Curtain wall construction has begun on what will become the world’s largest passive house office building, marking an important milestone in the drive to give Boston an iconic and environmentally friendly new skyscraper that could help the city meet its carbon-free mandate by midcentury. 

Winthrop Center is awaiting certification from the Passive House Institute conferring its office portion with a status that, once completed, will make it the largest of its kind anywhere in the world.

Handel’s vision for the 52-story center is a mixed-use development that includes a bar, restaurants, 321 luxury residences, and a triple-high atrium gathering space dubbed The Connector that seeks to blur lines between indoor and outdoor space in the through-block site along Federal Street to the adjacent Winthrop Square.

Only the 800,000 square foot office portion of Winthrop Center will seek the designation although the building’s overall energy use will be 65% less than projects of similar scale. 

Passive House building is especially difficult in a climate such as Boston’s, which sees an annual 48 inches of snow and winter temperatures that can reach into the negative twenties. The design team was able to work around this by producing an airtight triple-glazed glass curtain facade and heat-exchange wheel to better optimize the building’s thermal efficiency and energy consumption via a T-shaped floorplan and influx of natural light.

Now, with a major funding hurdle behind it, developers are looking to the future, and the impacts gained by committing to a project focused on environmental health with implications that could change tall building construction forever.

“We always envisioned this building’s impact would extend beyond improving the lives of its residents and office tenants,” Millennium Partners principal Rich Baumert said. “We want it to touch the larger Boston and even global community as a legacy-building rooted in visionary principles.”

The curtain wall portion will be completed in November with construction expected to end in May of next year.