Method
Conduction: Q = U × A × ΔT
Infiltration: Q = 1.08 × CFM × ΔT with CFM = ACH × Volume / 60
This tool approximates a rectangular envelope. For arched or gothic houses, use the surface area override for the most accurate results.
Results
Temperature difference ΔT
50.00 °F
Surface area used
1,350.00 ft²
Volume
4,500.00 ft³
Conduction loss
47,250 BTU/hr (85.4%)
Infiltration loss
8,100 BTU/hr (14.6%)
Total before safety
55,350 BTU/hr
Safety margin
15 %
Recommended heater size
63,653 BTU/hr
Equivalent
18.65 kW
Tips
- Use the surface area override for arched or gothic frames.
- Lower U-values reflect better insulation or thermal curtains.
- ACH rises with leaks and wind; sealing reduces heater size.
- Consider zoning and circulation to prevent stratification.
FAQs
Single glass is around 1.1, double poly film about 0.7, twinwall polycarbonate near 0.6, and triplewall around 0.35 BTU/hr·ft²·°F. Actual values vary by product and installation.
Many designs assume soil or slab floors lose less heat than glazing. Excluding the floor is common. If your floor is exposed to cold or uninsulated edges, include it.
Tight houses may be 0.5–1.5 ACH. Average builds 1.5–2.5. Older or leaky structures 2.5–4.0+. Local wind exposure and construction quality strongly affect ACH.
It is a useful approximation. Arched, gothic, and Quonset styles have more area than a box. If known, enter the measured wall and roof area as an override.
Heaters cycle and weather varies. A margin covers wind gusts, door openings, temperature sensor lag, and unexpected leaks to keep crops within setpoint.
Yes. Choose Metric to enter meters and Celsius. The tool converts to BTU/hr internally and also reports an equivalent kW value.
No. The calculator sizes for worst case night or cloudy conditions. Daytime sun can offset heating but is unreliable for peak sizing.
Select a model with rated output at or above the recommended BTU/hr. Consider redundancy with two smaller units for reliability and staged control.