In St. Paul, Penfield added a vegetated roof over a conventional membrane; monitoring found the green-roof membrane peaked about 51°F cooler than the conventional roof

It was a hot June afternoon in downtown St. Paul, Minnesota, and the black membrane atop an apartment complex called Penfield was hot enough to feel like a pizza oven. A two-year study report, published in 2015, on the Penfield Apartments by green-roof researchers Angie Durhman and John Buck, reveals that a conventionally built roof with a gravel-ballasted roof membrane reached more than 136°F on its hottest day, while the planted, vegetated, soil-laden roof peaked at about 85°F. In the same building, in the same sun, but with a recorded 51-degree gap. That difference should be more prominently featured at a time when we are increasingly feeling the impact of the AC and electricity bills and the metropolitan cities as they become concrete convection ovens in the summer.

The 51-degree difference sitting above your head

The Penfield study was not a single snapshot, but rather, temperature was recorded every five minutes for two years. And the heat spike wasn't the only extreme. The traditional roof membrane swings by as much as 75 °F in a single day and absorbs the heat from the sun, releasing it by night. The little movement by the green roof membrane was under 2.4°F, and 8 inches of soil and plants provided a layer of insulation. The reason why buildings get sweaty by the time it gets to August is essentially this.

This is not limited to one case for one courtyard in Minnesota, but to all American cities. The urban heat island effect is driven in part by how solar energy is absorbed by surfaces such as asphalt, dark roofing, and glass and then released over time. In addition to reducing building energy use for cooling, vegetated roofs can be 30 to 40°F cooler than conventional roofing during hot days, and can help lower the ambient temperature of the city, according to a U.S. Environmental Protection Agency (EPA) report. It's how Chicago-to-Washington, D.C. cities have been utilizing green roofs on federal buildings and city offices, and it's a landscaping trend, but it's also basic thermodynamics at work and a great tool for reducing energy costs.

Why should this matter

This extreme heat is no longer an inconvenience in the background but a mortality statistic. A 2024 peer-reviewed study titled “Projections of Extreme Temperature–Related Deaths in the US,” published in JAMA Network Open, by Sameed Khatana and his colleagues at the University of Pennsylvania, notes that extreme-temperature-related deaths in the contiguous United States (currently just over 8,249 each year) are expected to be more than double, about 19,300 annually by mid-century even in a low-emissions scenario, and could multiply more than three times in a high-emissions scenario. The researchers also discovered that the risk for Blacks and Hispanics was even higher than for Whites, making rooftop heat a “stealth” problem, rather than just an inconvenience, of inequality.

So why isn't every rooftop already green?

Well, for the most part, cost and inertia. Green roofs take more commitment in the initial design, irrigation, and maintenance than a simple roof consisting of another membrane and an additional gravel layer, which is why more of them are not found unless it is required or encouraged by a city. Partially because very little real performance data existed for cold-climate cities. The upside, as per the same EPA guidance, is that the payoff compounds: a roof that doesn't swing 75 degrees a day doesn't crack, expand, and need replacement nearly as often, which is the quiet offset to the additional up-front cost to install it on the roof over the roof's lifetime.

But none of it implies that all apartment complexes in the USA need to start developing a rooftop meadow out of the blue. However, the next time your building's HVAC unit is struggling in August, consider asking why your roof still resembles a rig that was built for some no longer existing climate. The data has been included in a report on Minnesota monitoring since 2015, but it's taken this long for the rest of us to see the temperature.