FROM 2001 TO 2007, the nation's affordable rental housing stock decreased by 6.3 percent while the high-rent rental housing inventory increased by 94 percent, according to a recent paper from the Department of Housing and Urban Development (HUD).

This translates into a loss of more than 1.2 million unassisted affordable rental units during that time.

The paper also reports that since 2000 low-income housing tax credit properties have comprised about 50 percent of all newly constructed multifamily rental units.

U.S. Rental Housing Characteristics: Supply, Vacancy, and Affordability