Estimate the right heater size for your greenhouse using surface area, temperature difference, and your glazing’s insulation value.
A greenhouse that stays too cold through a January night can undo months of work in a single freeze.
What Determines Your Greenhouse Heating Needs?
Three factors drive the final BTU number, and missing any one of them leads to an undersized heater that fails on the coldest night of the year.
Total exposed surface area. Heat escapes through the walls and roof, not the floor. Measure every exposed surface—sidewalls, end walls, and both roof slopes on a peaked design. For a gable greenhouse, approximate the volume as length × width × (wall height + half the peak rise above the wall).
The temperature gap (ΔT). Use the coldest winter night temperature your area sees—the design low that hits once every few years—and the minimum temperature your plants need inside. The difference between them is your ΔT.
The covering’s U-value. Glass, polycarbonate, and polyethylene film lose heat at very different rates. Single-pane glass sheds heat much faster than double-layer polycarbonate with an inflated inner skin, and that U-value is the single biggest variable in the equation.
How To Calculate Heater Size Yourself
Run it on a real example. An 8×10-foot gable greenhouse with 6-foot walls and a 4-foot peak rise has roughly 340 square feet of exposed surface area. With single-polycarbonate glazing (U-value about 1.2) and a 40°F temperature rise—60°F inside on a 20°F night—the math is 1.2 × 340 × 40 = 16,320 BTUs per hour.
| Glazing Material | Typical U-Value (BTU/hr·ft²·°F) | Notes |
|---|---|---|
| Single glass | 1.1–1.3 | Highest heat loss |
| Single polycarbonate | 1.0–1.2 | Similar to glass |
| Double polycarbonate | 0.6–0.8 | ~30% less loss than single |
| Polyethylene film (single) | 1.0–1.2 | Common for hoop houses |
| Double-layer poly with inflation | 0.5–0.7 | Inner liner cuts loss further |
Add a margin for air leakage. No greenhouse is airtight. That pushes the 16,320-BTU example to roughly 19,000 BTUs an hour.
Account for fuel-fired heater efficiency. The rated output on a propane or natural gas heater is what reaches the space—vented models lose some heat up the flue. Match the heater’s rated output to your load, not its fuel input rating.
Common Mistakes That Lead To The Wrong Size
The most frequent error is measuring floor area instead of surface area—that can cut the calculated load in half and leave you with a heater that can’t keep up on the worst nights. Omitting the roof from the surface calculation is nearly as common, and that’s the biggest heat-loss surface on most greenhouses. Using the average winter temperature instead of the coldest design night produces a heater that works fine in November but fails in January. Applying the wrong U-value for your actual glazing or forgetting to add the air-leakage margin are both easy misses that lead to a load estimate 15–25% too low.
Once you have your BTU target, you can match it against real options.
FAQs
Do I include the floor in the surface area?
No. Heat loss through the ground is negligible compared to the walls and roof, and the standard formula only uses exposed surfaces. Some cold-climate builders add a small perimeter-loss factor, but the floor itself never goes into the BTU calculation.
Can I use a regular space heater instead of a greenhouse heater?
You can, but most portable space heaters lack the safety features greenhouse units require—sealed combustion, pilot safety valves, and weatherproof housings. A standard indoor heater running in a damp, dusty greenhouse creates fire and carbon-monoxide risks that dedicated greenhouse heaters are designed to avoid.
What if my calculated size falls between two heater models?
Round up to the next larger size. A slightly oversized heater cycles less often on very cold nights and gives you margin for wind, unexpected temperature drops, or future insulation upgrades. Undersizing by even a few thousand BTUs can leave your plants exposed during the worst weather of the year.
References & Sources
- Purdue University. “Calculating Greenhouse Heating Requirements.” Provides the Q = U × A × ΔT formula and emphasizes U-value importance.