If you live in a city, the roof of your home or building can play an important role in managing outdoor temperatures amid record-breaking heatwaves. Painting the roof white or with a reflective coating material is an age-old that just works. But wait, wouldn’t it be better instead to vegetation or install solar panels on the roof for cooling?

This is a question that often confuses people who are looking for ways to keep their roofs cool in summer.

A new study clears this confusion and reveals the best way to keep both your roof and city cool. 

The study authors used computer models to analyze and compare the cooling effect of different roof cooling methods in the Greater London area. Here’s what they found:

Which is better — green roof, white roof, or solar panels?

If you’re one of those people who believe that green roofing is the best way to keep your roof cool, you’d be surprised to know that this isn’t the case. A green roof has multiple layers, each with a specific function. The top layer has soil in which vegetation is grown, and then there are layers for drainage, waterproofing, thermal insulation, etc.

While it is true that during the daytime, green roofs can keep the indoors cool and lower the outdoor temperature, this effect reverses at night when plants release the stored heat.

“Green roofs would increase the average temperature due to their increased heat-storage capacity that increases heat release at night. On average their effect is null in terms of temperature reduction,” the study authors said.

The computer models revealed that cool roofs — highly reflective roofs that can be composed of different roofing materials (e.g., concrete, metal, or single-ply membrane) — work best when it comes to cooling your city’s air. For instance, on a hot summer day, the air surrounding a cool roof would be 1.2 °C cooler than the air surrounding a normal roof.

“We find that on average cool roofs most effectively reduce temperatures (∼−1.2°C), outperforming green roofs (∼0°C), solar panels (∼−0.5°C), and street-level vegetation (∼−0.3°C). Application of air conditioning across London, on the other hand, increases air temperatures by ∼+0.15°C, ” the study authors added.

Why this is an important question

Urban areas such as cities and developed towns tend to become warmer than their surrounding rural areas. This is because buildings, roads, and other urban structures absorb and retain heat more effectively than natural landscapes like forests or fields. 

This phenomenon is called the urban heat island effect and it leads to higher temperatures in cities, especially during hot weather, which can adversely affect local climate, energy use, and health. The current study is part of a large project that aims to explore ways to mitigate the effects of the urban heat island effect.

During their analysis, the study authors conducted simulations based on the conditions of 26th and 27th July 2018. These were one of the hottest days London has ever witnessed with outdoor temperatures exceeding 35°C. 

Although their urban climate models (UCM) reveal that cool roofs would’ve been most effective in reducing the outdoor temperature on those days, the results could vary if they change the city. This means that while cool roofs can help mitigate the urban island effect in London, they might not work well for other cities. 

“Several limitations apply to this study. For instance, we used only one UCM, and another UCM may produce different results depending on its dynamics and physics. Moreover, our study only focuses on outdoor temperatures and does not consider the benefits or the detriments caused by these interventions on other aspects such as biodiversity, indoor temperature, increase in relative humidity, wind circulation, etc.,” the study authors note.

Further research is required to address all such limitations and test cool roofs in cities with climate and geographical conditions different than those of London.

The study is published in the journal Geophysical Research Letters.