"I'll move the system from the greenhouse into a closed room — everything will be the same, just without the wind." One month later: humidity at 95%, condensation on walls and ceiling, CO₂ at 180 ppm in the middle of the day, temperature swinging 8°C between the trays and the aisle. A grow room and a greenhouse are fundamentally different systems with different control logic. A greenhouse uses the external environment as a resource. A grow room is completely isolated from it and builds its climate from scratch.
Quick glossary: Grow room — a closed, opaque space for growing plants where all lighting is artificial and all climate parameters are maintained by equipment with no involvement from the natural environment. Greenhouse — a transparent structure that uses natural sunlight and interacts with the external microclimate through ventilation and heat exchange. Microclimate — the combined air environment parameters in the growing zone: temperature, humidity, CO₂, air movement; in a greenhouse — partial dependence on the outside; in a grow room — full autonomy.
The Core Difference: Open and Closed Systems
A greenhouse is an open system: internal temperature depends on the external temperature, CO₂ is replenished through ventilation, humidity is regulated by exchange with the atmosphere. Ventilation solves three problems simultaneously: removes heat in summer, introduces fresh CO₂-bearing air, and controls humidity. The downside — dependence on external conditions: in still, hot weather natural ventilation cannot keep up.
A grow room is a closed system: temperature is determined only by heat from lamps and climate equipment, CO₂ is consumed by plants and without active supply drops to zero, humidity rises from plant transpiration and has nowhere to go without dehumidification. Full independence from the outside means, simultaneously, full dependence on equipment.
On a power outage: in a greenhouse — temperature equilibrates with the outside over several hours (stress but not catastrophic in summer). In a grow room — lighting goes off, plants are in the dark; without ventilation, humidity reaches critical levels within 2–4 hours.
Lighting: 100% Artificial in a Grow Room
In a greenhouse, lamps are supplemental or seasonal support. Primary lighting is the sun. In a grow room, 100% of DLI comes from artificial sources. This means:
Electricity costs. For leafy crops (DLI 14–17 mol/m²/day): approximately 150–200 W/m² from fixtures at PPE 2.5–3.0 µmol/J. For fruiting tomato (DLI 25–35): 300–500 W/m². Compare with a greenhouse where in winter even weak sunlight partially offsets those watts.
Heat load. Every watt going into the lamp comes out as heat in the room. At 200 W/m² over 20 m² — 4 kW of heat load that must be removed by AC or ventilation. Heat load calculation is the basis for sizing climate equipment.
Full photoperiod control. Unlike a greenhouse where natural day length changes seasonally, a grow room has a fixed, year-round photoperiod. For photoperiod-sensitive crops — precise control with no seasonal adjustments needed.
Humidity and CO₂: The Main Challenges of a Grow Room
Humidity. Plants transpire continuously — water enters the air. In a greenhouse with ventilation, that moisture exits to the outside. In a grow room it goes nowhere without a dehumidifier or forced ventilation with an outdoor exhaust. At 20 m² with a dense canopy — 20–50 litres of moisture per day enter the air. Without dehumidification, within 4–6 hours after lights-on, humidity reaches 90%+.
Solution: a dehumidifier sized to the transpiration load and room temperature, or a ventilation system that exhausts humid air outside and draws in fresh air. The first is more expensive in equipment; the second wastes CO₂ and heat in winter.
CO₂. In a greenhouse with ventilation, CO₂ is replenished from the atmosphere (420 ppm). In a grow room with a closed loop, plants reduce CO₂ to 200 ppm within 2–3 hours under good lighting. At 200 ppm, photosynthesis is significantly limited — plants grow at roughly half their potential. CO₂ enrichment in a grow room is almost obligatory at DLI above 15 mol/m²/day.
In a greenhouse with good ventilation, CO₂ enrichment is impractical — it exits to the outside. In a grow room with a closed loop — it is effective and accumulates. This difference in CO₂ approach is one of the key agronomic distinctions between the two systems.
Where the Grow Room Wins and Where It Loses Against the Greenhouse
Grow room wins:
- Full independence from season and weather — stable year-round yield
- Precise control of all parameters — DLI, temperature, CO₂, humidity
- Effective CO₂ enrichment in a closed loop
- Vertical racking and maximum use of room volume
- Protection from external pests and diseases
Greenhouse wins:
- Significantly lower lighting costs thanks to the sun
- Natural ventilation reduces climate equipment costs
- Lower electricity dependence
- Greater area at the same investment
- Higher ceilings allow tall-crop growing (tomato, cucumber) without constraint
Conclusion: for high-DLI crops (tomato, cucumber) at large scale — a greenhouse is generally more economical. For microgreens, leafy greens, and vertical farms in urban settings — a grow room offers better control and scalability.
Three Mistakes That Cost the Most
Transferring greenhouse parameters to a grow room without recalculation. In a greenhouse, 70% humidity under ventilation is normal. In a grow room without dehumidification and ventilation, the same plant load produces 95% humidity. Every parameter — from humidity targets to CO₂ calculations — must be recalculated for a closed loop.
Not building redundancy for critical equipment. In a greenhouse, a pump failure — plants still have natural light and normal temperature for several hours. In a grow room, ventilation failure with lamps on — critical overheating within 20–40 minutes. A backup fan, UPS for critical equipment, and emergency blackout shading are mandatory elements of the safety system.
Not accounting for lamp heat load when sizing the air conditioner. "I'll buy a 7 kW unit for a 20 m² room" — without considering that the lamps contribute 4 kW of heat and the plants another 1–2 kW from transpiration. The actual heat load can be 6–8 kW on a summer day — and a 7 kW unit will be at its limit or fail to cope. Ventilation and air conditioning are calculated together.
How to Know the Grow Room Is Set Up Correctly
Temperature stable within ±1°C throughout the photoperiod regardless of season. Humidity in the plant zone 55–70% without spikes toward 90%. CO₂ does not drop below 400 ppm in a closed loop (or maintained at 800–1200 ppm with enrichment). DLI stable and matching the crop's requirement all year. On a power outage, critical systems have at least 30 minutes of reserve.
For deeper understanding: Greenhouse as a System: Structure, Glazing, and Microclimate — explains how greenhouse construction decisions determine to what extent the natural environment takes over part of the climate management work, and where these two systems overlap.