A greenhouse loses heat through every square foot of glass or plastic, and much faster than a house wall. To keep frost-tender plants alive on the coldest night, the heater has to replace that loss. The standard method, used by extension services, multiplies the covered surface by the heat loss coefficient of the covering and by the temperature difference you want to hold.
The formula
Heat loss (BTU/h) = surface area (sq ft) × U × (inside - outside temperature) × air leakage factor
In metric units, the same formula gives watts with the area in square meters, U in W/m²·K and temperatures in degrees Celsius.
U-values of common coverings
| Covering | U (BTU/h·ft²·°F) | U (W/m²·K) |
|---|---|---|
| Single glass | 1.1 | 6.2 |
| Single polyethylene film | 1.2 | 6.8 |
| Double inflated polyethylene | 0.7 | 4.0 |
| Twin-wall polycarbonate | 0.6 | 3.4 |
| Triple-wall polycarbonate | 0.5 | 2.8 |
The lower the U-value, the better the insulation. A double layer of film or twin-wall polycarbonate cuts heating costs by about 40% compared with single glass.
The surface
The calculator adds the two side walls, the two end walls with their gable triangles and the two roof slopes of a gable greenhouse. A lean-to against a heated house loses less: enter only the exposed walls by reducing the width or the wall height.
Example
A 12 × 8 ft twin-wall polycarbonate greenhouse with 6 ft walls and an 8 ft ridge has about 363 sq ft of surface. To hold 45 °F on a 20 °F night with average air tightness, it needs about 6,300 BTU/h, or 1.8 kW: a 2 kW electric heater with a thermostat is enough.
Choosing the heater
- Electric fan heaters are clean and precise, ideal for small greenhouses with power.
- Propane or natural gas heaters suit larger greenhouses but must be vented to the outside or rated for greenhouse use; unvented burners release ethylene and moisture that harm plants.
- Kerosene and paraffin heaters are cheap but give off fumes; ventilate.
Size the heater 10 to 20% above the calculated loss for very windy sites.
Cutting the bill
Bubble wrap on the inside of the glazing, a thermal screen at night, sealing gaps around doors and vents, and a thermostat at the lowest temperature your plants tolerate all cut the energy use. Every degree lower on the thermostat saves roughly 3 to 4%.