This paper presents the results from a modelling study that assessed the effectiveness of retrofitted green and cool roofs at reducing energy use for a typical office in Central London. These technologies were compared to application of retrofitting traditional insulation. Initial microclimatic modelling of the impact of the roofs showed that both green and cool roofs can reduce air temperatures directly above their surfaces, with maximum air temperature reductions of 1 °C. The results of the microclimatic modelling were then used as input into a whole building thermal simulation model, in order to assess the direct and indirect effects of the roofs.
The building modelling results indicate that in the current climate, green roofs reduce annual energy use within the building. The level of savings are reduced when green roofs are dry in the summer. Cool roofs are more effective in the summer, but result in an annual energy penalty due to their performance in winter. In a 2050 climate scenario, both green and cool roofs result in a reduction in annual energy use. The application of traditional insulation is the most effective technology at reducing annual energy use. Adding insulation and a green or cool roof reduces the relative effectiveness of the roofs.