The cost of renting a home in the U.S. has risen to its least affordable levels ever, taking up a record proportion of income in most major cities, according to a study from property website Zillow. 

"Rents are crazy right now," Dr. Svenja Gudell, chief economist at Zillow, said in a statement. "Unaffordable rents are making it hard for people to save for a down payment and retirement."

Renters in the U.S. can now expect to pay around 30.2 percent of their monthly income for rent, the highest percentage ever, up from pre-housing boom levels of around 24.4 percent, according to the analysis of second-quarter data on rental and mortgage affordability, which was released Thursday. The historical comparison period covered 1985-2000.

In some areas, rent is even more unaffordable. In Los Angeles, California, renters sent nearly 50 percent of their income to the landlord in the second quarter, while in the New York-Northern New Jersey and the Miami-Fort Lauderdale, Florida areas, that was hovering around 41-45 percent, the survey from the real estate listing and analytics company found.