For the majority of American homeowners, their house is their single largest asset. Despite the crash in home values in the last decade, that still holds true. That crash, however, created a much larger share of renters, and these Americans are not enjoying the new wealth that now-rising home prices afford. Ninety percent of metropolitan housing markets have seen a decline in their homeownership rates, while home values are rising and incomes are flat, and that is widening the wealth gap, according to a new study by the National Association of Realtors, which looked at homeownership, home values and income growth from 2000 to 2013.

File photo: A house for rent sign in Centreville, Virginia. Paul J. Richards | AFP | Getty Images

"Homeownership plays a pivotal role in the U.S. economy and has historically been one of the primary sources of wealth accumulation for middle-class families," said Lawrence Yun, chief economist for the Realtors. "Unfortunately, due to an underperforming labor market, insufficient housing supply and overly stringent underwriting standards since the recession, homeownership has plunged to a rate not seen in over two decades," Yun added. "As a result, the country has become more unequal as the number of homeowners has fallen while the number of renters has significantly risen."

Read MoreHouse flippers seeing record returns

The homeownership rate rose to a high of more than 69 percent during the housing boom in the mid-2000s and has now fallen to 63.7 percent, the lowest in 25 years, according to the U.S. Census. With millions of homes either in the foreclosure process or in a negative equity position, the real homeownership rate is actually lower. At the same time, renting is near an all-time high, as younger Americans stay in such housing longer than historical norms, older Americans downsize into rentals and the millions who lost their homes to foreclosure during the housing crash now rent.