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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →To keep a greenhouse warm overnight without overspending, first reduce heat loss, then measure the temperature where your plants are and heat only as much space as their needs require. Close and repair leaks without blocking necessary ventilation or a heater’s fresh-air supply. A well-fitted thermal curtain may help; supplemental heat should be greenhouse-rated, thermostatically controlled and sized for the structure and local conditions.
Start by reducing heat loss
In a greenhouse, preventing heat from escaping is usually the first cost-control step. Virginia Cooperative Extension says as much as 85% of greenhouse heat loss occurs at night; that is a source estimate, not a prediction for every backyard structure. Check that unused vents close fully and repair gaps. Insulate or repair unused fan openings, but do not seal the structure in a way that blocks required ventilation or a heater’s combustion air. Virginia Cooperative Extension warns: “Do not inhibit the fresh air supply to the greenhouse heater.” Virginia Cooperative Extension greenhouse heating guidance.
Use a thermal curtain where it fits
A greenhouse thermal blanket or energy screen can reduce the volume of space that must be heated overnight. Virginia Cooperative Extension recommends porous curtain material to limit condensation pooling above plants, and flame-resistant material (or a suitable alternating material in growing structures). Fit the curtain close to the walls, and keep heating or water lines below it or insulated. Open it gradually over chill-sensitive crops.
Reported savings vary by system and installation. Virginia Cooperative Extension says heat-retention blankets can reduce energy use by up to 50%, and combined thermal/shade blankets by 25% or more. UMass Extension reports 30–50% heating-cost savings for energy/shade screen systems. These are potential results, not a guarantee for a small greenhouse or a particular utility bill. UMass Extension energy conservation guidance.
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Measure the overnight low where plants are
Do not rely only on the heater’s thermostat reading. Place a maximum/minimum thermometer near the main plant area and check the recorded overnight low. The temperature at the thermostat can differ from conditions around the plants; the RHS notes that heater thermostats may be off by a few degrees when positioned close to the heater. RHS guidance on overwintering plants in greenhouses.
Set the target for the crop, not a universal number
Choose the minimum overnight temperature based on the plants you are protecting. There is no single suitable greenhouse night temperature for every crop. Lowering the setpoint can reduce heating demand, but Virginia Cooperative Extension cautions that colder conditions can affect growth and flowering and may increase disease problems. If plants have different temperature needs, prioritize the vulnerable group or consider heating a smaller protected area rather than raising the temperature throughout the greenhouse. Virginia Cooperative Extension greenhouse heating guidance.
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Use thermal mass only as part of a solar-heating design
Water or sand can absorb warmth during sunny hours and release it later, but thermal mass is not a dependable substitute for insulation or backup heat in every greenhouse. It is most relevant in a purpose-built passive-solar structure that captures daytime sun.
In a University of Georgia Cooperative Extension design documented in 2024, thermal mass was placed along the north wall. Each full 55-gallon water barrel weighed more than 450 pounds and was secured against shifting. The study found no noticeable difference in growing-degree-day accumulation between its water- and sand-mass sections. That result describes the studied design, not a universal performance comparison. University of Georgia Cooperative Extension passive-solar greenhouse design.
A different approach described by the University of Minnesota uses solar-heated air moved underground, where heat is stored in insulated soil or rock for deep-winter use. This is a construction-scale system rather than a quick, low-cost add-on. University of Minnesota deep-winter greenhouse guidance.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Add supplemental heat for the remaining need
If heat-loss reduction and the crop-appropriate setpoint are not enough, use a greenhouse-specific heater with thermostatic control. The RHS recommends electric fan heaters designed for greenhouses and professional installation of electrical connections. Virginia Cooperative Extension emphasizes heater maintenance and an adequate fresh-air supply. A heater’s capacity must be matched to greenhouse surface area, glazing, the local outdoor design temperature and the desired indoor temperature; the cited guidance does not establish a universal wattage. RHS greenhouse guidance and Virginia Cooperative Extension greenhouse heating guidance.
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Before choosing a heater, account for the structure’s heat load, available electrical or fuel infrastructure, control accuracy and any fresh-air requirements. Keep necessary ventilation clear, follow local electrical and building requirements, and maintain the heating system. For crop- or structure-specific advice, Ask Extension has addressed the question of shade cloth that also helps insulate overnight: Ask Extension.
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