One of the most complex parts of a grow room is the HVACD system. In addition to heating, ventilation and air conditioning, HVACD also provides dehumidification. These systems control temperature, humidity and airflow, which influence transpiration, disease prevention and other aspects of cannabis cultivation. Designing an HVACD system that balances these factors while remaining energy- and cost-efficient requires careful planning based on the grow room’s size, plant density and other characteristics.
This blog post provides an overview of why HVACD matters for cannabis grow rooms, the factors that shape HVACD system design and sizing, and best practices for operating them.
Why HVACD matters
Temperature, humidity and airflow are essential for healthy plant development, especially for transpiration, the process by which plants draw water from the roots and release it into the atmosphere through the leaves. Transpiration is critical not only for water and nutrient uptake but also for regulating plant temperature. It is driven by the vapor pressure deficit (VPD), which is the difference between the vapor pressure inside the leaf and the surrounding air. When the pressure inside the leaf is higher, moisture is released as vapor. The greater the VPD, the more plants can transpire.
VPD is directly influenced by temperature and humidity. If humidity is high and temperature is low, the air can’t hold as much moisture, resulting in a lower VPD. Grow lights warm the air inside the leaves, increasing pressure and promoting transpiration—which, in turn, raises humidity. Without the ability to control temperature and humidity, growers can’t influence VPD rates—and therefore can’t regulate transpiration or, ultimately, plant growth.
Heating and cooling the grow room also helps keep plants within the optimal temperature range of 25–30°C,[1] while controlling humidity and airflow helps prevent mold, mildew and pest outbreaks.
Why HVACD matters
Temperature, humidity and airflow are essential for healthy plant development, especially for transpiration, the process by which plants draw water from the roots and release it into the atmosphere through the leaves. Transpiration is critical not only for water and nutrient uptake but also for regulating plant temperature. It is driven by the vapor pressure deficit (VPD), which is the difference between the vapor pressure inside the leaf and the surrounding air.
When the pressure inside the leaf is higher, moisture is released as vapor. The greater the VPD, the more plants can transpire.
VPD is directly influenced by temperature and humidity. If humidity is high and temperature is low, the air can’t hold as much moisture, resulting in a lower VPD. Grow lights warm the air inside the leaves, increasing pressure and promoting transpiration—which, in turn, raises humidity. Without the ability to control temperature and humidity, growers can’t influence VPD rates—and therefore can’t regulate transpiration or, ultimately, plant growth.
Heating and cooling the grow room also helps keep plants within the optimal temperature range of 25–30°C,[1] while controlling humidity and airflow helps prevent mold, mildew and pest outbreaks.
Designing an HVACD system
Installing an HVACD system capable of managing climate requires calculating the peak heating and cooling loads.[2] This involves understanding the difference between sensible and latent heat:
- Sensible heat raises the air temperature[3] and can be measured with a thermometer. Sunlight hitting the facility and artificial light inside the grow room contribute to sensible heat. It is sometimes called the dry load.
- Latent heat comes from increasing moisture in the air.[4] Plant transpiration, humidity from the outdoors and evaporation from growing media and the irrigation system contribute to latent heat. It is sometimes called the wet load.
The peak heating load is the amount of energy the HVACD system must supply to compensate for heat lost to the outdoor environment (sensible heat). The peak cooling load is the amount of energy required to remove both sensible and latent heat.[5]
HVACD systems must also manage the moisture or humidity load, also known as the latent load.[6] Because plants release large volumes of water vapor through transpiration, a well-designed HVACD system must have enough dehumidification capacity to maintain the target humidity and VPD, especially during peak transpiration periods when the lights are on.
Calculating these loads can be complex. For instance, crop canopy size and density at full maturity, and the intensity and duration of lighting, affect the overall climate load. Seasonal changes also matter, as growers must heat the grow room in the winter and cool it in the summer.
Growers should seek a knowledgeable, reputable HVACD company experienced in designing HVACD systems to address these complexities and install an appropriately sized HVACD system.
