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Greenhouse Heating Load Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Formula based on ASAE EP406 and ASHRAE Fundamentals heat loss methodology. Last verified 14 June 2026.

Sizing your greenhouse heater correctly ensures crops survive cold nights without overspending on equipment or fuel. The peak heating load calculation from ASAE EP406 multiplies the surface area, glazing U-value (heat loss coefficient), and design temperature difference to find the BTU per hour needed at the coldest outdoor design temperature. Add an infiltration allowance of 10 to 25 percent for realistic sizing.

Total vertical rise of gable (both slopes)
ASHRAE 99% winter design temp for your location
Typically 10 to 25% (ASAE EP406)
2,688.00 sq ft
94,080.00 BTU/hr
108,192.00 BTU/hr

Greenhouse heating load formula (ASAE EP406)

Q(conduction) = U x A x (T(inside) - T(outside))
Q(total) = Q(conduction) x (1 + infiltration fraction)
Where: U = glazing U-value (BTU/hr/sq ft/F), A = total surface area (sq ft)

Surface area is calculated as the sum of end walls, sidewalls, and roof panels. End wall area for a gable greenhouse includes the triangular gable area above the sidewall. This formula follows ASAE EP406 (Heating, Ventilating, and Cooling Greenhouses).

Glazing U-values (ASAE EP406)

  • Single glass: U = 1.13 BTU/hr/sq ft/F (R-0.88).
  • Double-layer polyethylene film with air space: U = 0.70 (R-1.43).
  • Double-wall 8 mm polycarbonate: U = 0.58 (R-1.72).
  • Triple-wall polycarbonate: U = 0.38 (R-2.63).

Greenhouse heating calculator: frequently asked questions

How is greenhouse heating load calculated?

The peak heating load (BTU/hr) = U-value (BTU/hr/sq ft/F) x Surface area (sq ft) x Design temperature difference (F). The U-value depends on glazing type: single glass is approximately 1.13, double-layer polyethylene is approximately 0.70, and double-wall polycarbonate is approximately 0.58 BTU/hr/sq ft/F, per ASAE EP406.

What is the design temperature difference?

The design temperature difference (delta-T) is the inside design temperature minus the outside design temperature. The outside design temperature is the 99th-percentile winter low temperature for your location, available from ASHRAE Fundamentals (Chapter 14) or the NOAA Climate Design Data tables. For example, if you want to keep the greenhouse at 60 F and your design winter low is 10 F, delta-T = 50 F.

What U-value should I use for double-wall polycarbonate?

Double-wall 8 mm polycarbonate panels have a published U-value of approximately 0.58 BTU/hr/sq ft/F (R-1.72). Triple-wall polycarbonate is approximately 0.38 BTU/hr/sq ft/F (R-2.6). These values are from ASAE EP406 and manufacturer published data. Use the manufacturer's published U-value for your specific panel product.

Should I add an infiltration allowance to the calculated load?

Yes. Greenhouse structures typically have higher infiltration than residential buildings. ASAE EP406 recommends adding 10 to 25 percent to the conduction load to account for air infiltration through vents, doors, and gaps. The calculator includes an infiltration factor field for this purpose.

How do I choose the right heater size?

Select a heater with a rated output equal to or greater than the calculated peak heating load. Always size for the peak load, which occurs on the coldest night. Adding 10 to 20 percent safety margin is common practice. For large greenhouses, multiple smaller heaters provide redundancy and more even heat distribution.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.