Heat a mini greenhouse by sealing leaks and adding insulation first; then add a low-watt, thermostat-controlled heater sized for the space.
Most people buy a heater before they seal the cracks, and that’s exactly backwards. Every dollar of heat you produce escapes through a gap you could have plugged for a few cents. The effective way to heat a mini greenhouse is to reduce heat loss first, then add low-output, controllable heat only where it’s needed. For many small greenhouses, a low-watt electric fan or tube heater is the most predictable option, while thermal mass and bubble wrap are the main passive aids.
Why Sealing Beats Heating Every Time
Heat loss is the real problem, not heat production. A small greenhouse loses warmth through broken panes, gaps around doors and vents, and cracked seams long before the air inside has a chance to warm up. The Royal Horticultural Society’s guidance on heating greenhouses efficiently starts with the structure, not the heater.
- Replace broken panes and repair damaged glazing.
- Check that vents and doors fit snugly and close fully.
- Seal cracks with transparent silicone sealant.
- Add bubble wrap insulation to the interior glazing.
- Renew or tape leaking plastic sheeting, but keep ventilation working.
Bubble wrap is the workhorse of mini greenhouse insulation. It traps a layer of still air against the glass, which slows heat transfer dramatically. The trade-off: it cuts light transmission by about 10%, so remove it in early spring when growth picks up.
What Is Thermal Mass and Does It Work?
Thermal mass is anything dense that absorbs heat during the day and releases it slowly at night. Black water barrels are the standard choice for a mini greenhouse. Place them on the sunniest side or along the north wall, where they won’t block light from your plants. Each barrel acts as a heat battery, smoothing out the overnight temperature drop.
Compost piles generate heat as they break down, and a working compost bin inside a greenhouse can contribute noticeable warmth, though it’s hard to control. Row covers and temporary insulation over plants inside the greenhouse add another layer of protection during cold snaps. Paraffin heaters need ventilation, so crack a vent when one is running.
How to Choose a Heater for a Mini Greenhouse
If passive measures aren’t enough, the heater type matters. The Royal Horticultural Society recommends buying heaters sold specifically for greenhouse use, not repurposing household units. Two types dominate for small greenhouses: fan heaters and tube heaters.
| Heater Type | Best For | Key Trade-Off |
|---|---|---|
| Fan heater | Even heat distribution and air circulation | Not ideal in very wet conditions unless moisture-protected |
| Tube heater | Background frost protection at low wattage | Slow to warm; not for rapid temperature lifts |
Size the heater to the space, not to what’s on sale. A heater built for a large garage will overshoot and waste power in a 6×8 ft greenhouse.
Run any electric heater through a thermostat rather than leaving it on full. Place a maximum/minimum thermometer at plant height in the middle of the greenhouse, because a thermostat dial sitting near the heater can read a few degrees off.
For readers ready to move from heating to housing, our tested mini indoor greenhouse picks cover models that hold heat well in the first place.
Common Mistakes to Skip
- Heating before sealing leaks, which wastes most of the heat you pay for.
- Using heaters sized for larger structures, which overshoot and cycle constantly.
- Trusting the thermostat dial instead of a plant-level thermometer.
- Forgetting that bubble wrap cuts light by about 10%, which slows winter growth.
- Blocking airflow while insulating; fan heaters help avoid cold corners but need dry conditions.
FAQs
What temperature should a mini greenhouse stay at in winter?
Heat only when frost is forecast, and keep a max/min thermometer at plant height to verify conditions rather than relying on the heater’s dial.
Can I use a regular space heater in a greenhouse?
No. The Royal Horticultural Society advises buying heaters sold specifically for greenhouse use, because household space heaters aren’t built for humid, damp conditions. Fan heaters need moisture protection, and paraffin or gas heaters require ventilation.
Do water barrels actually keep a greenhouse warm?
Yes, modestly. Black water barrels absorb heat during the day and release it overnight, smoothing temperature swings. They work best alongside sealing and insulation, not instead of them, and are the main passive heat option for off-grid greenhouses.
References & Sources
- Royal Horticultural Society. “Heating Greenhouses Efficiently.” Official guidance on insulation, heater selection, and safety for greenhouse heating.