That trend is continuing as young people and doubled-up families move out on their own. “They’re creating a lot of incremental demand,” Mr. Humphries said. But new households rarely plunge straight into homeownership, especially given that mortgages are much harder to obtain than they were before the financial crisis. “The expectation is that when they strike out into their own units they’ll be moving into rental as opposed to the owner side,” he said.

And as rents head higher in the tightest markets, many are discovering that living on their own is proving unaffordable, forcing them to double up again. Arturo Breton, a 37-year-old waiter in Miami Beach, said that after years living on his own, he was joining forces with a roommate who works as a manager at J. C. Penney. “I’ve come down to the conclusion that in this country, it’s easier for two people to pay the rent than for one person,” he said.

For many middle- and lower-income people, high rents choke spending on other goods and services, impeding the economic recovery. Low-income families that spend more than half their income on housing spend about a third less on food, 50 percent less on clothing, and 80 percent less on medical care compared with low-income families with affordable rents, according to a new report by the National Low Income Housing Coalition. And renters amass less wealth, even non-housing wealth, than homeowners do.

The problem threatens to get worse before it gets better. Apartment builders have raced to build more units, creating a wave of supply that is beginning to crest. Miami added 2,500 rental apartments last year, and 7,500 more are expected in the next two years, according to the CoStar Group, a real estate research firm.