Greenhouse climate control means storing daytime heat in thermal mass for the night, venting the surplus before it overheats the crop, and shading in the strongest sun. Overheating kills plants faster than cold does, and a rule of thumb is to size total vent area at 15-25% of the floor area.
Greenhouse climate control is the management of heat and air so that the interior stays inside the range plants tolerate.
Without Control, A Greenhouse Swings Between Two Dangers
A glazed box with nothing inside it does two bad things. In the sun it heats until plants cook: a closed greenhouse in full sun can exceed 50 °C. At night, once the sun is gone, the heat leaks out through the glazing and the interior can freeze. Three tools keep it between those limits.
- Thermal mass soaks up surplus heat by day and gives it back at night. See Thermal Mass for Night Warmth.
- Ventilation removes heat and humidity before they build up. See Ventilation & Overheating Prevention.
- Shading cuts the incoming sun on the hottest days. See Shading Strategies.
What You Need
- A thermometer, ideally one that records the day’s minimum and maximum
- Vents you can open and close from a distance
- Heat-storing mass — water barrels, stone or dark floors
- Shade material for summer
Method: Run The Daily Cycle
Step 1: Open the vents when it warms. Manual: open doors, windows or cold frame lids when the interior exceeds 25-30 °C.
Step 2: Close in late afternoon. Close up while the air is still warm so the mass has time to store heat for the night.
Step 3: Read the minimum. Check the night minimum each morning. If the interior is nearer freezing than you want, add mass or insulation; if it never drops, you may be over-sealing.
Step 4: Adjust through the year. In summer open everything and add shade; in winter close early and add mass.
Worked Example
Vent area for a small greenhouse. At 15-25% of floor area, a greenhouse with a 2.5 m × 4 m floor (10 m²) needs 0.15 × 10 = 1.5 m² up to 0.25 × 10 = 2.5 m² of total vent opening. The reasoning behind the fraction, and the wind speed it assumes, is in Ventilation & Overheating Prevention.
Common Mistakes
- Closing in the middle of the day — the interior is already hot; you trap the excess.
- No thermometer — judging by feel misses fast changes.
- All mass, no vents — mass without vents just slows the overheat.
Safety
Heat illness and combustion heaters
A closed greenhouse in sun can exceed 50 °C, and anyone working inside can suffer heat illness quickly. Open vents before entering. Do not run a fuel-burning heater in a closed greenhouse: carbon monoxide builds up and the plants and people inside are at risk; see Carbon Monoxide Risk.
What’s Next
- Learn next: Thermal Mass for Night Warmth — the night side of control
- Combine with: Passive Solar Design — the same principles at building scale