Environmentally-friendly buildings are on the rise, and it’s not just shiny new glass towers grabbing green headlines.

The U.S. Green Building Council, in partnership with Dodge Data & Analytics, announced the most recent findings of the Green Building Trends 2018 SmartMarket Report on Tuesday. The report—which surveys architects, contractors, consultants, developers, engineering firms and investors from 86 countries around the world—found that 45% of participants expect green projects to constitute a majority of their construction projects by the year 2021. Currently, 32% report a majority of green projects in their portfolios.

That expected growth is slightly less than worldwide averages. Currently, 27% of global respondents report a majority green projects, jumping up to an expected 47% in 2021.

Interestingly, the leading sector of growth for green construction in the U.S. is the retrofit of existing buildings. 50% of respondents selected this category, far exceeding the global average of 37%.

“If we want our cities to be truly sustainable, we have to find a way to reduce the environmental impact of the existing building inventory,” says Convene’s Brian Tolman, SVP of design and construction. “Clearly, you can’t start a city over from scratch with all new buildings.”

What’s driving this growth? Market demands. 44% of respondents say client demands are their top trigger for future green projects, compared to a global average of 34%. Another leading factor was the desire for healthier buildings (32% vs global average of 27%).

While the projected growth is strong, there are still major hurdles for further growth. Nearly three quarters of the respondents cited higher first costs as a top barrier to green building activity, while less than half of global respondents cited this as a top concern. Meanwhile, political support for green initiatives appear to be paying dividends in the U.S. Only 26% of respondents cited a lack of political support or incentives as a top barrier, compared to a 33% global average.

“With more and more people demanding and expecting healthier places to live and work, more and more leaders around the globe are committing to green building, which is now a trillion-dollar industry,” said Mahesh Ramanujam, president and CEO of USGBC. “We need to get all buildings on a path to sustainability in order to raise the standard of living for all people around the world, regardless of their circumstances. And the results of this study show we are on the right path.”