Think rents are getting unaffordable? It's about to get even worse, a new study has found.

The number of households severely burdened by rent—or those spending more than 50 percent of their income on rent—is set to rise at least 11 percent from an estimated 11.8 million this year to 13.1 million by 2025, with the largest increases expected among older adults, Hispanics and single-person households, according to new research from Harvard University's Joint Center for Housing Studies (JCHS) and Enterprise Community Partners.

More than one in four renters, or around 11.2 million households, were severely burdened, according to 2013 data, up by more than 3 million since 2000, the study noted. Adding in those facing "moderate" burdens—or spending 30-50 percent of income on housing—brings the total to just under half of all renters, the study said.

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"Even in the unlikely event that income growth greatly outpaces rent gains, the number of severely cost-burdened renters will remain near current record levels," said Christopher Herbert, managing director of Harvard's Joint Center for Housing Studies, in a statement.

The study's base case—which assumes both rents and incomes rice 2 percent annually—expects the number of severely rent-burdened households of those over the age of 65 will rise by around 39-42 percent over the next decade, compared with an around 27 percent increase among Hispanic households and around 12 percent for single-person households.