BTU/Hour Needed
33,800
Daily BTU
811,200
ΔT
65°F
Floor Area
240 sq ft
BTU/Hour Required
33,800
811,200 BTU/day
Surface Area
800 sq ft
Temp Diff
65\u00B0F
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Inputs
Result
Wall area = 2(20×8)+2(12×8) = 512. Roof = 20×12×1.2 = 288. Total = 800 sq ft. BTU = 800 × 0.65 × 65 = 33,800.
Inputs
Result
Wall = 2(12×7)+2(8×7) = 280. Roof = 12×8×1.2 = 115. Total = 395. BTU = 395 × 0.58 × 35 = 8,018.
BTU needs depend on surface area, glazing type, and temperature difference. A 20×12 ft double-glazed greenhouse in Zone 6 (-10°F low) targeting 55°F needs about 23,000–28,000 BTU/hour. Single-glazed needs almost double that.
Twin-wall polycarbonate has the lowest U-value (0.58) of common greenhouse glazing. Double glass is close at 0.65. Single glass and polyethylene film are poor insulators with U-values above 1.0.
| Glazing | U-Value | Relative Heat Loss | Cost/sq ft |
|---|---|---|---|
| Polycarbonate | 0.58 | Lowest | $1.50–$3 |
| Double Glass | 0.65 | Low | $3–$6 |
| Single Glass | 1.13 | High | $1–$3 |
| Poly Film | 1.20 | Highest | $0.05–$0.10 |
Natural gas is cheapest per BTU where available ($10–15/million BTU). Propane costs $20–30/million BTU. Electric is most expensive at $30–50/million BTU but requires no venting. Many growers use propane with a backup electric heater.
Most cool-season crops survive at 40–45°F minimum. Warm-season seedlings need 55–65°F. Tropical plants need 60–70°F. Every 5°F increase in target temperature roughly doubles your heating cost.
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Last Updated: Mar 19, 2026
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