Venture capitalists have poured more than $4 billion into the green building industry since 2000, according to a new report released by Boston-based Lux Research.

"Early venture capital investors are looking for exits for the first wave of successful green buildings startups, and the seeds of the next crop are being sown in on-site generation and sustainable materials," says Ryan Castilloux, Lux Research analyst and the lead author of the report titled, "Building a Green 21st Century: Tracking Venture Investments in Green Buildings to Uncover New Opportunities."

Of the 332 venture investment rounds in technology and materials developers for green buildings since 2000, Lux Research determines that 152 were series A investments and 83 were series B. Last year, nearly half of all funding - totaling $445 million - went into 15 late-stage investments, which Lux Research sees as a signal for the maturation of the first wave of green building sector start-ups.

However, Lux Research warns that this sector may be contracting. Since 2010, 11 of the 16 venture capital-backed companies tracked in the report have been acquired. Other start-ups - including Digital Lumens, Redwood Systems, Adura Technologies, Metrolight, EnOcean, Serious Energy, Control4 and Aspen Aerogels - have been dubbed "ripe" for acquisition.