Greenhouse Heating Calculator

Heat leaves a greenhouse through its glazed surface, not through its volume. That distinction matters more than it sounds, because a great many greenhouse calculators multiply cubic feet by a made-up coefficient and hand you a number three or four times too large. Buy a heater on that basis and you have paid for capacity you will never use, and it will short-cycle all winter instead of running steadily.

The formula:

Heat loss = Glazed surface area × U-value × ΔT

Add 10% on top for infiltration, the cold air that works its way in around doors, vents and the base rail. A greenhouse is not an airtight box, and no amount of good glazing changes that.

What each variable means:

• Glazed surface area is walls plus roof plus gable ends. Not the floor, which is the number everyone reaches for first, and not the volume.
• U-value is how fast heat crosses the glazing, per square foot, per degree of difference. Lower is better.
• ΔT is your target inside temperature minus the coldest night you design for. Use the coldest night you actually expect, not the average winter low, because the one night a heater fails to keep up is the night you lose the crop.

Working out the surface area

For a traditional gable greenhouse of length L, span W, wall height Hw and ridge height Hr, with rise r = Hr − Hw:

Side walls = 2 × L × Hw Gable ends = 2 × [(W × Hw) + (½ × W × r)] Roof = 2 × L × √((W/2)² + r²)

For a hoop house or polytunnel, the cross-section is a half circle of diameter W:

Arch = π × (W/2) × L, Ends = 2 × (π × W² / 8)

Worked example: A 20 ft long, 12 ft wide gable greenhouse with 6 ft walls and a 10 ft ridge. Rise is 4 ft.

• Side walls: 2 × 20 × 6 = 240 ft²
• Gable ends: 2 × [(12 × 6) + (½ × 12 × 4)] = 192 ft²
• Rafter length: √(6² + 4²) = 7.21 ft, so roof = 2 × 20 × 7.21 = 288 ft²
• Glazed surface: 720 ft², against a floor of only 240 ft²

Holding 50°F inside on a 20°F night is a ΔT of 30°F. With twin-wall polycarbonate: 720 × 0.62 × 30 = 13,400 BTU/hr, and 14,700 BTU/hr once infiltration is added. A 20,000 BTU propane heater covers it with headroom for a colder-than-design night.

Note the surface-to-floor ratio: three to one. That ratio is why greenhouses are expensive to heat and why doubling the glazing pays for itself faster than almost any other upgrade. Going from single poly to twin-wall on this greenhouse cuts the heater from 26,900 to 14,700 BTU/hr, close to half the fuel for every hour of every winter.