it easy to maintain stable warmth in your greenhouse. Its rapid 3-second PTC heating element quickly warms spaces up to 160 sq ft, ideal for winter plant protection.
To heat a greenhouse efficiently in winter, you need the right tools and parts before you start. Efficient heating means controlling temperature precisely and avoiding wasted energy, which is similar to heating a home efficiently by sealing drafts and using thermostats.
Here’s a practical checklist of what to have on hand. Start with a tape measure to size your greenhouse and check clearances. A reliable thermometer or temperature probe is essential to monitor conditions. Weather sealing supplies and row covers help reduce heat loss. A timer or thermostat lets you control the heater’s run time and temperature without wasting power. Make sure your heater plugs directly into a wall outlet—no extension cords. If you use a fuel-burning heater, ventilation and a working carbon monoxide alarm are required for safety.
Greenhouse Heating Kit Checklist
Tape measure
Thermometer or temperature probe
Weather sealing materials
Row cover fabric
Timer or thermostat
Extension-free wall outlet access
Chosen heater unit
Electric heaters are often the easiest and safest option for small to medium greenhouses. They plug straight into an outlet and offer precise temperature control. Fuel-burning units need more planning for ventilation and safety gear.
Compact electric heater with thermostat
Air Choice Greenhouse Heater with Digital Thermostat
The Air Choice Greenhouse Heater has a built-in digital thermostat and temperature probe, making it a simple, efficient choice for controlling heat in up to 160 square feet without extra gear.
Reducing heat loss is the cheapest and most effective way to keep your greenhouse warm. Before upgrading your heater, seal gaps around doors and windows with weather stripping or caulk. Adding a second layer of clear plastic or polycarbonate inside the existing glazing can cut heat loss by trapping an insulating air space.
Thermal mass stores heat during the day and releases it after sunset. Filling a barrel with water inside your greenhouse is a simple example. Water’s high heat capacity means it can smooth temperature dips overnight, reducing how often the heater needs to run.
Row covers made of lightweight fabric draped over plants create a microclimate that holds warmth close to leaves and soil. This is a low-cost way to boost temperatures in critical zones without heating the entire space.
Method
Cost
Effectiveness
Best Use
Seal gaps
Low
High
Stops drafts to keep warm air inside
Double glazing layer
Medium
High
Improves insulation of greenhouse shell
Thermal mass (water barrels)
Low
Medium
Stores daytime heat for nighttime release
Row covers
Low
Medium
Protects plants directly by trapping heat
Infrared heat
High
Spot heating
Warms plants and surfaces locally, not whole greenhouse
Comparison of heat retention and supplemental heating methods
Myth
Infrared heaters warm the whole greenhouse efficiently
Fact
Infrared heat works well for warming plants or surfaces in small zones but won’t compensate for a leaky, poorly insulated greenhouse shell.
Infrared heating tips
Use infrared panels to warm benches or seedling areas, especially where you want to avoid raising air temperature too much. Combine with sealing and thermal mass for best results.
Choose the Right Heater and Fuel Source
The best heat source for a greenhouse depends on its size, seal quality, and how much you want to manage fuel or electricity costs.
Choosing the right heater is crucial for heating efficiency.
For small, well-sealed greenhouses under about 160 square feet, a 1,500 watt electric heater often covers the heat load. Its output equals roughly 5,100 BTU per hour, which suits tight spaces or acts as backup heat in larger setups.
Infrared heaters can work for small greenhouses too, but only if you want targeted warmth on plants or people. The warmth comes straight from the heater to nearby objects without heating the air much, so infrared doesn’t replace insulation or circulation—you still need those to avoid cold spots.
Propane and natural gas heaters offer higher output and can suit large or drafty greenhouses, but they require ventilation and carbon monoxide precautions that usually mean professional installation. We cover those safety steps later.
Electric Heater
Clean, easy to plug in, good for small to medium greenhouses
Precise thermostat control cuts running costs
No combustion gases, no venting needed
Fuel-Burning Heater
Higher operating cost per BTU than propane or natural gas
Limited to 1,500 W on typical circuits, not ideal for large spaces
Needs reliable electricity supply
To heat large spaces efficiently, consider fuel-burning heaters that deliver 20,000 BTU or more, paired with good insulation and airflow. Electric heaters are best for small or medium zones where you want fine temperature control and simple setup.
Best electric heater for small greenhous
Air Choice Greenhouse Heater with Digital Thermostat
The Air Choice Greenhouse Heater has a digital thermostat, rapid heating modes, and safety features. It covers up to 160 sq ft, ideal for well-sealed small greenhouses or backup heat.
A 1,500 W electric heater is enough for small, sealed greenhouses. Infrared adds local warmth but doesn’t replace insulation. Propane and natural gas suit larger spaces but need venting and professional care.
