Washington ? Rates for 30-year mortgages fell to a new low this week amid growing signs of a sluggish economic recovery.

Freddie Mac, the mortgage company, reported Thursday that the average interest rate on a 30-year fixed-rate mortgage dropped to 6.31 percent, the lowest level since the company began its nationwide survey in April 1971. Last week, rates on 30-year mortgages averaged 6.43 percent.

A year ago this time, 30-year mortgages averaged 7 percent.

This week’s rate surpassed the previous low rate of 6.34 percent reached just two weeks ago.

Low mortgage rates is spurring a further boom in mortgage refinancing. Savings or extra cash coming out of refinancing deals is helping to support consumer spending and offset some potentially negative factors, such as the volatile stock market and eroding consumer confidence.

The average interest rate on 15-year mortgages, a popular option for refinancing, dipped to 5.69 percent this week, the lowest level since Freddie Mac began tracking these rates in August 1991. That compared with last week’s rate of 5.84 percent. A year ago, 15-year mortgages averaged 6.54 percent.

On one-year adjustable-rate mortgages, lenders were asking an average initial rate of 4.37 percent, down from 4.45 percent the previous week. Last year this time, one-year ARMs averaged 5.70 percent.

These rates do not include add-on fees known as points. Each loan type carried an average 0.6 point this week.

The economy grew by just 1.1 percent in the second quarter of this year, down from a brisk 5 percent pace in the first quarter. That suggests “the economy faces weak growth,” said Freddie Mac’s chief economist, Frank Nothaft. That spurred speculation that the Federal Reserve might cut interest rates by the end of the year, if not sooner, he added.

“That expectation, in turn, has created a boon for potential and existing homeowners in the form of lower mortgage rates,” Nothaft said. “Additionally, low mortgage rates open the window to homeownership for more first-time homebuyers.”