A greenhouse can lose heat very quickly on a clear, cold night because thin glazing has far less insulation than a house wall. Keeping a greenhouse warm during a freeze is therefore a heat-loss problem first and a heater-selection problem second. The goal is usually not to make the greenhouse feel like a living room; it is to keep plant-zone temperatures above the minimum your crops can tolerate.
TL;DR
- Reduce greenhouse heat loss before increasing heater size: seal gaps, use appropriate insulating layers and minimize unnecessary heated volume.
- Use a thermostat-controlled heater suitable for the greenhouse environment and keep leaves, trays, plastic and other combustibles safely away.
- Measure temperature at plant level, because the thermostat near a heater can read warmer than the coldest part of the greenhouse.
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Why greenhouses cool so fast during a freeze
Greenhouse glazing intentionally lets light in, but it also allows heat to escape rapidly. Wind increases losses through gaps and infiltration, while clear nights can create additional radiative cooling. A heater must replace this lost heat continuously if outdoor temperatures remain below freezing.
Reduce heat loss before adding more heater wattage
- Seal obvious gaps around doors, vents and panels without defeating required ventilation.
- Use greenhouse-appropriate insulating film or bubble insulation where it will not interfere with safe equipment operation.
- Create a smaller protected zone around sensitive plants when practical instead of heating unused air volume.
- Use thermal mass such as water containers only as a supplement; it cannot replace adequate heating during prolonged hard freezes.
- Keep cold-sensitive pots off extremely cold floors when appropriate for the crop.
What type of heater works in a greenhouse?
The correct choice depends on moisture, greenhouse size and electrical service. A thermostatically controlled electric heater avoids combustion gases and is straightforward where a suitable protected electrical supply exists. Infrared heat can be useful for targeted plant or work zones, while a fan heater improves air mixing. Any heater must be approved for the conditions where it will operate; ordinary indoor bedroom heaters should not be assumed suitable for damp greenhouse environments.
How to size greenhouse heat
Do not size a greenhouse heater from floor area alone. Heat demand depends on glazing area, glazing material, outdoor design temperature, wind exposure and the indoor target temperature. A small hobby greenhouse may need far more heat per square foot than a well-insulated room in a house because its envelope loses heat much faster.
Thermostat placement matters
Measure temperature near the plants you are protecting rather than directly above or in front of the heater. Warm air rises, so a thermostat mounted high can allow the lower plant zone to become colder than expected. A separate min/max thermometer is useful for checking the coldest overnight temperature.
Using an infrared heater in a greenhouse
The Dr Infrared Heater DR-968 is a popular indoor infrared/convection heater, but greenhouse use should only be considered if the manufacturer permits the specific environment and the greenhouse remains dry enough for the product. For a dedicated greenhouse heater, prioritize models specifically designed or approved for greenhouse or damp-location use rather than choosing only by wattage.
Freeze-night checklist
- Check the heater, thermostat and circuit before nightfall.
- Close doors and vents that should be closed for the night.
- Confirm that plants, plastic, fabric and stored materials are safely clear of the heater.
- Place a thermometer at plant height in the coldest zone.
- Have a backup plan if power fails or the heater cannot maintain the target temperature.
Final verdict
The most reliable way to keep a greenhouse warm during a freeze is to combine heat-loss reduction, accurate plant-level temperature monitoring and a heater that is genuinely suitable for the greenhouse environment. Adding wattage without fixing leaks or checking moisture suitability is a poor substitute for a complete freeze plan.
FAQs
What temperature should I keep my greenhouse during a freeze?
It depends on the plants. Protect to the minimum temperature required by the crop rather than heating the whole greenhouse to household comfort temperatures.
Can I use a normal space heater in a greenhouse?
Only if the manufacturer permits the environment. Many ordinary indoor heaters are not designed for damp greenhouse conditions.
Is infrared heat good for a greenhouse?
Infrared heat can warm objects and plant zones directly, but suitability depends on greenhouse layout, moisture rating and safe clearances.
Will bubble insulation help a greenhouse in winter?
Greenhouse-appropriate insulating layers can reduce heat loss, but installation must preserve safe ventilation and clearance around heating equipment.
Where should I put the greenhouse thermostat?
Near the crop zone you need to protect, away from direct heater airflow and direct sun, so it reflects the actual plant temperature.
Can a 1500W heater protect a greenhouse from freezing?
Sometimes in a small greenhouse and mild freeze, but heater size must be based on heat loss, glazing, outdoor temperature and desired indoor temperature, not floor area alone.
