How to Heat a Greenhouse in Winter: Insulation, Heaters and BTU Math
Last updated: November 2026 · From the KRMPLANS team, designers of DIY building plans
Quick answer: To heat a greenhouse in winter, first cut heat loss with bubble wrap or a second glazing layer, add thermal mass such as black 55-gallon water barrels, then size a heater with the formula BTU per hour = surface area x temperature difference / R-value. A 10x12 hobby greenhouse often needs 6,000 to 15,000 BTU per hour on a cold night. Use a thermostat, and vent any fuel-burning heater.
Heating a small greenhouse is mostly a math problem with a safety side. This guide is for backyard growers who want to keep seedlings, citrus, or winter greens alive through freezing nights without a shocking power bill. You will learn the order that saves the most money, how to size a heater in plain numbers, and how to use propane or gas safely.
Key Takeaways
- Reduce heat loss before buying a heater. A second layer of glazing or film can cut the heater size you need by roughly 40%.
- Heat loss depends on three numbers: glazing surface area, the temperature difference inside versus outside, and the covering's R-value.
- A 1,500-watt electric heater gives about 5,100 BTU per hour, which is often not enough for a 10x12 greenhouse on a single layer of film.
- Fuel-burning heaters need a flue or fresh-air supply, because combustion gases can harm plants and people.
- Soil heating cables warm roots for seed starting and use far less energy than heating all the air.
How Do You Reduce Greenhouse Heat Loss First?
Reduce greenhouse heat loss by adding insulation to the glazing, sealing air leaks, and insulating the north wall and foundation. Every BTU you keep inside is one you do not have to buy.
The covering matters most. University of Georgia Cooperative Extension lists overall R-values (resistance to heat flow, higher is better) of 0.91 for single-layer glass and 2.00 for double-layer glass with a 1/4-inch space, and 0.83 for single polyethylene film versus 1.43 for double-layer film. Going from single to double layer cuts conduction loss by about 40% to 55%.
- Horticultural bubble wrap on the inside of glass or polycarbonate adds an insulating air layer. Use UV-stabilized greenhouse bubble wrap, which lasts several seasons.
- Double film: On hoop or Gothic arch houses, a second layer of poly inflated with a small blower creates an insulating air gap.
- North wall: If the north side gets little winter sun, insulate it with rigid foam board on the inside.
- Perimeter: Bury 1- to 2-inch rigid foam vertically around the foundation, 12 to 24 inches deep, to stop heat leaking into cold soil.
- Air leaks: Weatherstrip doors and vents and caulk gaps at the base and ridge.
Does Thermal Mass Like Water Barrels Help?
Yes. Thermal mass absorbs heat on sunny days and releases it at night, which softens temperature swings and can keep a greenhouse a few degrees warmer before dawn. Water is the most common choice because it stores a lot of heat for its volume.
In a University of Georgia passive solar greenhouse project, the east side used 55-gallon plastic barrels stacked two high and filled with water, painted black to absorb more energy. The same publication notes that each full barrel weighs more than 450 pounds, so set barrels on a level, solid base.
A common rule of thumb in passive solar designs is 2 to 5 gallons of water per square foot of south-facing glazing. Place barrels where winter sun hits them directly, usually along the north wall. Thermal mass helps most in sunny climates; after several cloudy days it has little heat left to give.
Can Compost Heat a Greenhouse?
Compost can add some heat, but it is a supplement, not a main heat source. An active hot compost pile of at least 1 cubic yard can reach 130 to 160 degrees Fahrenheit in its core and warm the air and benches around it for several weeks.
The trade-offs are real. Compost needs regular turning and fresh material to keep working, and a hot pile releases moisture and ammonia that can damage tender plants in a closed space. Keep piles in a bin with a lid or venting, away from seedlings, and do not rely on compost alone during a hard freeze.
How Do You Size a Greenhouse Heater?
Size a greenhouse heater by multiplying the total glazed surface area by the temperature difference you need, then dividing by the covering's R-value. University of Georgia Extension gives the conduction heat loss formula as Q = A (Ti minus To) / R, where Q is heat loss in BTU per hour, A is surface area in square feet, Ti is inside temperature, and To is outside temperature.
In plain terms: BTU per hour = area x temperature difference x heat-loss factor, where the heat-loss factor (often called the U-factor) is simply 1 divided by the R-value.
- Measure the surface area. Add up the area of every wall, gable end, and roof panel that is glazed. A 10x12 greenhouse with 6-foot walls and a 9-foot peak has about 430 square feet of surface.
- Pick your temperatures. Decide the minimum inside temperature (for example 45 degrees F) and your typical coldest night (for example 15 degrees F). The difference is 30 degrees.
- Find the R-value of your covering from the table below or the manufacturer.
- Calculate. 430 x 30 / 0.83 = about 15,500 BTU per hour for single-layer film, or 430 x 30 / 1.43 = about 9,000 BTU per hour for double film.
- Add a margin of 10% to 25% for air leaks and wind, which this simple formula does not include.