Avoid The Mistakes That Waste Heat Or Damage Plants
Safety First
Never plug electric heaters into extension cords or power strips; always use a dedicated wall outlet. Keep at least 3 feet of clearance around heaters. Only indoor-rated fuel-burning heaters belong indoors, and they require proper ventilation and a carbon monoxide alarm.
Common mistakes waste heat and can harm plants. Oversizing the heater causes temperature swings and wasted fuel. Blocking airflow with clutter or poor placement traps cold air in pockets. Directing hot air at sensitive foliage can scorch leaves. Skipping a thermostat means the heater runs full blast or not at all, wasting energy.
Sealing the greenhouse too tightly might sound smart but raises humidity and condensation. Condensation on cold glazing signals heat loss and invites mold, so ventilation remains essential even in winter.
Humidity and Ventilation
A well-ventilated greenhouse keeps condensation low and prevents mold. Even in cold months, a small controlled airflow exchange reduces moisture without losing much heat.
If you use fuel-burning heat, carbon monoxide is a serious hazard. Only use indoor-rated propane heaters with the ventilation the manual requires. A working carbon monoxide alarm is mandatory. Any heater that burns fuel needs fresh air to keep plants healthy and you safe.
Avoid These Heat-Wasting Mistakes
Oversized heater cycling on and off too frequently
Heater placement blocking air circulation
Aiming heater directly at delicate plants
Operating without a thermostat
Sealing so tightly that humidity builds unchecked
When Permits Or A Licensed Pro Are Required
Heating a greenhouse efficiently often means knowing when to call in a licensed pro and when you can handle the job yourself.
Plug-in electric heaters and removable insulation usually fall into the DIY category if your existing electrical circuit can handle the load. Check the breaker panel to confirm the circuit’s amperage. Most portable heaters top out at 1,500 watts, drawing about 12.5 amps on a standard 15-amp circuit—running more than one heater or other devices on that circuit risks tripping breakers or worse.
Electrical Load Warning
Never plug a 1,500-watt heater into an overloaded circuit or extension cord. Doing so risks fire or damage.
Installing new 120-volt outlets, hardwiring heaters, running gas piping, or installing venting almost always requires a licensed electrician or plumber. Local codes vary, so check with your city or county building department before starting work. Vent-free gas heaters can be banned or restricted in some places, so confirm their legality for greenhouse use in your area.
Check Local Codes
Codes cover vent-free gas heaters, greenhouse electrical work, and structural changes. Permits may be required even for minor modifications.
If your project involves a new circuit, gas line, or roof or wall penetration for venting, stop and call a pro. Mistakes here can be costly and dangerous, especially when gas and electricity mix. For simple heating using existing outlets and portable electric heaters, you can usually proceed without a permit, but confirming your setup’s safety is key.
Do These Next In Order
1
Measure the Greenhouse
Check your greenhouse’s dimensions and calculate heat loss to size your heater properly.
2
Seal Leaks and Add Insulation
Fix drafts, seal gaps, and add removable insulation where possible to keep heat in.
3
Choose the Heat Source
Pick a heater that matches your greenhouse size and fuel preferences.
4
Verify Electrical or Fuel Setup
Confirm your circuit can support the heater or that gas piping and venting are installed safely and per code.
5
Set the Thermostat
Program your thermostat to maintain target night temperatures without running constantly.
6
Test on the First Cold Night
Run the system through a full cold night to spot issues and adjust settings.
In short
Measure, seal, choose heat, check power or fuel, set controls, then test. Follow this order for a warm greenhouse all winter.
Questions People Ask
Can infrared heater heat a small greenhouse?
Yes, an infrared heater can heat a small greenhouse effectively by warming the plants and objects directly rather than the air. This reduces heat loss and can save energy, but sizing must be accurate to avoid cold spots or overheating.
What is the cheapest way to heat a greenhouse?
The cheapest way is to improve insulation and reduce heat loss before adding any heater. Using passive solar gain, thermal mass, and sealing leaks cuts heating needs dramatically, which lowers fuel or electricity costs over time.
How do I size a heater for my greenhouse?
Calculate the volume and factor in insulation quality, outside temperature, and desired inside temperature. For example, a well-insulated 100 sq ft greenhouse about 8 ft high needs roughly 3,000 to 5,000 BTU per hour. Always add some margin for coldest nights.
Can I use a propane heater in a greenhouse?
You can use a propane heater only if it’s rated for indoor use with an oxygen depletion sensor and you provide proper ventilation. Without these, carbon monoxide buildup risks plant damage and safety hazards.
Do I need a permit to install a greenhouse heater?
Permits depend on local code and the heater type. Electric heaters on existing circuits usually don’t, but adding new wiring, gas lines, or venting often requires a permit and licensed pro. Check with your local building department.
