How to Keep Greenhouse Warm in Winter | Heat Without the Waste

Winter greenhouse success combines sealing drafts with insulated glazing, thermal mass like water barrels, and safe heater placement low and central—the first night you hit 39°F tells you it’s working.

The greenhouse that grows through January isn’t the one with the biggest heater. It’s the one where every gap is sealed, the glass is wrapped, and a few black water barrels soak up daytime sun to release it overnight. The heater is the safety net, not the system. Here’s how to layer all of it so your plants stay warm and your electric bill doesn’t ruin the project.

Seal and Insulate Before You Heat

Cold air leaking through a loose pane or vent opening is the single biggest reason greenhouse heaters waste electricity. Track down drafts the old way—a match or lighter near seams: if the flame wavers, cold air is coming in. Seal that spot with weatherproof tape or silicone sealant, and replace any cracked glass.

Cut polythene bubble wrap to size and fix it to the frame on the inside, running as close to the glass as possible. This is the most effective single method for cutting heat loss. If cladding the whole interior feels like too much, at least section off the growing area with a bubble-wrap curtain or a polythene sheet to trap warmth where plants sit.

RHS guidance emphasizes that a sealed greenhouse holds heat far longer than a drafty one, even with the same heater. For readers ready to upgrade, our tested best greenhouse cover for winter roundup covers insulation blankets and poly films that outperform bubble wrap on the coldest nights.

Thermal Mass: Free Heat You Build Once

Water barrels are the cheapest thermal battery you own. Place dark-colored drums—55-gallon is ideal—along the north wall where they get direct sun. During the day they absorb heat; after dark, they radiate it back into the space. The rule of thumb: 2 to 4 gallons per square foot of floor area, with the lower number for milder winters and the higher one for harsh climates.

Keep soil in pots moist, not wet—moist soil stores more heat than dry soil and adds to the greenhouse’s overall thermal mass. A flagstone pathway or concrete blocks along the south-facing side also absorb peak sun and release it through the night.

Active Heating: Choose the Right Method

Electric fan heaters are the most controllable option. Place the heater in an open, central spot at one end of the greenhouse, set low—warm air rises naturally, so low placement moves it across the plants. Buy heaters specifically listed for greenhouse use, which include damp-environment safety features.

If electricity isn’t available, a paraffin heater or propane heater can supplement on especially cold nights. A kerosene or wood-burning stove is a heavier option but works for off-grid setups. Whichever fuel you use, keep a small circulation fan running to mix warm air and prevent cold pockets from forming.

Protect Plants During Severe Snaps

Maintain a night minimum of 39–41°F (4–5°C) for most hardy greenhouse plants. That range buffers against sudden dips. When the forecast says single digits, drape one or two layers of horticultural fleece directly over vulnerable plants at dusk and remove it during the day so light and air reach the leaves.

Low tunnels—hoops covered with plastic film or row cover—create a second warm air pocket inside the greenhouse, adding several degrees of frost protection. Group plants with similar temperature needs together; keep potted plants slightly drier in winter to help roots handle cold. Water sparingly and early in the day so foliage dries before night, because wet leaves and still air invite rot.