- Convert for electric heaters. Divide BTU per hour by 3.41 to get watts.
| Covering (10x12, about 430 sq ft) | R-value | Heat loss at 20 F difference | Heat loss at 30 F difference |
|---|---|---|---|
| Single-layer poly film | 0.83 | About 10,400 BTU/hr | About 15,500 BTU/hr |
| Single-layer glass | 0.91 | About 9,500 BTU/hr | About 14,200 BTU/hr |
| Double-layer poly film | 1.43 | About 6,000 BTU/hr | About 9,000 BTU/hr |
| Double-layer glass, 1/4-in space | 2.00 | About 4,300 BTU/hr | About 6,500 BTU/hr |
R-values from University of Georgia Extension; heat loss figures are conduction only, before the leak margin.
A standard 1,500-watt plug-in heater delivers about 5,100 BTU per hour. That covers a well-insulated 10x12 on a moderate night, but not a single-layer house at a 30-degree difference. This is why insulation usually pays off before a second heater does.
Are Electric Heaters Safe in a Greenhouse?
Electric heaters are the simplest and cleanest option for small greenhouses, because they produce no combustion gases. Choose a heater rated for damp or greenhouse use, with tip-over and overheat shutoff, and control it with a separate plug-in or line-voltage thermostat set at plant height.
Plug heaters into a GFCI-protected outdoor outlet, never into a light-duty extension cord. A 1,500-watt heater draws about 12.5 amps, so it should have a circuit to itself. Have a licensed electrician add a dedicated circuit if you plan to run more than one heater.
How Do You Use Propane or Natural Gas Heaters Safely?
Use propane or natural gas heaters safely by choosing a vented model with a flue to the outside and by supplying fresh combustion air. UMass Extension explains that unvented heaters are designed without a flue connection, so heat and products of combustion are exhausted into the greenhouse. Those gases can include carbon monoxide, nitrogen dioxide, and ethylene, and UMass notes ethylene levels as low as 0.01 ppm can damage leaves and flowers.
- Provide a fresh-air inlet; UMass recommends at least 1 square inch of free inlet area per 1,000 BTU per hour of heater input.
- Install a battery carbon monoxide alarm if you work inside a greenhouse with any fuel-burning heater.
- Have heaters cleaned and inspected each fall, and have a qualified gas technician run any natural gas or permanent propane lines.
- Keep propane cylinders outside the greenhouse unless the heater is specifically designed otherwise.
Do Soil Heating Cables Work?
Soil heating cables work well for seed starting and rooting cuttings because they warm the root zone to about 70 to 75 degrees F without heating the whole greenhouse. Lay the cable in a serpentine pattern in a 2- to 3-inch sand bed on the bench, cover it with sand, and run it on a soil thermostat. Never cut or overlap the cable, and follow the maker's coverage chart for length per square foot.
Warm roots often let plants tolerate cooler air, so pairing bottom heat with a lower air setting can save energy.
Common Mistakes to Avoid
- Buying a heater before adding bubble wrap or a second glazing layer.
- Running an unvented propane or kerosene heater in a closed greenhouse.
- Placing the thermostat in direct sun or right next to the heater.
- Using indoor extension cords on wet floors.
- Stacking water barrels on soft soil that settles under 450-plus pounds each.
DIY 12x16 Monitor Roof Greenhouse Plans, Large Wood Frame PDF: instant PDF download with illustrated steps, a shopping list and cut lists.
Planning a new build? A KRMPLANS plan like the 10x12 Gothic arch greenhouse with polycarbonate or the 12x20 Victorian greenhouse lets you measure glazed area from the elevations before you buy a heater. New to greenhouse framing? Start with our DIY wood greenhouse beginner's guide, and see the shed insulation and electrical guide for wiring basics.
Frequently Asked Questions
What is the cheapest way to heat a greenhouse in winter?
The cheapest approach is to cut heat loss first with bubble wrap or a second layer of film, then add water barrels as thermal mass. After that, a thermostat-controlled electric heater sized to the remaining heat loss is usually the lowest-cost active heat for a small greenhouse.
How many BTUs do I need to heat a greenhouse?
Multiply the glazed surface area in square feet by the temperature difference you need, then divide by the covering's R-value. A 10x12 greenhouse with about 430 square feet of surface needs roughly 6,000 to 15,500 BTU per hour at a 20 to 30 degree difference, depending on the covering.
Can I use a propane heater in a greenhouse?
Yes, but choose a vented model with a flue to the outside and provide fresh combustion air. Unvented heaters release carbon monoxide, ethylene, and other gases into the greenhouse that can harm plants and people.
Will a 1,500-watt heater heat a greenhouse?
A 1,500-watt heater produces about 5,100 BTU per hour. That can hold a small, double-glazed greenhouse a modest amount above outdoor temperatures, but it is often too little for a single-layer greenhouse on a very cold night.
How warm should a greenhouse be in winter?
It depends on the plants. Cool-season greens and overwintering perennials often do fine with nights at 35 to 45 degrees F, while tropical plants and warm-season seedlings usually need 55 to 65 degrees F or bottom heat.
Sources
- University of Georgia Cooperative Extension: Greenhouses, Heating, Ventilation and Cooling (B792)
- University of Georgia Cooperative Extension: Constructing a Passive Solar Greenhouse for Season Extension (B1566)
- UMass Extension Greenhouse Crops and Floriculture: Problems With Using Unvented Greenhouse Heaters
Plan a Greenhouse That Is Easier to Heat
The easiest greenhouse to heat is one designed with tight framing, good glazing, and a sensible size. Browse our greenhouse plans for Gothic arch, monitor roof, octagon, and Victorian designs with shopping lists and framing drawings.
