Nearly one-third of Americans are unlikely to qualify for a mortgage because their credit scores are too low, making homeownership out of reach for many. This is according to an analysis of more than 25,000 loan quotes and purchase requests on Zillow Mortgage Marketplace during the first half of September.

Borrowers with credit scores under 620 who requested purchase loan quotes for 30-year fixed, conventional loans were unlikely to receive even one loan quote on Zillow Mortgage Marketplace, even if they offered a relatively high down payment of 15 to 25 percent. Nearly one-third of Americans, or 29.3 percent, has a credit score this low, according to data provided by myFICO.com.

Meanwhile, the lowest interest rates went to mortgage borrowers who were among the 47 percent of Americans with excellent credit scores of 720 or above.

In the first half of September, borrowers with credit scores of 720 or above got an average low annual percentage rate (APR) of 4.3 percent for conventional 30-year fixed mortgages. Borrowers with mid-range credit scores between 620 and 719 received APRs between 4.73 and 4.44 percent, with the APR rising as credit score drops. Those with credit scores below 620 received too few loan quotes to calculate average low APR.

For those with mid-range credit scores of 620 to 719, improving one's credit score can mean a significant savings in interest over time. For each 20-point credit score increase, the average low APR declines 0.12 percent, which for a $300,000 home, with a 20 percent down payment, equates to a savings of $6,400 over the life of a 30-year loan.

"We are in an era of historically low mortgage rates, reaching levels not seen in decades. Coupled with four years of home value declines, homes are more affordable than we've seen for years. But the irony here is that so many Americans can't qualify for these low rates, or can't qualify for a mortgage at all," said Zillow Chief Economist Dr. Stan Humphries. "Four years ago, in the era of easy-to-get subprime loans, many borrowers with low scores did buy homes, which in turn helped contribute to a housing bubble. Today's tighter credit is a predictable response by banks after the foreclosure crisis, but also keeps a cap on housing demand, which is important for the greater housing market recovery."