The U.S. housing market remains in the doldrums, says Karl Case, co-creator of the S&P/Case-Shiller home price index.

A high vacancy rate combined with a low construction rate makes the Wellesley College economics professor pessimistic, he told Bloomberg.

The S&P/Case-Shiller index for 20 cities gained 3.8 percent in April from a year earlier, its biggest increase since September 2006.

But much of that appreciation stemmed from a flurry of home sales before the April 30 expiration of the home buyer tax credit.

Meanwhile, housing starts have remained at 15-year lows for the past 18 months, Case points out. “Home building is dead flat in the mud,” he said.

That stagnancy in building has come in the face of an increase in households.

“The census is telling us that households are being formed, but they don’t seem to be showing up (in new homes),” Case said.

This conundrum stems partly from the “doubling-up phenomenon,” he said. That describes people who have decided to live together or with their parents to reduce their housing costs.

With foreclosures continuing to spread, the housing market probably won’t stabilize until the second half of 2013, Rick Sharga, vice president of research firm RealtyTrac, tells the Los Angeles Times.

"It is a much longer recovery cycle than we have seen in housing," he said. "But the boom was also unprecedented."