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How to Heat a Greenhouse Without Electricity
The cheapest heat is the heat you stop losing. Everything else is a supplement.
Direct answer: Heat a greenhouse without electricity by insulating first, then adding thermal mass, then a heat source: a compost pile, a rocket mass heater, or a wood stove with a water jacket. None of these will keep a greenhouse at tomato temperatures through a hard freeze without backup, but they will keep hardy crops alive.
Insulation first
- Twin-wall polycarbonate instead of single film — roughly doubles the R-value.
- Insulated north wall, ideally with a dark surface to re-radiate.
- Bubble-film curtain pulled at night — the best dollars-per-degree improvement.
- Seal gaps at doors, vents, and glazing bars. A 1/8-inch gap along a 10-foot panel leaks more heat than you expect.
- Perimeter insulation — 2 inches of foam 18 inches down around the foundation.
Compost heat
A compost pile inside the greenhouse releases heat as it breaks down: 120–160°F internally for several weeks. Build a bay 3x3x3 ft with a mix of greens and browns, turn it every few days, and vent the gas away from plants. It is not controllable, but it is nearly free.
Expect useful heat for 3–6 weeks per batch, then rebuild. Many growers run two bays in rotation.
Thermal mass recap
Water barrels, stone walls, and soil floors absorb daytime heat and release it at night. A rough rule is 2–3 gallons of water per square foot of glazing. Place mass where it gets direct sun in winter and where it will not shade summer crops.
Rocket mass heater
A rocket mass heater burns wood hot and fast, then stores heat in a thermal mass — typically cob or stone. It produces almost no smoke once running and needs no electricity. The trade-offs are real: it needs a chimney, a fireproof surround, and constant attention during burn cycles.
Any solid-fuel heater in a greenhouse needs proper clearances, a chimney above the roofline, and a carbon-monoxide alarm. Do not improvise a flue.
What not to do
- Run a propane or kerosene heater without ventilation — CO and moisture problems.
- Seal the greenhouse completely; you need air exchange to control humidity.
- Rely on compost heat through a week of single digits.
Frequently asked questions
How much heat does a compost pile produce?
A 3x3x3 ft pile can hold 120–160°F internally for several weeks, raising adjacent air temperature a few degrees. It is a supplement, not a furnace.
Will a wood stove keep my greenhouse above freezing?
Yes, if it is sized correctly and you tend it. An unattended stove goes out overnight — which is exactly when you need heat.
Is a rocket mass heater safe in a greenhouse?
Yes when built to spec with proper clearances, a chimney, and a CO alarm. Improvised flues are the main risk.
Start with the self-heating greenhouse guide, then size your mass with water barrel thermal mass.
Frequently asked questions
How much heat does a compost pile produce?
A 3x3x3 ft pile can hold 120–160°F internally for several weeks, raising adjacent air temperature a few degrees. It is a supplement, not a furnace.
Will a wood stove keep my greenhouse above freezing?
Yes if sized correctly and tended. An unattended stove goes out overnight, which is when you need heat.
Is a rocket mass heater safe in a greenhouse?
Yes when built to spec with clearances, a chimney and a CO alarm. Improvised flues are the main risk.