The history of 20th-century architecture abounds with buildings that critics love and users hate, but there aren't many that have suffered such extreme public mood swings as the Richards Medical Research Laboratory at the University of Pennsylvania.

Designed by the acclaimed Philadelphia architect Louis Kahn, the clutch of brick towers was celebrated in a solo show at New York's MoMA in 1961, a year before it opened. The museum deemed Richards "the most consequential building constructed in the United States" since World War II.

The dirty secret is that Penn's scientists hated Richards from the day they moved in. They complained there was no privacy in the large, open-plan labs, and no respite from the natural light that flooded in from Kahn's generous windows. When papering the glass with computer printouts didn't solve the problem, scientists fled in droves, and Richards became a place where, as Penn's university architect David Hollenberg so delicately put it, "people who had less prominence found themselves."

I suspect the pecking order is about to be reversed now that Hollenberg's office has finished renovating two of Richards' four towers. Having long ago acknowledged that Kahn's masterpiece wasn't right for sensitive experiments involving bubbling beakers of chemicals, Penn decided to convert the complex into offices and computer labs for researchers.

The renovation has pared Kahn's spaces down to their essence, restoring a Zenlike calm, and revealing the muscular concrete structure that made the design such a revelation in the early 1960s, when International Style glass towers were all the rage. As Kahn, who was known for his mystical pronouncements, might say, Richards has become the building it always wanted to be.

To nonarchitects, it may be hard to understand why the subdued brick-and-concrete complex is so important. Even Kahn's own son, Nathaniel, seems perplexed when he visits the building in My Architect, his Oscar-nominated documentary about his father's life, and gazes up at windowless, almost medieval, ventilation towers.

But that perceptible heaviness is what made Richards so ground-breaking. For decades, mid-20th-century architects had been turning out high-rises with thin glass skins that seemed light enough to float way. The facades revealed nothing about the building's structure. At Richards, you can see how the building is made and feel its weight.

Kahn took his cues for Richards from ancient European buildings, such as the windowless watchtowers in the Italian town of San Gimignano, where the spare, heavy forms make the architecture feel eternally rooted in the hillsides. Kahn transferred that timeless sensibility to the modern high-rise. Half a century after it opened, the 1960s building still looks at home next to Penn's late-19th-century dormitories, the Quad, which have their own dramatic towers.

Because of Richards' exalted status in architectural history, Hollenberg admits he had some trepidation about undertaking the renovation in July 2014. Richards is still revered by architects, and it appears in the curriculum of virtually every architecture program in the U.S. and beyond. It easily qualifies as Penn's most important 20th-century building.

Hollenberg further complicated his task by advising Penn to nominate Richards for National Historic Landmark status. After it received the honor in 2009, the renovation became subject to strict oversight from federal preservation agencies.

Richards would have been a tricky building to renovate under any circumstances. One of the things that makes the design so pioneering is that Kahn invented a new way of incorporating the mechanical systems - electrical, plumbing, ventilation. They were segregated into the thin brick towers that are attached to the lab buildings. Kahn called them "servant spaces."

The advantage of his approach is that it kept the lab floors - the "served" space - open and airy. To provide connections to the labs, Kahn designed an open, concrete grid ceiling and threaded the pipes and ducts above the labs. All that had to be reconfigured.

Like many architects, Kahn worked in an open-plan studio with lots of light, and he assumed scientists would enjoy the same arrangement. (Unlike today, the users weren't always consulted.) But scientists in the 1960s were more territorial, and they wanted privacy for their experiments. Over time, they colonized the spacious labs with partitions.

The renovation - executed by a large cast of architecture and engineering firms - has stripped the labs of their manic clutter, banishing the mess of ventilation hoods, ad-hoc walls, and tangle of overhead pipes, which were notorious for leaking.

Removing those accretions was just the beginning. Hollenberg's team has cleaned all the concrete, giving it the glow of travertine, and replaced every window. After much debate, Hollenberg says they decided against installing energy-efficient insulated glass, which would have changed the building's look, and opted for thicker glass plates. They've carefully restored the elegantly thin window frames that give the jutting office windows their distinctive crisp corners.

Though a large part of each floor remains an open work area, Penn did insert a few private offices. But glass partitions help maintain the natural light in the common area. The windowless "servant" towers now house coat closets, kitchens, and seminar rooms.

Despite all the effort, the project was a relative bargain, costing $13.4 million for the first two towers, less than most new office buildings. There's a huge need for "dry" office space for Penn scientists, who are increasingly doing their research on computers. Although the second phase hasn't been approved yet, Penn expects the renovation of the other two towers will cost $16.4 million.

Maja Bucan, a professor of genetics at the medical school, says she loved the feel of the renovated space - even the open plan - when she toured the building. The views are "magnificently green and relaxing."

After spending most of her career hearing horror stories about Richards and avoiding the building, she's been thinking she wouldn't mind moving her office there.

Hollenberg says that, "for an architect of my generation, Kahn means so much." Once nonarchitects see Richards, they might start to think the same thing.