Greenhouse Heating: How to Keep a Greenhouse Warm in Winter
Pick the target temperature, spend the free measures first, then size the heater from the actual heat loss. In that order, winter growing is affordable. In any other order it is not.
Decide the target temperature first
Almost every expensive winter mistake starts with skipping this step. Heating cost scales with the difference between inside and outside, so the question is never "how do I heat a greenhouse" but "to what, and for what".
There are three sensible answers, and they are wildly different projects.
| Regime | Held at | What it gets you | Heating burden |
|---|---|---|---|
| Frost-free | 35 - 40F | Overwintering dormant and tender plants, hardy salads, garlic, brassicas | Low. Often achievable with passive measures plus a small heater on the coldest nights. |
| Cool house | 45 - 50F | Continuous winter salad and herb harvests, early propagation from late winter | Moderate. A thermostatic heater will run most nights in zone 5 or 6. |
| Warm house | 60F and up | Winter tomatoes, tropicals, year-round seedling production | High. This is a genuine utility line item in a cold zone. |
If your goal is to keep a fig, some pelargoniums and a bench of winter lettuce alive, you need frost-free. That is a completely different equipment and cost picture from a warm house, and the passive measures below may get you most of the way there on their own. Do not buy a warm-house heater to do a frost-free job.
Everything free comes first
Glazing choice is the biggest single lever, and it is decided at purchase. Twinwall polycarbonate insulates because of the sealed air gap between its two skins - a genuine thermal break that single-layer glass or film does not have. Stepping from 4 mm to 6 mm twinwall meaningfully reduces heat loss for a one-time cost, and against a recurring fuel bill that is the cheapest efficiency you will ever buy. If you know you will grow through winter, buy the 6 mm.
Thermal mass costs almost nothing and works every night. Black-painted water barrels or containers along the north wall absorb solar energy during the day and release it slowly after sunset, flattening the overnight low by several degrees. Around 2 to 5 gallons of water per square foot of south-facing glazing is the conventional target. Stone, brick or a concrete slab floor does the same job more slowly.
Seal the leaks. Door gaps, panel joints and the base line are where most of the heat actually goes. Weather-strip the doors, tape panel edges with the proper vented tape at the ends so condensation can still drain, and pack the gap between the base rail and the ground.
Insulate the surfaces that do not grow anything. Horticultural bubble insulation on the north wall and the lower north-facing roof costs little light and cuts loss where it matters. Do not bubble-wrap the south face - you need that light far more in December than you need the insulation.
Heat a smaller volume. A fleece tent or a cold frame inside the greenhouse creates a second envelope around exactly the plants that need protection. Heating 20 cubic feet to 40F is trivially cheaper than heating 1,200.
How to size a heater without guessing
The working formula is BTU per hour = glazed surface area (sq ft) x temperature difference (F) x heat loss factor.
The heat loss factor is roughly 1.1 to 1.2 for single-layer glass or film, and roughly 0.6 to 0.7 for twinwall polycarbonate. The temperature difference is your target inside minus your design low outside - the coldest night you actually intend to defend against, not the average.
Worked example. A 10 ft x 19.5 ft arched greenhouse has roughly 420 sq ft of glazed surface. Holding 40F inside when it is 0F outside is a 40 degree difference. In twinwall: 420 x 40 x 0.7 = about 11,800 BTU per hour, which is around 3.4 kW. In single-layer film the same job would need close to 20,000 BTU per hour. That single number is the whole argument for twinwall.
For reference, a 1,500 W electric fan heater on a standard household circuit delivers about 5,100 BTU per hour. So the example above needs either two of them on separate circuits, a propane heater, or - far more sensibly - a lower target temperature and better insulation.
