How to Keep a Small Greenhouse Warm? | Cut Heat Loss First

For the full breakdown, see our best Small Heated Greenhouse guide.

A small greenhouse loses heat quickly because it has a large surface-area-to-volume ratio. That’s why the most reliable strategy for how to keep a small greenhouse warm starts with stopping that heat loss, then adding thermal mass, and only then bringing in active heat if the temperature still drops too far. Sealing gaps, insulating glazing, and using water barrels as heat batteries can often make a heater unnecessary.

Seal Drafts and Insulate Before Adding Heat

This is the single most effective step. Heating a greenhouse that leaks warm air through gaps around frames, doors, and vents wastes energy and money. STIGA’s winter greenhouse guidance recommends using weather-proofing tape around vents, doors, and windows to reduce draughts, and pinning cut-to-size bubble wrap to the inside wooden frame.

For best results, put bubble wrap or similar glazing insulation on the side that gets the least sun. Dividing larger interior areas with bubble wrap creates smaller pockets of air that retain warmth better. Mulching over soil also helps protect plant roots from cold.

Add Thermal Mass to Store Daytime Sun

Thermal mass is the cheapest heat source you already have: the sun. Water barrels painted black, placed along a wall that receives sunlight, absorb heat during the day and slowly release it at night. Stone, brick, gravel, or masonry work the same way when positioned where they catch sun.

At night, adding row covers, fleece, or translucent sheeting directly over plants traps the day’s warmth around the foliage. STIGA also recommends orienting a greenhouse to maximize winter sun exposure — true south in the Northern Hemisphere gives the best solar gain from late autumn through early spring.

Choosing the Right Greenhouse Heater

If passive measures don’t hold the temperature high enough, a controlled heater is the reliable next step. The Royal Horticultural Society (RHS) recommends buying heaters specifically sold for greenhouses as a safety measure, and distinguishes between two main types:

• Tube heaters deliver lower-energy background heat, mainly for frost protection.

For seedlings and root zones, heat mats or soil-heating cables work well, but they must be outdoor-rated for damp greenhouse conditions. Position any heater away from direct contact with foliage or trays, since it can dry out or damage plants. STIGA recommends keeping a minimum of 4–5°C at night to protect against snap frosts.

Electric heaters require mains power. If you’re off-grid or want resilience during a power failure, the non-electric methods above — insulation, thermal mass, and row covers — are your main tools. A compost pile can add gentle biological heat, but it needs space and active management.

Common Mistakes to Avoid

• Adding a heater before sealing leaks and insulating — this wastes the most energy.
• Failing to ventilate on warm, sunny days, which causes overheating and humidity that encourages fungal growth.
• Using paraffin or kerosene heaters without daily ventilation. Their combustion byproducts produce fumes and soot that can harm plants — and you.
• Relying on unregulated DIY heat sources instead of thermostats, which risks overheating or inadequate frost protection.

For a ready-to-buy option, our tested roundup of the best small heated greenhouse models compares top picks side by side.