NLIHC analysed data from the 2008 American Community Survey. It found that the shortage of homes affordable to ELI families grew from 2.7 million in 2007 to 3.1 million in 2008 - about 400,000 units.
ELI families are defined as those earning less than 30 percent of the Area Median Income.
According to the report, released earlier this month, the number of all renter households in the U.S. increased by 2.4 percent between 2007 and 2008, but the number of ELI renter households increased even more, by 3.5 percent.
"It`s a lot of people going through foreclosures entering the rental market, the result of the current economic situation," explained Megan DeCrappeo, a research analyst for NLIHC. "People are sort of being pushed out of home ownership."
The number of ELI households has increased because people`s incomes are decreasing, DeCrappeo told IPS.
"They`re kind of dropping into a new income group there. People are becoming unemployed because of the current situation, the recession - more and more people are becoming extremely low-income," DeCrappeo said.
"For every 100 extremely low income renter households, there were 39 rental housing units affordable and available for them in 2007. By 2008, the number of affordable and available units had declined to 37," the report said. "A scarcity of housing that the poorest families can afford is the principle cause of homelessness in the United States."
During the same period, the supply of all rental homes increased by 2.2 percent overall. However, the increase in supply was experienced by every income category except the ELI group. For ELI households, the supply of affordable rentals decreased by 1.8 percent.
"In general, we`ve noted this kind of a trend for years. New housing generally is built in other income groups, not for ELI households," DeCrappeo said. "While the demand grows in that sector, the supply doesn`t keep up because it can be expensive to construct and operate housing for ELI households."
"There have been all these foreclosures and increase in vacancy rates, but these [vacant units] are not at levels affordable to people of ELI groups," she added. "They`ve had to take other measures, cut out food or health care costs, doubling up with friends and family, things like that."
DeCrappeo said that some people with higher incomes are also moving into the units affordable to ELI households.
"We`re all kind of competing for low-cost housing," she said.
Asked if landlords raising rents has been part of the problem, DeCrappeo said NLIHC has observed a decrease in rents overall, but not enough to reach the poorest households.
Ironically, even as the number of families needing affordable housing has grown, there is also an increase in vacant housing, including foreclosed homes as well as vacant upscale condominiums and apartments.
So why don`t the owners of the condos just lower their rents? "The reason why so much housing for ELI households needs to be subsidised by the government is it`s difficult to operate the housing [with such small revenues]," DeCrappeo said.
Another factor contributing to the decrease in ELI units is the mass demolition of public housing in cities and counties across the U.S.
"We`ve lost tens of thousands of public housing units over the years and that`s just contributing to the shortage," Linda Couch, deputy direct