Personal finance experts often advise people to spend no more than 30 percent of their income on housing. But for a growing number of renters, spending only 30 percent would be a welcome prospect, a new study finds.

Nearly 28 percent of all renters were "severely cost-burdened" in 2011, meaning they spent over half their incomes on rent, according to the study from Harvard University's Joint Center for Housing Studies. The study, which analyzed U.S. Census Bureau data, additionally found that more than half of all renting households – 50.8 percent – were either moderately or severely cost-burdened, spending more than 30 percent of their incomes on rent.

That share of renters facing potential affordability problems has grown substantially in recent years, the study says. As recently as 2000, only 38.1 percent of renters spent more than 30 percent of their incomes on rent. As of 1960, only 23.8 percent of renting households did.

A person looking for a rental right now faces an even more daunting affordability challenge. The median asking rent for a new apartment right now is $1,090 per month, compared to the $861 per month in median gross rent for all apartments.

The gap between rents and incomes has growth particularly wide since the turn of the millennium, the study notes. After decades of moderately diverging and converging, gross rents and incomes finally split more decidedly over the last 12 years. Rents grew by an inflation-adjusted 6 percent between 2000 and 2012, the report says, while real median renter incomes fell by 13 percent.

"As a result, the gap between rental costs and renter incomes in 2012 was wider than in any year except 2010," the study says.

Affordability issues for renters are among the great ironies to come out of the recession, says Shaun Donovan, secretary of Housing and Urban Affairs, as plummeting home prices caused many Americans to worry that housing had become too cheap.

"In the midst of the broader crisis you have a silent crisis in many ways going on among renters, and particularly among the lowest-income renters," Donovan said at a Monday event releasing the report.

The share of the population renting homes has grown sharply in recent years. In the 2000s, the number of renting households grew by 500,000 annually, but in the last three years, that rate increased to 1.25 million annually.

The increased interest in renting was driven in part by the recession. The downturn pushed some homeowners from their homes and prevented some potential buyers from purchasing houses, but the report also points out that the recession "highlighted the many risks of homeownership," like the potential for a drop in home value, as well as benefits of renting, such as the ease renting allows in relocating and the fact that it lessens the burdens of home maintenance.

While increased demand may have driven rents up, a dearth of low-income housing has also contributed to a lack of affordability for many renters. The number of low-income renters has grown alongside the number of high-income renters, meaning tougher competition for apartments and homes on the rental market. More than 30 percent of all rentals that "extremely low-income renters," defined as those with less than 30 percent of area median income, could afford in 2011 were occupied by higher-income households, according to the report.

This is helping to create a "supply gap," the report says. Between 2001 and 2011, the number of "extremely low-income renters" grew by 3 million, but the number of rentals affordable to them remained unchanged.