Quick answer: Size the heater from the greenhouse's heat loss: Q (W) ≈ U × surface area × (inside − outside temperature), plus air leakage. Typical U-values are about 6 W/m²·K for single glass or single poly, about 4 for double inflated poly and about 3.4 for twin-wall polycarbonate. Seal leaks, use energy screens at night, and distribute heat low near the crop. Remember that heating lowers RH but removes no water: in winter, humidity and condensation peak at night when vents are shut, so combine heating with short heat-and-vent cycles or dehumidification.
Quick answer
- Heat loss ≈ U × glazing area × temperature difference (+ leakage).
- U-values (W/m²·K): single glass/poly about 6, double poly about 4, twin-wall PC about 3.4.
- Heating alone does not dry the air; plan moisture removal for winter nights.
Step 1: Estimate heat loss
| Glazing | Approximate U-value (W/m²·K) |
|---|---|
| Single glass | about 6.2 |
| Single polyethylene film | about 6.0 |
| Double inflated polyethylene | about 4.0 |
| Twin-wall polycarbonate | about 3.4 |
Values are typical figures used in greenhouse heating guides; manufacturer data and wind exposure change them.
Worked example: a 10 × 30 m double-poly greenhouse with about 600 m² of glazed surface (roof, walls and ends). Design inside 15 °C, outside −5 °C (ΔT = 20 K):
- Conduction: 4.0 × 600 × 20 = 48 kW.
- Leakage: add about 10–25 % for a well-sealed house, more for older or leaky ones: roughly 55–60 kW total.
The USDA NRCS guide on controlling the high tunnel environment covers heating and ventilation for tunnels (USDA NRCS).
Step 2: Reduce the loss
- Seal gaps, doors and vents; fix torn film.
- Energy screens drawn at night reduce heat loss substantially.
- Double layers or bubble insulation on walls that don't need light.
- Windbreaks on the exposed side.
- Insulate the base (foundation walls).
Step 3: Choose and place heating
| Heating | Notes |
|---|---|
| Hot-water pipes near the crop | Even heat at plant level; dries the canopy; common in commercial houses |
| Unit heaters (gas, oil) with fans | Lower cost; must vent combustion gases outside unless designed otherwise |
| Heat pumps | Efficient where electricity is affordable; can also dehumidify in some designs |
| Electric heaters | Small greenhouses, frost protection |
| Under-bench or floor heating | Root-zone warmth for propagation |
Use horizontal airflow fans to mix warm air and avoid cold spots.
Winter humidity: why heating isn't enough
On a winter night the house is closed, the crop keeps transpiring, and the glazing is the coldest surface. Water condenses on the roof and drips onto plants.
- Heating lowers RH but the water stays.
- Venting works because cold winter air holds little water: air at 0 °C and 90 % RH holds about 4.4 g/m³, compared with about 10.9 g/m³ in house air at 15 °C and 85 % RH. Short heat-and-vent cycles at dusk and dawn exchange moist air cheaply.
- Dehumidifiers remove water without venting, returning the condensation heat to the house, useful when screens are closed or outside air is not dry enough.
See greenhouse night-time humidity, heat-recovery ventilation vs dehumidifiers and our greenhouse dehumidifiers. Wageningen research covers heating and dehumidification in production greenhouses (WUR).
Winter checklist
- Heater sized for design cold night plus margin
- Backup heat or frost alarm
- Energy screen with a gap control for humidity
- HAF fans running
- Heat-and-vent or dehumidification plan for nights
- Sensors at canopy height; check condensation on glazing in the morning
FAQ
Frequently asked questions
How do I calculate greenhouse heating needs?
U-value × glazed surface area × (inside − outside temperature), plus 10–25 % or more for leakage.
What is the cheapest way to heat a greenhouse?
Reduce losses first (sealing, screens, double layers), then choose the lowest-cost fuel available; distribute heat evenly near the crop.
Why is my greenhouse so humid in winter?
The house is closed, plants transpire, and the cold glazing condenses moisture. Heating lowers RH but does not remove water; vent briefly or dehumidify.
Sources
- USDA NRCS, Controlling the high tunnel environment.
- Wageningen University & Research, Heating and dehumidification in production greenhouses.
- ASHRAE Handbook—HVAC Systems and Equipment.
