Abstract: Glass greenhouses suffer severe heat loss in winter. Inner insulation films (secondary thermal curtains) represent a cost‑effective energy‑saving solution. However, many projects install films purely based on field experience without optimized spacing between films and building envelopes, resulting in compromised insulation performance. Combining Computational Fluid Dynamics (CFD) simulation and field measurement, this article highlights two critical parameters — side clearance and top mounting height — and delivers practical engineering references for inner‑insulation systems in Venlo‑type glass greenhouses.
In cold winter conditions, Venlo‑type glass greenhouses experience substantial nocturnal heat dissipation through highly conductive glass cladding. Low indoor temperatures restrict over‑winter production of solanaceous fruits and leafy vegetables. Installing inner insulation films (secondary thermal curtains) creates stationary air gaps between the film and greenhouse frames to suppress heat transfer, which is a widely adopted low‑cost insulation measure for glass greenhouses in China.
A common industry misconception is that hanging an insulation film alone guarantees satisfactory heat retention. In real‑world projects, identical insulation films often deliver drastically different performance across sites. The root cause lies in non‑quantified installation: mounting distances between insulation films and side walls / roof structures are determined by construction experience rather than design calculation. Improper air gaps trigger intense cold‑air disturbance and prevent insulation materials from delivering full performance.
To address this engineering challenge, research teams conducted field temperature measurements and CFD thermal‑environment numerical simulations on Venlo‑type glass greenhouses. Validated against on‑site sensor data, the numerical model achieves a maximum absolute temperature error of 0.68 °C, a root‑mean‑square error of 0.36 °C and an average relative error of merely 6.0%. The simulation output matches real‑world greenhouse conditions closely and reliably predicts temperature and airflow patterns under varied installation configurations.
Two working conditions were compared: greenhouse without inner film, and greenhouse fully enclosed with side‑and‑top inner insulation films. Under low outdoor temperature scenarios, installing inner insulation films raises the average temperature at crop canopy height by 2.21 °C, effectively mitigating risks of nocturnal frost damage.
Nevertheless, research indicates that inner insulation films alone cannot meet baseline thermal requirements for thermophilic vegetables under extreme cold weather. Inner films serve as auxiliary energy‑saving components to reduce heating load instead of replacing heating systems in frigid zones.
Thermal simulation contour plots visualize this contrast. Without inner insulation films, the overall greenhouse temperature stays low. When inner films are deployed, a thermally buffered growing zone forms within the enclosed film boundary. Cold air originating from glass envelopes is trapped inside the air gap, limiting convective heat exchange between two zones and preserving heat for cultivation areas. If films are mounted too close to glass walls, narrow gaps generate violent airflow disturbances from cold surfaces that penetrate into growing zones, causing uneven temperature distribution and harming crop development.
Multiple orthogonal simulation sets were carried out, testing side clearances (distance between insulation film and greenhouse side wall) of 0.2 m, 0.6 m and 1.0 m, paired with top‑film mounting heights of 2.5 m, 3.0 m and 3.5 m. Key engineering conclusions are summarized below:
Larger side clearance improves canopy temperature uniformityWhen side clearance increases from 0.2 m to 1.0 m, high‑speed airflow turbulence between cold glass walls and inner films weakens significantly. The buffering effect of the air gap is enhanced, narrowing temperature differentials inside insulated zones. A 1 m side clearance greatly offsets localized cooling effects induced by cold glass surfaces.
Higher top mounting height reduces vertical temperature gradientsElevating the top insulation film optimizes vertical thermal distribution and diminishes temperature stratification. Among all tested combinations, the configuration with 1 m side clearance and 3.5 m top mounting height delivers the best overall performance. Temperature deviation within insulated zones drops substantially, and canopy‑height temperature non‑uniformity reaches as low as 5.38 °C, creating the most thermally homogeneous environment for consistent crop growth.
Range analysis reveals that side clearance exerts greater influence over average indoor temperature; meanwhile, top mounting height weighs more heavily on canopy temperature uniformity. Both parameters deserve equal attention during design. Optimizing side spacing without considering top‑film height will limit overall insulation performance.
For Venlo‑type glass greenhouses equipped with secondary inner insulation curtains, avoid mounting films flush against glass walls. Where layout permits, adopt 1 m side air‑gap clearance and 3.5 m top‑film height to maximize thermal benefits. If constrained by site or planting layout, maintain sufficient buffer gaps and avoid tight contact between films and glazing.
The core insulation mechanism of inner films depends on stationary air‑layer thermal resistance. Seals at film joints and retraction mechanisms must remain intact. Air leakage breaks static air gaps and drastically degrades insulation performance.
Inner insulation films are energy‑saving auxiliary devices that cut heating consumption. Supplementary heating equipment is indispensable for over‑winter production in severely cold regions; do not rely solely on insulation films.
Apply CFD thermal‑environment simulation at the greenhouse design phase. Pre‑emptively evaluate temperature and airflow fields for different thermal‑curtain layouts to avoid costly post‑construction retrofits.
Modern protected agriculture moves past experience‑driven construction. Insulation performance is not determined merely by plastic‑film material; invisible structural parameters such as gap width and hanging height directly shape winter energy consumption and crop yields. Introducing CFD simulation in early‑stage greenhouse design translates academic research into practical installation standards, supporting stable year‑round cultivation and lowering operational energy costs for growers.
Business Call‑to‑Action (for website footer)We provide complete environmental‑control solutions for Venlo‑type glass greenhouses and multi‑span greenhouses. Custom layout validation and CFD simulation assessment for inner thermal curtains / secondary films are available according to local winter climate conditions. Feel free to contact our technical team for project proposals.
Contact: Mr. Zhuang
Phone: +8613974932555
Email: info@xlagritec.com
Address: Changsha, Hunan, China