Heater types, honestly compared
| Type | Good for | Watch out for |
|---|---|---|
| Electric fan heater with thermostat | Small to mid greenhouses, frost-free and cool regimes | Needs a properly installed outdoor-rated circuit and RCD. Output capped by the circuit. |
| Electric tube or convector heater | Gentle background heat, frost protection along a bench | Low output. Best combined with thermal mass and insulation. |
| Propane or natural gas heater | Larger volumes, cold zones, where power is limited | Combustion releases water vapour and needs a permanent air supply. Unvented units demand real ventilation and a CO alarm. |
| Paraffin or kerosene heater | Emergency frost protection, no power available | Same moisture and fume issues, plus soot on glazing. A stopgap, not a system. |
| Soil warming cables or heat mats | Propagation benches, germination | Warms the root zone only, which is often all you actually need. |
| Compost or biological heat | Enthusiasts, low-tech setups | Real but modest, and it demands active management. |
Any heater that burns fuel consumes oxygen, produces carbon monoxide, and adds a surprising amount of water vapour to the air. Fit a carbon monoxide alarm in the greenhouse, and keep a permanent air inlet open even on the coldest night. The moisture is not a minor detail either - it raises humidity exactly when cold, still air makes botrytis most likely, so you will need to vent briefly at midday to shed it.
Control beats capacity
A big heater running unregulated wastes more money than a small heater running on a good thermostat saves. Put every heat source on a thermostat sited at plant height in the middle of the house, away from the heater outlet and away from the door - the two places that will lie to it.
A max-min thermometer is the other essential, and it costs almost nothing. A week of readings tells you what your greenhouse actually does overnight, which is nearly always different from what you assumed. Buy the heater after that week, not before.
A remote temperature alarm is worth it if you are protecting plants you cannot replace. The failure mode that hurts is not a cold night; it is a heater that quietly stopped working on Friday and was discovered on Sunday.
Getting a greenhouse ready for winter
- Clean the glazing inside and out - winter light is the scarce resource and dirty panels cost you real growth.
- Check and reseal panel edges and the base line, and weather-strip both doors.
- Insulate the north wall and lower north roof with horticultural bubble; leave the south face clear.
- Fill and place water barrels or containers for thermal mass along the north side.
- Hang the max-min thermometer and the thermostat at plant height, mid-house.
- Run the circulation fan on low all winter to prevent still, humid pockets.
- Size and fit the heater from real readings, and test it before the first hard frost.
- Fit a carbon monoxide alarm if the heater burns anything, and keep a permanent air inlet.
Greenhouses built for winter use
Twinwall polycarbonate on hot-dip galvanized steel, rated to shed 5 ft of snow. Choose 6 mm panels if you are growing through a cold winter.
8.3ft Gable Roof Walk-In Polycarbonate Greenhouse Kit - Gable Series
From $1,589.00
10ft Straight Wall Walk-In Polycarbonate Greenhouse Kit - Loft Series
From $1,589.00
10ft Walk-In Polycarbonate Greenhouse Kit - Arc Series
From $1,489.00
Greenhouse heating questions
How do I keep a greenhouse warm in winter without electricity?
Stack the passive measures: twinwall glazing, thermal mass such as black water barrels, bubble insulation on the north wall, sealed door gaps, and a fleece tent around the plants that actually need protection. In a mild zone this can hold frost-free on its own. In a cold zone it substantially reduces what a heater has to do rather than replacing it.
What size heater do I need for my greenhouse?
Multiply the glazed surface area in square feet by the temperature difference you want to hold in degrees F, then by about 0.7 for twinwall polycarbonate or about 1.15 for single-layer glass or film. That gives BTU per hour. A 1,500 W electric heater supplies roughly 5,100 BTU per hour.
Is a polycarbonate greenhouse warmer than glass?
Twinwall polycarbonate loses heat considerably more slowly than single-glazed glass because of the sealed air gap between its skins. Glass transmits slightly more light; twinwall keeps more heat. For winter growing the insulation usually matters more.
Can I use a propane heater in a greenhouse?
Yes, and it is common in larger or colder installations, but combustion consumes oxygen, releases carbon monoxide and adds water vapour. Fit a CO alarm, keep a permanent air inlet open, and vent briefly at midday to shed the added humidity.
What temperature should a greenhouse be kept at in winter?
It depends on what you are protecting. 35 to 40F keeps dormant and tender plants alive. 45 to 50F supports continuous salad and herb harvests. 60F and above is needed for winter tomatoes and tropicals and costs several times as much to hold.