Other factors to consider
In addition to handling the different types of loads, a good HVACD system for a grow room should provide sufficient air exchange, accommodate CO2 supplementation as needed, and use filtration to remove airborne pathogens and particles to maintain healthy air quality. A grower may need multiple HVACD systems if the grow room includes multiple zones, allowing them to fine-tune the environment for specific cultivars and growth stages. For example, flowering plants need lower humidity than those in veg to prevent mold and optimize bud development. The drying and curing processes also have distinct HVACD requirements, including gentle airflow to avoid over-drying and to preserve cannabinoids and terpenes. Multiple HVACD systems are often necessary to meet these different needs, especially in large operations.
Growers may also need multiple dehumidifiers to support the system during high transpiration. Proper placement of these units is critical to avoid microclimates and ensure even drying in the canopy zone. HVACD systems are also among the highest expenses in a grow room, so it pays to invest in energy-efficient units. Systems that use variable-speed fans, compressors, heat recovery, geothermal cooling, economizers and VRF (variable refrigerant flow),[7] can improve efficiency. Monitoring tools or building management systems can help optimize runtime and reduce energy waste.
Best HVACD practices
The first step to operating an HVACD system in a grow room is to determine temperature and humidity setpoints based on the plant’s growth phase and VPD targets. Installing sensors allows for constant monitoring and ensures the system turns on and off as needed.
Next, monitor airflow. Research shows that photosynthesis and vapor exchange between the air and plants may be enhanced when airflow above the canopy is 0.3–0.5 meters per second.[8] An anemometer is the most precise tool for measuring airflow.[9] Adjust air dampers if necessary.
Finally, consider integrating HVACD into an automated system that also controls lighting and irrigation. These components affect one another, and understanding their interaction is crucial to maintaining an ideal grow-room climate. For instance, switching from HPS to LED lights usually requires HVACD adjustments, as LEDs produce less heat, so the grow room may need to be kept warmer. Transpiration patterns also change due to the lack of infrared radiation from HIDs, which can affect humidity settings. Automation ensures that all systems work together to support healthy plant growth and yield.
Choosing a well-designed, energy-efficient HVACD system is one of the best investments a grower can make. The ability to fine-tune temperature and humidity to match plant genetics and growth stages can make all the difference in cannabis cultivation.
Emerald Harvest Team
[1] Chandra, Suman, Hemant Lata, and Mahmoud A. ElSohly. 2020. “Propagation of Cannabis for Clinical Research: An Approach Towards a Modern Herbal Medicinal Products Development.” Frontiers in Plant Science 11: 958. https://doi.org/10.3389/fpls.2020.00958.
[2] Burdick, Arlan. 2011. “Strategy Guideline: Accurate Heating and Cooling Load Calculations.” U.S. Department of Energy, June. https://digital.library.unt.edu/ark:/67531/metadc843609/m2/1/high_res_d/1018100.pdf.
[3] Cornell University Ergonomics Web. n.d. “DEA3400: Ambient Environment: Thermal Conditions.” Accessed August 29, 2025. https://ergo.human.cornell.edu/studentdownloads/DEA3500notes/Thermal/thcondnotes.html.
[4] Ibid.
[5] Burdick, Arlan. 2011. “Strategy Guideline: Accurate Heating and Cooling Load Calculations.” U.S. Department of Energy, June. https://digital.library.unt.edu/ark:/67531/metadc843609/m2/1/high_res_d/1018100.pdf.
[6] Mallay, Dave. 2024. “Advanced HVAC Humidity Control for Hot-Humid Climates.” U.S. Department of Energy Building America Program, April. https://docs.nrel.gov/docs/fy24osti/83357.pdf.
[7] A ductless system that connects a single outdoor unit to multiple indoor units, controlling the amount of refrigerant flowing to each zone for individual heating and cooling.
[8] Gao, Han, Zhi-Cheng Tan, Ming Yang, Cheng-Peng Ma, Yu-Fei Tang, and Fu-Yun Zhao. 2025. “Microclimate Air Motion and Uniformity of Indoor Plant Factory System: Effects of Crop Planting Density and Air Change Rate.” Applied Sciences 15 (8): 4329. https://doi.org/10.3390/app15084329.
[9] Both, A. J., and Eileen Fabian Wheeler. 2002. “Instruments for Monitoring the Greenhouse Aerial Environment Part 2 of 3.” Rutgers Cooperative Extension. https://horteng.envsci.rutgers.edu/factsheets/e276.pdf.