Impact of vegetation architecture on green roof thermal inertia: An empirical study of apparent thermal diffusivity

Green roofs (GR) are critical for reducing building energy demands, yet thermal interactions between specific vegetation architectures and substrate layers remain difficult to quantify for building services design. This study applies the amplitude method over an annual cycle to calculate the apparent thermal diffusivity of GR simulators. Using distinct species as models for creeping ( Modiola caroliniana ) and bunchgrass ( Nassella tenuis ) architectures, we demonstrate how plant morphology fundamentally alters heat transfer mechanics. Results revealed creeping vegetation caused significantly higher diffusivity in upper substrate layers (0–5 cm), accelerating surface heat transfer. Conversely, bunchgrass restricted surface diffusivity but increased it in the deeper 5–12 cm profile. Crucially for building load calculations, this divergence yielded a substantial difference in thermal inertia: the bunchgrass system provided a 3.5 h thermal lag, significantly outperforming the 2.7 h lag in the creeping system. These findings establish that plant architectural traits are critical parameters modulating heat flux into the building envelope. This study provides actionable data to optimise GR thermal performance and peak load attenuation in variable climates.