Student loan debt is preventing millions of Millennials and Gen Xers from buying their first homes -- and that is having a big impact on the housing market.

According to one recent analysis, student loan debt has caused an 8% decline in home purchases among Americans ages 20 to 39. That represents $83 billion worth of real estate transactions, estimated Rick Palacios, director of research for John Burns Real Estate Consulting.

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Total outstanding student loan debt now stands at $1.1 trillion, with the average debt for a grad under the age of 30 now reaching $21,000, according to the Federal Reserve Bank of New York.

Palacios figures that for every $250 a month in student loan debt that a household owes, it reduces their power to purchase a home by $44,000. And, he notes, that there are nearly 6 million households that pay more than $250 a month on their student loans.

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Mark Zandi, chief economist for Moody's Analytics, believes there are too many moving parts, such as economic conditions and changing lifestyles, to put an accurate dollar estimate on the impact of student loan debt on the housing market. But he -- like many other industry watchers -- agrees that the trillion-dollar-debtload has had definite repercussions.

Last year, the National Association of Realtors (NAR) conducted a housing market survey of 2,000 Americans, in which nearly half referred to student loan debt as a "huge obstacle to homeownership."

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The percentage of homes sold to first-time homebuyers, the most likely buyers to owe student debt, fell to 29% this year compared with the long-term average of 40%. Meanwhile, the homeownership rate among those under the age of 35 has plummeted -- to 36.2% in mid-2014 from a record 43.6% just 10 years ago, according to the Census Bureau.

The New York Fed contends that most student loan debtors who can't afford to buy a home are those who failed to complete their educations. Piling up debt without getting a high-paying degree is a poor recipe for homeownership.

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Other factors, such as high unemployment, could also be at play, said Zandi.

Leaving school to try to make a living during the bumpy Great Recession years had a "scarring effect" on young Americans, said Stan Humphries, chief economist for real estate site Zillow.

"When students graduate during periods of high unemployment, long-term homeownership is affected," he said.

Millennials live at home with their parents longer and postpone things like marriage and having children -- one of the prime reasons people buy homes.

If such trends continue, homeownership for Millennials could be even lower in the future, said Humphries.