A model of vegetated exterior facades for evaluation of wall thermal performance

A mathematical model of an exterior wall covered with climbing vegetation has been developed to evaluate the thermal effects of plants on heat transfer through building facades. This model allows for analysis of how various plant physiological parameters such as leaf area index, average leaf dimension, and leaf absorptivity can improve facade thermal performance by reducing the exterior wall surface temperatures and heat flux through the façade. The model has been verified with a set of experiments that measured both bare and vegetated facade thermal performance of an educational building in Chicago, IL, during the summer. A sensitivity analysis was also conducted to elucidate the relative impacts of plant characteristics, weather conditions, climate zones, wall assembly types, and facade orientation on vegetated facade thermal performance. Overall, results herein show that a plant layer added to the facade can improve its effective thermal resistance by 0.0–0.7 m2 K/W, depending on a range of inputs for wall parameters, climate zones, and plant characteristics (particularly leaf area index). These improvements are especially pronounced in predominantly warm climates with high solar radiation and, to a lesser extent, low wind speeds. The model developed herein can ultimately be used both to access facade thermal improvements in existing buildings retrofitted with green walls and to design green walls for optimal energy efficiency in new construction.