Only 10 floors have been completed in what is intended to be the tallest residential building in the Western Hemisphere — a slender, 84-story tower on Park Avenue at 56th Street in Manhattan. But the top penthouse is already under contract for $95 million.

Other buyers have snapped up apartments on lower floors for prices that are almost as breathtaking. While their identities are not known, it is likely that many are the rootless superrich: Russian metals barons, Latin American tycoons, Arab sheiks and Asian billionaires.

Ultraluxury housing and construction is booming across Manhattan, which is now beginning to rival London in popularity with the world’s wealthy. The number of condominium buildings in the borough with apartments selling for more than $15 million has risen to 49, up from 33 in 2009, according to CityRealty.

And an additional 20 or so are under construction or in planning.

“There’s a great deal of interest in New York, which is seen as relatively cheap compared to other global cities,” said Yolande Barnes, director of research for Savills, an international real-estate firm.