Greenhouse Heater Calculator

The Greenhouse Heat Loss Formula

Heat loss through the glazing is BTU/hr = surface area × temperature difference × U-value. Surface area is the total exterior surface of the greenhouse in square feet, calculated as 2 × (LW + LH + WH) for a rectangular structure. Temperature difference is the desired inside temperature minus the lowest expected outside temperature. U-value is the material constant — 1.15 for single poly, 0.65 for double glass or twin-wall polycarbonate, and so on. Lower U-value means better insulation.

Choosing Glazing Materials

Single-layer polyethylene is the cheapest option and the least insulating — a 6 mil sheet runs 0.10 per square foot but has a U-value of 1.15. Double-inflated poly (two layers with a fan blowing air between them) cuts U to 0.70 and is the commercial hoop-house standard. Twin-wall polycarbonate is rigid, long-lasting, and has a U-value around 0.65, comparable to double-pane glass. Glass is heavy, expensive, and fragile but beautiful. Fiberglass was once popular but yellows with age. For a home hobby greenhouse, twin-wall polycarbonate is the best balance of cost, durability, and insulation.

Factors That Increase Heat Loss

Wind is the biggest additional factor — a 15 mph wind roughly doubles effective heat loss, while still air matches the calculated number. Infiltration (air leaking through gaps, seams, and vents) typically adds 10 to 20 percent to the base number. A greenhouse that is partly buried or bermed loses much less heat through the below-grade walls. Orientation matters too — a greenhouse with its long axis running east-west captures more winter sun and needs less supplemental heat than one running north-south.

Passive and Renewable Strategies

Before buying a big heater, consider thermal mass (barrels of water), row covers inside the greenhouse, and simply accepting 35-40°F minimum instead of 50°F for frost-hardy crops. A small greenhouse with 50 gallons of water inside, double-poly glazing, and row cover over seedlings can stay above freezing down to 20°F outside without any active heat. For longer cold spells, a small propane heater sized to the worst night is enough. Commercial growers increasingly use geothermal loops and biomass boilers for large greenhouses to cut fuel costs by 50 percent or more.

Frequently Asked Questions

How many BTU per square foot of greenhouse?

Roughly 40 to 80 BTU/hr per sq ft of floor area, depending on glazing and ΔT.

Can I use a propane heater?

Yes — vented propane heaters are the most common choice. Unvented propane releases water that raises humidity.

Is electric cheaper than propane?

Usually no for big greenhouses, yes for small hobby structures under 200 sq ft.

Do I need a backup heater?

If you're growing high-value plants, yes — any cold snap can kill an entire crop in hours.

How do thermal curtains save energy?

Pulled at night, thermal curtains cut heat loss by 30 to 50 percent.