Tree canopy shade impacts on solar irradiance received by building walls and their surface temperature

Abstract

Shading coefficients are used to predict the seasonal and diurnal benefit of architectural shading devices. It is more difficult to model the impact of tree shade on building thermal performance, and data is rare and varies greatly with species and season. We established a range of tree shade amounts and shade qualities from which to develop simple, robust models that predict external wall surface temperatures. We measured percentage shade cover, solar irradiance and external surface temperature on north and west sun-bearing walls of three identical buildings in spring and summer 2010/11. One building was shaded by tall Angophora trees, another by smaller Fraxinus trees and one was unshaded.Tree shade reduced wall surface temperatures by up to 9 °C and external air temperatures by up to 1 °C. The smaller trees did little to reduce external wall surface temperatures, and moving the tall trees further away from the building wall eliminated their cooling benefits. Wall surface temperatures were best predicted by shade cover and solar irradiance, and was most poorly predicted by shading coefficients, that varied greatly through the day and the season, as tree height and leaf area index increased.Trees can reduce external solar irradiance loads when they are close enough and tall enough to shade the majority of the wall. To simulate the thermal performance benefits that trees provide, it is necessary to account for seasonal, growth and phenological changes in tree shade amount and quality.