Chicago planted a 20,300-square-foot green roof on City Hall; EPA says its surface can be about 100°F cooler than nearby conventional roofs and saves nearly $5,000 in energy each year

In 2000, Chicago began doing something unusual with its own city hall. Instead of leaving the rooftop bare, city officials planted grass, shrubs, vines and trees across 20,300 square feet, creating a roof with about 20,000 plants. The EPA's Heat Island Community Actions Database shows that this particular rooftop’s surface temperature now runs about 100 degrees Fahrenheit cooler compared to the other conventional roofs nearby, while saving almost $5,000 in energy expenses annually. Even more than 20 years later, it continues to be an example of a functioning green roof.

What a green roof actually is

A green roof is more than a few pots of plants scattered across a terrace. Instead, it is a complex structure involving waterproofing, a drainage system, growing medium, and vegetation, capable of enduring the weather fluctuations and thriving throughout the year. The EPA's page on green roofs explains that green roofs may range from a few inches of ground cover up to an entirely accessible park with trees.

Why bare rooftops get so hot in the first place

Cities are full of dark and hard surfaces like asphalt roads, tar rooftops, and concrete walls. These surfaces absorb heat during the day and release it slowly, keeping neighborhoods warm even after sunset. Such a phenomenon is termed the urban heat island effect and is one of the reasons why cities tend to be several degrees warmer than rural areas surrounding them. Green roofs help in breaking this cycle. Rather than trapping solar energy and radiating it back out as heat the way a bare dark roof does, plants use much of that energy for photosynthesis and release water through evapotranspiration, which helps keep heat from radiating directly into the building or surrounding air.

The research behind the numbers

Chicago's rooftop is not an isolated case. A 2016 study published in the journal Urban Forestry & Urban Greening studied green roofs in metropolitan Adelaide in Australia and found that green roofs provided a cooling effect during summer and acted as an insulator to warm buildings during winter. In the study, two types of roofs were investigated: shallow extensive roofs and intensive roofs with shrub plantings, and the intensive roofs provided better thermal insulation than the extensive roofs. The authors estimated that an increase in the ratio of green roof coverage by 30% would save up to 2.56 W/m2/day of electrical energy, a small but real dent in a building's energy load. Adelaide's climate is drier and milder in winter than Chicago's, which sees hard freeze-thaw cycles the Australian study didn't have to account for, so these numbers are best read as a general pattern rather than a direct prediction for Chicago specifically.

Where the $5,000 in savings comes from

A separate EPA resource on green roofs explains why energy bills can drop. The green roof serves as an insulator that separates the sun from the ceiling of the building and makes it easier for the air conditioners to operate in summer while making sure not too much heat is lost in winter. Chicago's building managers credit the City Hall roof with steady annual savings, a figure consistent with the EPA's own database entry for the project mentioned above, and the EPA says those savings could rise as energy costs increase.

More than just a temperature number

The benefits go beyond cooling. Chicago's green roof was created to test how native and non-native plants survive in this condition and to estimate how much stormwater can be absorbed by the green roof before reaching city storm drains. The same EPA database emphasizes that, apart from being an experiment, it was also serving as an educational initiative where Chicago's Department of Environment gave presentations regarding it.

Why millennials and city planners both care

For people facing hotter summers and rising utility bills, green roofs are more than a design trend. Green roofs combine climate adaptation and cost savings by cooling buildings, easing pressure on the electrical grid during heat waves and also offering people living in concrete jungles a little patch of nature. Chicago's City Hall roof shows that a rooftop garden is far from ornamental; it can get some serious work done, one flowerbed at a time. Since then, other US cities have followed suit by offering various kinds of green roof incentives, such as Washington DC's rooftop rebate program and Cincinnati's low-interest green roof loans. These projects do not claim that they can single-handedly address urban heat island problems. However, combined, these projects seem to share the same lesson that Chicago learned in 2000: sometimes the simplest fix for a hot city is to let something grow on top of it.