Where the heat actually goes
A greenhouse loses heat almost entirely through its glazing, and the rate is straightforward: the U-value of the material, times the glazed area, times the temperature difference between inside and outside. Single glass has a U-value around 5.8 W/m²K, which is very poor — roughly four times the loss of a modern house window.
The floor barely features. Ground under a greenhouse stays substantially warmer than night air and acts as a heat store rather than a loss surface, which is why the calculation here counts walls and roof only. The roof is taken as about 1.15 times the footprint to allow for the pitch.
The temperature difference is the term people underestimate. Holding 5 °C when it is −5 °C outside is a 10-degree lift. Holding 10 °C on the same night is a 15-degree lift — half as much again, for a greenhouse that most winter crops do not need.
| Glazing | U-value | Relative loss |
|---|---|---|
| Single glass | 5.8 | 100% |
| Twin-wall polycarbonate | 3.5 | 60% |
| Glass + bubble wrap lining | 3.2 | 55% |
| Triple-wall polycarbonate | 2.4 | 41% |
Bubble wrap is the best return in this calculation
Lining the inside of a single-glazed greenhouse with horticultural bubble wrap takes the effective U-value from about 5.8 to about 3.2 — a 45% cut in heat loss for the price of a roll and an afternoon.
Nothing else available comes close on cost per watt saved. Replacing the glazing costs hundreds and saves less. A bigger heater saves nothing at all; it costs more to run.
The trade-off is light. Bubble wrap cuts transmitted light by roughly 10%, which matters in midwinter when there is little to spare. The usual compromise is to line it in autumn and take it down in early spring, so the dark months get the insulation and the growing months get the light.
Seal the gaps as well. A greenhouse with a door that does not close properly or a missing pane loses far more through that gap than the glazing calculation suggests, because it is losing warm air rather than conducting heat.
Frost-free costs a fraction of warm
There are two quite different reasons to heat a greenhouse and they cost very different amounts. Keeping it frost-free — holding around 5 °C so overwintering plants survive — is cheap. Keeping it warm enough to actively grow in, 12 to 15 °C, can cost three or four times as much for the same building.
The calculator shows the cost of one cold night and the cost across 120 cold nights, which is roughly a northern winter. That second figure is the one worth looking at before buying a heater, because it is often larger than people expect and it changes what is worth overwintering.
The duty cycle assumption matters here. A thermostatically controlled heater does not run continuously; over a cold night it might run 45% of the time, which is what this uses. A heater without a thermostat runs constantly and costs more than double. If you take one thing from this page, it is that a thermostat pays for itself in the first month.
For a small greenhouse there is a cheaper option again: heat the plants rather than the building. A heated propagator bench or a fleece tent over one section holds a few square metres above freezing for a fraction of the cost of the whole volume.
Common questions
What size heater do I need for my greenhouse?
Multiply the glazed area by the U-value of the glazing by the temperature lift you want. An 8 × 6 ft single-glazed greenhouse held at 5 °C when it is −5 °C outside needs roughly 1.5 kW.
Is bubble wrap worth it in a greenhouse?
It is the best return available. Lining single glass takes the U-value from about 5.8 to 3.2 — a 45% cut in heat loss for the price of a roll. The cost is roughly 10% of transmitted light.
How much does it cost to heat a greenhouse over winter?
Frost-free at 5 °C in a small insulated greenhouse is usually modest. Holding 12–15 °C for active growing can cost three to four times as much in the same building.