Insulation Design and Thermal Performance of Polyurethane-Based Walls in Prefabricated Solar Greenhouses

The increasing adoption of prefabricated solar greenhouses in modern agriculture offers advantages in construction efficiency and land use; yet they face a critical challenge—thermal bridging at panel joints—which compromises their insulation performance. Polyurethane (PU) spray foam presents a promising seamless insulation solution, but its effectiveness depends on the selection of spraying position, material, and thickness. This study systematically investigates the thermal performance of PU-sprayed walls in a prefabricated solar greenhouse located in Shenyang, China, using computational fluid dynamics (CFD) simulations. The effects of different spraying positions, insulation materials, and spraying thicknesses were evaluated. The results demonstrate that spraying PU on the north roof alone is the most thermally efficient strategy among the tested cases, substantially reducing heat loss through the thermally weak north roof without hindering daytime heat storage in the back wall. Among the materials, PU foam outperforms rock wool and cement layer, raising the minimum nightly temperature by 2.8 °C at a thickness of 50 mm. 50 mm is identified as the best-performing thickness among the tested cases, as thickness increases beyond this point, yield diminishing marginal benefits. Furthermore, a regional crop suitability analysis indicates that a 30 mm spray suffices for Shouguang, 50 mm is recommended for the harsher winter in Shenyang, while at least 70 mm is necessary for Tacheng to accommodate the low-temperature requirements of tomato and cucumber. This study provides a practical, simulation-based parametric screening framework for PU spraying in prefabricated solar greenhouses, enabling targeted insulation design that addresses thermal performance and regional crop needs.