The average rate on the 30-year fixed mortgage fell to 3.91 percent this week, Freddie Mac said today. That matches the record low reached two weeks ago.

The average on the 15-year fixed mortgage ticked down to 3.23 percent from 3.24 percent. That’s up from 3.21 percent two weeks, also a record low.

Mortgage rates are lower because they tend to track the yield on the 10-year Treasury note, which fell below 2 percent this week. They could fall even lower this year if the Fed launches another round of bond purchases, as some economists expect.

Still, cheap mortgage rates have done little too boost the depressed housing market. For eight straight weeks at the end of 2011, the average fixed mortgage rates hovered around 4 percent. Yet many Americans either can’t take advantage of the rates or have already done so.

High unemployment and scant wage gains have made it harder for many people to qualify for loans. Many don’t want to sink money into a home that they fear could lose value over the next few years.

Previously occupied homes are selling just slightly ahead of 2010’s dismal pace. New-home sales in 2011 will likely be the worst year on records going back half a century.

Builders are hopeful that the low rates could boost sales next year. Low mortgage rates were cited as a key reason the National Association of Home Builders survey of builder sentiment rose in December to its highest level in more than a year.

But so far, rates are having no major impact. Mortgage applications have fallen slightly in recent weeks, according to the Mortgage Bankers Association.

To calculate the average rates, Freddie Mac surveys lenders across the country Monday through Wednesday of each week. The average rates don’t include extra fees, known as points, which most borrowers must pay to get the lowest rates. One point equals 1 percent of the loan amount.

The average fee for the 30-year loan rose to 0.8 from 0.7; the average on the 15-year fixed mortgage was unchanged at 0.8.

For the five-year adjustable loan, the average rate declined to 2.86 percent from 2.88 percent. The average on the one-year adjustable loan rose to 2.80 percent from 2.78 percent.

The average fee on the five-year adjustable loan rose to 0.7 from 0.6; the average on the one-year adjustable-rate loan was unchanged at 0.6.