How to Heat a Greenhouse Efficiently

Insulation Comes First

Before you invest in a heater, minimize how much heat your greenhouse loses. Heating an uninsulated greenhouse is like running the furnace with the windows open—expensive and ineffective.

Bubble wrap insulation is the classic greenhouse hack. Standard large-bubble horticultural bubble wrap (UV-stabilized) applied to the interior walls and ceiling with clips adds a dead-air layer that significantly reduces heat loss. It cuts light transmission by about 10–15% but can reduce heating costs by 30–50%. Apply it in fall and remove in spring.

Seal gaps and leaks. Walk through your greenhouse on a cold day and feel for drafts around door frames, vent edges, panel joints, and the base. Foam weather stripping, silicone caulk, and panel clips address most leaks. A poorly sealed greenhouse can lose 10–20% of its heat through air infiltration alone.

Polycarbonate glazing matters. If you’re still using single-pane glass or single-layer poly film, the single biggest efficiency upgrade is switching to twin-wall (or thicker) polycarbonate. The insulation improvement is dramatic—roughly 2× the R-value of single glass. See our Greenhouse Types guide for a full comparison.

Using Thermal Mass

Thermal mass absorbs heat during the day and releases it slowly at night, buffering your greenhouse against temperature swings for free. Common thermal mass approaches:

• Water barrels: Paint 55-gallon drums black and line them against the north wall. Water holds roughly 4× more heat energy per volume than air. Four barrels can moderate overnight lows by 5–10°F in a small greenhouse.
• Stone or concrete floors: A concrete slab or stone paver floor absorbs daytime heat and radiates it back at night. Also provides clean, durable, well-drained flooring.
• Soil beds: In-ground growing beds act as thermal mass. Soil is a decent heat store, and raised beds with dark mulch absorb even more solar energy.

Right-Sizing Your Heater

An oversized heater wastes energy cycling on and off. An undersized heater runs constantly and still can’t maintain temperature. The sizing formula:

BTU/hr needed = Total surface area (sq ft) × Desired temp rise (°F) × Heat loss factor

Heat loss factors: single glass/poly ≈ 1.0–1.2, twin-wall polycarbonate ≈ 0.6–0.7, multi-wall PC (8mm+) ≈ 0.4–0.5.

For most small hobby greenhouses (6×8 to 8×12 with twin-wall polycarbonate), a 1,500W electric heater (5,120 BTU) handles moderate winters. Cold climates or large structures need proportionally more. See our Best Greenhouse Heaters roundup for specific product recommendations.

Reducing Heat Loss

Beyond insulation, several practical steps minimize heat escape:

• Close vents at night. If you have manual vents, close them before sunset. Automatic vent openers close naturally as temperature drops.
• Use a thermal curtain. A reflective or heavy-duty fabric curtain drawn across the peak of the greenhouse at night reduces the volume of heated air and reflects radiant heat back down to plant level.
• Avoid heating unused space. If you’re only growing on one bench, partition off the growing area with plastic sheeting and heat just that zone.
• Windbreaks. A fence, hedge, or building wall on the windward side reduces convective heat loss. Wind multiplies heat transfer through glazing surfaces.

Automating Heat Control

A thermostat is essential. Without one, you’re either overheating (wasting energy) or underheating (losing plants). Options range from affordable plug-in thermostats (BN-LINK, Inkbird) to WiFi-enabled smart controllers that let you monitor temperature from your phone and set different day/night targets.

Set your thermostat to the minimum temperature your plants need, not the maximum you’d prefer. Every degree above the minimum costs energy. Most vegetables survive overnight lows of 45–50°F; setting a thermostat to 60°F “just in case” adds significant cost over a winter season.

Keeping Costs Down

Heating a greenhouse year-round can be expensive, but these strategies keep the bill manageable:

• Heat only when you’re growing. If you’re not growing through the coldest months, don’t heat. Restart in late winter when you begin seed starting.
• Zone heating. Heat seedling mats rather than the entire greenhouse for seed starting. Heat one bench rather than the whole space.
• Solar supplementation. A small solar panel setup can power fans and monitoring equipment. See Solar Panel Kits for off-grid options.
• Choose the right fuel. Electric is cleanest and easiest for small spaces. Propane is often cheaper per BTU for larger greenhouses in areas with expensive electricity.