How to Heat a Small Greenhouse | Insulate First, Then Add Heat

Most small greenhouses lose heat through cracks, gaps, and single-pane glass long before the heater gets a chance to work. The smartest sequence for heating a small greenhouse is to reduce heat loss first, then add the smallest heat source that meets your needs. This approach keeps your plants alive through frost without turning your electric bill into a second mortgage.

Why Insulation Comes Before Heat

Every bit of escaping warmth is wasted money, so sealing the greenhouse shell should always come first. The Royal Horticultural Society’s heating guidance recommends checking vents and doors fit snugly, replacing broken panes, and sealing cracks with transparent silicone sealant before you even plug in a heater.

Bubble wrap is the gardener’s best friend here.

If your greenhouse is larger than your plant collection, don’t heat the whole volume. Move plants to one end and screen off the rest with bubblewrap and battens to shrink the heated area — you’ll warm a fraction of the space for the same cost.

Passive Heat: Thermal Mass and Solar Gain

Thermal mass is free heat storage. Black or water-filled containers — barrels, jugs, or tanks — absorb sunlight during the day and release it slowly at night, smoothing out temperature swings without using a single watt of electricity.

Place water barrels along the north wall or in direct sun where they’ll catch the most daylight. This passive approach is cheapest to run, but it’s also less controllable than powered heat — on cloudy stretches, thermal mass alone won’t keep a greenhouse above freezing.

Active Heat: Electric Heaters That Work

When passive methods aren’t enough, a small electric heater with a thermostat is the most predictable option for spaces with mains electricity available. Low-watt models sized to the greenhouse keep temperatures steady without the cycling and overheating that come with oversized units.

The RHS recommends two distinct electric options, and they serve different jobs:

• Fan heaters push more heat and circulate air, which reduces cold corners. Buy a greenhouse-sold model for safety, since these units live in damp, splash-prone environments.
• Tube heaters use lower energy and are better for background frost protection than for rapid warming. They’re the quiet workhorse for keeping things above freezing.

For seedlings and root-zone warmth, skip air heating entirely. Heating mats under trays warm the soil directly, which is more efficient and better for germination than heating the whole greenhouse volume.

If you’re ready to compare specific models, our roundup of small greenhouse heating systems breaks down the tested options by size and budget.

Thermostat Placement and Common Mistakes

Thermostat placement makes or breaks your heating setup.

The most common heating mistakes all trace back to skipping the basics:

• Heating before insulating — warmth escapes as fast as you create it, and costs climb.
• Skipping the thermostat — increases overheating risk and wastes energy.
• Oversizing the heater — small spaces need low-output, controllable heat, not a system built for a commercial range.
• Warming all the air — mats or thermal mass handle specific needs more efficiently.

In climates where mains electricity isn’t available, paraffin or gas heaters work but require ventilation to clear combustion gases — never treat them like sealed indoor heaters. The University of Florida’s IFAS Extension notes that electric options work well for moderate climates, while colder regions may need propane or natural-gas units sized to the structure.

For most small greenhouses, the winning formula stays the same: seal every gap, add bubble wrap and water barrels, then let a small thermostatic heater handle the rest.