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Loss depends on surface area, U-value of glazing, and temperature difference between inside and outside.
BTU/hr ≈ U × area × ΔTAir leaks and frequent door openings add load. Vented poly houses lose more heat than double-wall polycarbonate.
Add 10% to 25% margin so the heater can recover after a cold snap or windy night.
Updated: July 2026
Compare heat load at 45°F inside when outside drops to 15°F in USDA zone 5.
See how better glazing cuts BTU needs and heater cost over a season.
Model lower night temperatures to right-size heaters for frost protection only.
Design for the lowest expected night temperature plus wind, not the monthly average.
Open fields and ridge-top sites increase infiltration loss. Add margin or use windbreaks.
Size a heater before cold nights arrive. Enter greenhouse dimensions, insulation, and temperature targets to estimate heat loss and required BTU output.