Seasonal influence of leaf area index (LAI) on the energy performance of a green facade

Double-skin green facades using deciduous climbing plants are easy-to-implement construction systems stated to be effective energy-saving tools for buildings during cooling periods. Although the leaf area index (LAI) has been identified as a key parameter for characterizing foliar density and, consequently, the green facade's potential as a passive tool for energy savings, a lack of knowledge still remains on this index's values and measurement methods. The present paper aims to characterize the annual LAI evolution of a Boston ivy double-screen green facade under Mediterranean continental climate (Csa), by using an original non-destructive methodology during two consecutive years. Moreover, the influence of the green facade's foliage density, characterized by LAI, on the external building wall temperatures and the energy consumption by season and orientation was addressed. From the results it can be noticed that LAI changed seasonally over the course of five periods with a related differentiated energy performance: early summer (LAI of 4.8; 54% savings for cooling), late summer (LAI of 4.4; 30% savings for cooling), autumn (LAI of 1.7; 5.4% increase for heating), winter (LAI of 0.9; 5.4% increase for heating), and spring (LAI of 3.6; 11.9% increase for heating). The increase of energy consumption during leaf-off stage was directly linked to woody material and remaining leaves. Two crucial effects were identified and characterized: firstly, the influence of facade orientation and, secondly, a slight "insulation effect" at night, with the green screen acting as a thermal barrier.