What is our assessment?

There is strong evidence that green roofs sequester carbon and may reduce building energy consumption, although emissions reduction data are limited and vary with geography, roof design, and other factors. The potential climate impact of increasing green roofs is likely too small to be globally significant (>0.1 Gt CO₂‑eq/yr ). The solution, however, is considered “Worthwhile” because it can reduce energy use in buildings and sequester carbon while helping communities adapt to climate change and benefiting human health, the environment, and building owners.

What is it?

Vegetation planted on specially engineered rooftops sequesters CO₂ through photosynthesis and provides indirect cooling for buildings through evapotranspiration, reflecting heat back to the atmosphere, and shading. This cooling plus the added insulation inherent in the design can reduce the air conditioning loads of the building, particularly compared to dark rooftop surfaces, and therefore reduce emissions from the electricity used to power cooling systems. Green roofs can also reduce heating energy use and corresponding GHG emissions due to the insulation that soils and plant matter provide. Green roofs are in use in all regions of the globe, but concentrated in high-income countries. 

Does it work?

There is strong evidence that green roofs sequester carbon and can reduce the energy consumption and therefore emissions from cooling and heating buildings. Carbon sequestration by vegetation on green roofs has been documented in several studies. A study in Germany found that plants absorbed 141 g carbon/m²/yr (517 g CO₂ /m²/yr) over a 5-year period. However, carbon sequestration rates are difficult to generalize due to variations in design, plant types, and climates. 

Reported building energy savings from green roofs can range from negligible to 60% or more for cooling. For heating the savings can reach 45% or more, but some studies also show a roughly 10% increase in heating energy use with a green roof. The large variability in energy savings outcomes is due to differences in climate; existing insulation and other properties of buildings; green roof design, vegetation and maintenance practices; and measurement and modeling approaches. The highest energy savings potential has been calculated in dry-winter subtropical highlands for cooling and in humid subtropical climates for heating. Areas with short and mild winters are most likely to see heating energy use increase with green roofs, but these areas often have net energy savings when heating and cooling are combined, and most studies of green roofs show a reduction in heating energy use. 

When combined with the carbon sequestration effect of vegetation, green roofs appear to consistently reduce GHG emissions. 

Why are we excited?

Green roofs and other urban green spaces (see Increase Urban Vegetation) provide valuable climate adaptation, human health, environmental, and economic benefits. Green roofs can help cities adapt to climate change because the vegetation reduces heat exposure during extreme heat, while the soil and root systems absorb stormwater – thereby reducing runoff and flooding risks during extreme rainfall. Green roofs improve human health because vegetation filters the air and reduces noise transmission, and interactions with green spaces, including green roofs, have been shown to improve mental well-being. Green roofs can increase biodiversity and habitat and remove water pollution. They also can increase the property value of a building and prolong the longevity of the roof.

Why are we concerned?

Increasing green roofs can be challenging due to high up-front cost, lack of supportive policies, structural and climate limitations, maintenance requirements, and lack of awareness. A green roof can cost three to six times more than a conventional roof, and although it can save energy for cooling and heating, the returns on investment can be lengthy and savings may not be enough to fully offset the higher costs. In addition, not all roofs can support vegetation, rooftop plants can struggle to survive in hot and dry climates, and green roofs may increase heating energy use in buildings in climates with short and mild winters. A green roof also requires maintenance such as watering, plant care, weed control, pruning, and regular inspections. Finally, a lack of awareness is a major barrier to greater adoption. We also noted a lack of measured, rather than modeled emissions reduction data and on current and potential green roof adoption globally.