Find the right heater size for your greenhouse based on your dimensions, covering material, and local cold temperatures. Sized for your worst-case winter night.
The formula is: BTU/hr = (1/R-value) × surface area × temperature difference. Calculate total surface area (walls + roof), divide by your covering material's R-value, multiply by the temperature difference between your desired inside minimum and the coldest expected outdoor temperature. Add 15–25% for air infiltration.
Triple-wall polycarbonate (R-2.5) provides the best insulation among common coverings. Double-layer inflated poly film (R-1.5) is the most cost-effective option for cold climates. Single-pane glass (R-1.0) loses heat quickly but is durable and traditional.
It depends on what you're growing. Tropical plants need 55–65°F minimum nights. Cool-season crops like lettuce, spinach, and kale survive at 28–40°F. Hardy greens can tolerate light frost. Most hobby greenhouses target 35–45°F as a minimum to prevent freezing while keeping heating costs reasonable.