A clear look at cool, green and solar roofs through the lens of urban heat and building energy.
The urban heat question
Air-conditioning helps people indoors, but the heat rejected outdoors can add to the urban heat cycle. Cities are therefore testing roof strategies that reflect sunlight, add vegetation or produce electricity.
What the cited studies reported
A University College London simulation reported that broad adoption of cool roofs during London’s hot 2018 summer could lower average outdoor temperature by 1.2°C, with local reductions reaching 2°C. An Argonne National Laboratory comparison in Chicago reported a 1.5°C reduction during the hottest period and a 16.6% reduction in air-conditioning energy for cool roofs; the cited green-roof and solar-roof results were 1.2°C / 14% and 0.6°C / 7.6%, respectively.
Three roofs, three roles
Cool roofs use reflective coatings or light-coloured surfaces to reduce solar absorption. Green roofs bring rainwater management, habitat and vegetation into the building envelope. Solar roofs prioritise electricity generation and can reduce reliance on the grid. The best choice depends on the climate, building, available space and the outcome the project needs to improve.
Design the condition, then compare
The figures above belong to the cited studies and their specific assumptions. They should not be treated as a universal product guarantee. A useful project comparison records the baseline roof, local conditions, material, geometry, measurement method and time period together.