What's causing the shortage of affordable rentals

You don't have to be a recent college graduate just entering the job market to know that the U.S. housing market is in a bind, especially when it comes to affordable rental properties.

Despite the economic recovery and a rebounding housing market, the national rate of homeownership continues to decline, to just 64.5 percent last year.

On the flip side, a recently released report by Harvard University's Joint Center for Housing Studies noted that the drop in homeownership rates means an "exceptionally strong demand for rental housing," with the national vacancy rate for rental properties at its lowest level in decades.

And this exploding demand for rentals isn't affecting just the millennials, ages 18 to 34, who are starting out on their own. The Harvard study found that people ages 45 to 64 make up "about twice the share of renter growth," compared to households under 35.

At the same time, the latest National Rent Report by the rental website Zumper, which looks at data from over 1 million active rental listings nationwide, is reporting a surge of rental prices all over the U.S. -- not only in cities like San Francisco, where rentals have been notoriously expensive for decades.

Zumper noted the median cost for a one-bedroom rental in Austin, Texas, has risen by 17 percent this year. A similar rental in Kansas City, Missouri, is up 16.7 percent compared to last year, while a one-bedroom rental in Atlanta will cost you 13.6 percent more than it did in 2014.

But the rental housing boom isn't playing out everywhere. Zumper said in Nashville, Tennessee, median one-bedroom rentals dropped 19.6 percent over the past year. They're also down in Cleveland (-14.7 percent) and Minneapolis (-14.2 percent).

Zumper economist Devin O'Brien said those weaker rental markets are generally due to a metro area's own particular sets of economic dynamics as it deals with issues such as flat demand or scant housing development.

As for the broader nationwide rental boom, O'Brien told CBS MoneyWatch several trends are currently at play: millennials migrating to city centers, the rapid development of higher-end rental properties, new regulations that make it harder to secure a home mortgage and the growing number of would-be renters.

In the major metro regions, "You have this problem with the middle of the market -- your average person not being able to find affordable rent options anymore," he said.

Zumper CEO and co-founder Anthemos Georgiades said while the American dream of owning your own home isn't going away, its realities are changing as more people wait until later in life to get married, start families and purchase a home.

"Typically, most people will buy three houses in their lifetimes, but going forward it might now be two," he said. "And given the average age of marriage is now a decade older than (it was) half a century ago, that's had a massive impact on homeownership. So it isn't really a blip. It's a fundamental restructuring of the way society functions."