Managing temperature in a Canadian greenhouse is not simply a matter of adding heat. The goal is to maintain a stable growing environment across seasons that pull in opposite directions — a January night in Manitoba may require the full output of a heating system, while a June afternoon in the same structure may require maximum ventilation to prevent heat stress. A well-designed climate system addresses both extremes.
Heating Systems: Options and Considerations
The three most widely used heating systems in Canadian greenhouses are radiant hot-water (hydronic) heating, forced-air unit heaters, and electric heating. A fourth option — biomass or wood boilers — is used in some rural and agricultural settings where fuel is available at low or no cost.
Radiant Hot-Water (Hydronic) Systems
Hydronic systems circulate heated water through pipes running beneath the floor, along the perimeter, or in overhead distribution manifolds. They are considered the most efficient option for large and mid-scale greenhouses because radiant heat warms the root zone directly, reduces temperature stratification, and provides a stable, consistent temperature with slow swings between setpoint deviations.
The primary components are a boiler (typically natural gas or propane), a circulation pump, and distribution piping. Natural gas boilers are the most cost-effective in urban and suburban areas with gas service. Propane serves rural locations but at a higher per-BTU fuel cost. Modern condensing boilers recover heat from combustion exhaust and operate at efficiencies of 90 percent or above.
The upfront cost of a hydronic system is higher than unit heaters, but the operating efficiency advantage compounds over years of use. For a greenhouse larger than roughly 500 square feet operated year-round, hydronic heating is commonly recommended by agricultural engineers.
Forced-Air Unit Heaters
Unit heaters — suspended from the roof structure and fuelled by natural gas or propane — are the most common heating system in small and mid-scale greenhouses across Canada. They are less expensive to install than hydronic systems and can be operational within a day of mounting. The trade-off is that they heat air rather than surfaces, which leads to greater temperature stratification (warmer air near the roof, cooler air at root level) and more frequent cycling.
Horizontal airflow (HAF) fans are often used alongside unit heaters to break up stratification and improve temperature uniformity. A well-positioned HAF fan system typically reduces heating demand by mixing warm overhead air downward throughout the growing space.
Electric Heating
Electric resistance heating is straightforward to install and requires no fuel storage or combustion venting, but it is generally the most expensive per BTU in Canadian energy markets. It is occasionally used as a supplemental or backup heat source, or in very small hobby greenhouses where the convenience outweighs the higher operating cost. Heat pump systems — particularly ground-source heat pumps in appropriate geological settings — can make electric heating more competitive, but the capital cost is significant.
Biomass and Wood Boilers
In agricultural contexts where wood waste, sawmill residuals, or crop residues are available, biomass boilers provide a low fuel-cost option. They require more daily management than gas or electric systems (fuel loading, ash removal), and the boiler must be sized to handle the peak heating load without frequent cycling that accelerates wear. Several Canadian provinces offer support programs for agricultural biomass heating under their climate and energy programs.
Many crops tolerate a 4–6°C night temperature drop compared with the daytime setpoint. Programming a night setback reduces heating demand during the coldest hours while having minimal impact on growth rates for most vegetables. This is a standard practice in commercial greenhouse operations across Canada.
Insulation and Heat Retention
No heating system works well in a poorly insulated structure. The north wall of a greenhouse (in the northern hemisphere) receives no direct solar gain and is consistently exposed to prevailing winter winds in many Canadian locations. Insulating the north wall with rigid foam board — to R-20 or higher — and covering it with a reflective surface redirects light and reduces heat loss through what would otherwise be a significant source of thermal bridging.
Thermal curtains — insulating blankets drawn across the interior of the greenhouse at night — can reduce overnight heating consumption substantially. Commercial curtain systems are motorized and controlled by timers or greenhouse climate computers. Manual curtain systems work in small structures. The curtain material must be permeable to moisture to prevent condensation buildup beneath it.
The perimeter foundation should be insulated on its exterior face with rigid foam board to the local frost depth. Uninsulated foundations allow frost to penetrate the floor, reducing root zone temperatures and increasing heating demand disproportionately.
Ventilation: Managing Summer Heat and Year-Round Humidity
Overheating in summer is a genuine risk in Canadian greenhouses. A well-glazed structure can reach 40°C or above on a clear July afternoon without ventilation. The primary ventilation tools are roof vents (ridge vents), side vents or roll-up sides, and HAF fans.
Ridge Vents
Ridge vents at the apex of the greenhouse roof allow hot air to escape by natural convection. For effective passive ventilation, a total vent opening area of roughly 15 to 20 percent of the floor area is commonly recommended. Motorized vent openers controlled by a thermostat allow the vents to respond automatically to temperature, which is particularly useful when the grower is not present.
Roll-Up Sides and Side Vents
Poly-covered hoop houses typically use roll-up sides that allow full airflow when summer temperatures peak. This is one of the most effective low-cost ventilation approaches for single-bay structures. Permanent glass or polycarbonate structures use hinged or sliding side vents that perform similarly, though with a smaller total opening area.
Horizontal Airflow Fans
HAF fans circulate air continuously within the greenhouse regardless of whether vents are open. Continuous circulation reduces localized humidity buildups around leaf surfaces, which is the primary driver of fungal diseases including botrytis and powdery mildew. Fans are typically placed in a circular or oval pattern to encourage a gentle, consistent air movement of roughly 0.5 metres per second across plant surfaces.
Humidity Management
Greenhouses accumulate moisture from plant transpiration, irrigation runoff, and condensation on cooler surfaces. Relative humidity above 85 percent sustained over several hours promotes the development of fungal pathogens on many vegetable and herb crops. Controlling humidity is as much about structural decisions — drainage, floor material, glazing material — as it is about active dehumidification.
In practice, the most effective humidity management tools for small-scale Canadian growers are: proper plant spacing to allow air movement between leaves; early-morning watering so foliage has time to dry before nighttime temperatures drop; and sufficient ventilation to exchange humid interior air with drier outdoor air during appropriate weather windows. Dedicated dehumidifiers are used in some commercial operations but add to operating costs.
Thermostats and Climate Controllers
At minimum, a greenhouse requires separate thermostats controlling heating and ventilation. A heating thermostat with a night setback function and a high-limit thermostat that opens vents or triggers fans if temperature exceeds a setpoint covers the basics. More sophisticated climate controllers integrate all inputs — temperature, humidity, CO2, light — and manage multiple outputs simultaneously. These systems are standard in commercial operations and increasingly accessible to small-scale producers as costs have declined.
Data loggers that record temperature and humidity over time are useful for diagnosing climate problems. Consistent records reveal patterns that are not apparent from spot observations — a recurring cold spot near a door, a humidity spike in the pre-dawn hours, or an afternoon temperature overshoot that correlates with low cloud cover reducing ventilation efficiency.
"The best heating system is one that responds smoothly and consistently — not one that cycles between extremes."
Greenhouse climate management is ultimately about reducing variance. Crops that experience consistent temperature and humidity grow more predictably and with fewer disease events than those subjected to frequent swings. Investing in monitoring before spending on additional heating capacity often reveals that existing equipment is adequate but poorly controlled.