Rates should stay low for another month or two as government efforts to keep them low remain effective, predicts Michael Larson, an interest rate and real estate analyst with Weiss Research.

But it won't last forever. Rates will eventually trend upward, Larson said, as the economy starts to turn around and concerns return about how long overseas investors can stomach massive levels of U.S. debt.

"Over the longer-term, there's going to be general upward pressure on interest rates across the spectrum," he said. "Mortgage rates will be caught up in that."

To prop up the housing market and help the economy revive from the worst recession since the 1930s, the Federal Reserve is spending $1.25 trillion on mortgage-backed securities, which has driven down rates on home loans.

That money is set to run out by winter, though some analysts expect the central bank to add more money to the program or allow it to last longer by gradually reducing its purchases.

"What the Fed does not want is to see a sharp pickup in mortgage rates," said Greg McBride, senior financial analyst with Bankrate.com. "That would throw cold water on a recovering housing market."

With rates low, borrowers are seizing on the opportunity. Mortgage applications for refinancing surged 22.5 percent for the week ending Sept. 4 in the biggest one-week jump since mid-March. According to the Mortgage Bankers Association, applications for home purchases were up 9.5 percent.

Despite government efforts to prop up the mortgage market, qualifying for a loan is still tough. Lenders have tightened their standards dramatically, so the best rates are available to those with solid credit and a 20 percent downpayment.

Freddie Mac collects mortgage rates on Monday through Wednesday of each week from lenders around the country. Rates often fluctuate significantly, even within a given day.

The average rate on a 15-year fixed-rate mortgage fell to 4.5 percent, from 4.54 percent last week, according to Freddie Mac.

Rates on five-year, adjustable-rate mortgages averaged 4.51 percent, down from 4.59 percent a week earlier. Rates on one-year, adjustable-rate mortgages rose to 4.64 percent from 4.62 percent.

The rates do not include add-on fees known as points. The nationwide fee for loans in Freddie Mac's survey averaged 0.7 point for 30-year and 15-year loans, 0.5 point for five-year loans and 0.6 point for one-year loans