"Bought a 3×6 polycarbonate greenhouse, put it up — now protected from rain and frost." First summer: 48°C inside in July, plants burning out every day after 11 a.m. First winter: condensation dripping from the ceiling onto plants every night, Botrytis on every crop. A greenhouse protects from the external environment — but simultaneously creates an internal environment that must be managed. Good construction and cladding minimise the management workload. Poor choices shift all the compensation onto equipment and the grower.
Quick Glossary
- Greenhouse — a structure with a transparent cladding that transmits sunlight and retains heat; functions as a system where construction, cladding, and equipment together determine the microclimate
- Cladding — the transparent material of the walls and roof (glass, polycarbonate, film); determines light transmission, heat retention, condensation behaviour, and durability
- Heat balance — the relationship between heat entering from the sun and heating system and heat lost through the cladding and ventilation; determines heating costs and overheating risk
Cladding: What Drives the Choice of Material
Cladding selection is the first decision that defines greenhouse characteristics for 10–25 years:
Glass: highest light transmission (90–92% PAR), 30+ year lifespan, best for maximum natural DLI. Downsides: weight requires a stronger structure, higher cost, breakable. Condensation on glass runs evenly downward — does not drip on plants at the correct roof pitch. The standard for Dutch commercial greenhouses.
Polycarbonate: lighter and cheaper than glass, impact-resistant. Twin-wall (4–6 mm) and triple-wall (8–10 mm) offer different thermal performance. Light transmission 70–82%, declining by 10–15% over 5–7 years due to UV degradation. Key problem: condensation accumulates inside the channels and drips unevenly onto plants — fungal issues under drip points. Solution: anti-drip coating on the inner surface and correct roof pitch.
Film (polyethylene or EVA): the cheapest option and simplest to install. Double or triple-layer construction gives acceptable heat retention. Service life 3–5 years. Worst condensation behaviour without anti-drip coating. Standard for tunnel greenhouses.
Practical choice: 10 mm polycarbonate on a limited budget with heat retention as a priority. Glass — for maximum DLI requirements and a 20+ year planning horizon. Film — for a seasonal greenhouse or tunnel where construction cost is the primary concern.
Structure and Orientation: Where Most Growers Lose Light
Orientation. For greenhouses in a temperate climate (Ukraine) — ridge oriented east–west gives maximum solar exposure during the low-sun winter and autumn months. A north–south ridge delivers more even summer light distribution but less in winter when the sun is low. For year-round production: east–west.
Roof pitch. Steeper pitch (25–35°): better self-cleaning, less snow load, better condensate runoff. Shallower pitch (10–15°): less interception of direct winter sunlight by structural elements at low sun angles. For winter production — a 20–28° pitch is a practical compromise.
Structural elements. Metal profiles and trusses cast shadows. For greenhouses where every photon counts in winter — "wide span" (12–16 metres between supports) with minimum internal vertical columns is the commercial standard. For small greenhouses — an acceptable compromise.
Heat Balance: Heating and Overheating
In winter: retain heat. Heat loss through the cladding is the primary heating cost. Twin-wall polycarbonate and modern EVA film have a U-value of 3–4 W/m²·K. Glass — 5–6 W/m²·K (higher heat loss). For Ukraine at −20°C outside and +18°C inside over 100 m² — heating demand is 5–10 kW just to maintain temperature, not including infiltration.
In summer: prevent overheating. Polycarbonate or glass under direct July sun — internal temperature reaches 45–55°C without active ventilation. Natural ventilation through ridge and side vents lowers temperature but depends on wind. Forced ventilation (exhaust fans + intake openings) — a reliable solution. External shade netting at 30–50% or white shading compound applied to the cladding — the most effective way to reduce thermal load without eliminating all light. Internal shading is less effective: the heat is already inside.
Ventilation as Part of the Structure
Effective ventilation starts at the design stage — not when you buy fans. Ridge vents at the roof peak allow hot air to exit naturally upward while cool air enters through open side vents below. Effective natural ventilation requires vent area of at least 15–20% of floor area.
With forced ventilation: exhaust fans at one gable end, intake openings at the opposite end. Air change rate for greenhouses on a hot day: 30–60 room volumes per hour. Calculate the greenhouse volume and select fans accordingly — "bigger is not always better."
Connection to CO₂: with open ventilation, CO₂ enrichment is not practical — all the supply exits immediately. CO₂ enrichment and effective ventilation are mutually exclusive in the same zone. Practical approach: enrich in closed mode in the morning when temperatures are low; switch to ventilation as temperatures rise during the day.
Three Mistakes That Cost the Most
Choosing a structure without considering orientation and climate. A greenhouse from a nice catalogue installed on a north–south axis with no ridge vents — minimum light in winter and overheating in summer simultaneously. Choose the structure for the specific climate and specific production.
Not planning a shading and reflection system at the design stage. External shade netting that can be easily installed and removed — relatively inexpensive during construction. External netting retrofitted to a completed structure — significantly more expensive and often less convenient. Ventilation and shading together solve overheating more reliably than either alone.
Relying on natural ventilation in a hot climate without a backup. Natural ventilation depends on wind. In calm conditions at +35°C — ridge vents are open but air is not moving. Without backup forced ventilation fans, plants reach critical heat stress within 30–40 minutes.
Signs That the Greenhouse System Is Configured Correctly
- Internal temperature on peak summer days does not exceed outdoor temperature by more than 4–6°C with ventilation open
- Target temperature is maintained in winter at the calculated heating load
- Condensate runs down the cladding and does not drip onto plants
- Natural DLI is measured and corresponds to the calculated value for the orientation and cladding type