Green Roofs and Urban Heat Islands

Guest blogger: Karina Bree, Program Manager at Sustainable Kingston

Most people don't think about roofs unless they need to be replaced, however roofs can do more than just keep the rain out. With the increase of hot days during the summer, roofs can also be used to decrease the heat island effect in urban environments. What is an urban heat island effect and how can changing our roofs help?

Urban Heat Island Effect

Urban heat islands typically occur in cities with high concentrations of pavement, buildings, and other surfaces that absorb and retain heat. The temperatures in the heat island are typically 1-3°C than the surrounding areas as well as have worse air and water quality.

This increase in temperature causes an increase in energy use due to more air conditioning use which can cause rolling blackouts and more emissions are generated. Cities can combat heat island effects by increasing green infrastructure such as lighting streets with trees, or encouraging green or cool roofs.

Green Roofs

There are two types of green roofs (extensive, and intensive) with the growing medium depth being the defining difference. Green roofs will reduce the heat absorbed and lower the ambient temperature using vegetation. Green roofs can be 10-15°C (30-40°F) cooler than conventional roofs, helping to reduce ambient air temperatures, and help reduce building energy costs.

Having more green areas in urban environments does more than just help lower the surrounding temperature. Vegetation helps trap and filter rainwater while slowing storm-water runoff which can reduce flooding. Having plants, especially flowering plants, spread throughout cities also helps support urban pollinator populations. Greenspaces are also known to reduce stress levels. That's quite a few benefits for increasing green spaces in our environment!

Cool roofs

A cool roof looks like a white roof, but there is more happening than just a change in colour - the roof may be 25°C (50°F) cooler than a traditional roof. This is because the lighter colour is more reflective allowing the roof to reflect more light and reduce heat transfer to the building. Overall, this reduces heating costs and extends the life of your roof since less heat is transferred to the building.

Both types of roofs reduce greenhouse gas emissions by lowering demand for air-conditioner use, save money by reducing energy use, and improve city comfort by reducing urban heat islands. Changing your roof can impact more than just how it looks!