Mortgage rates fell this week, with the average rate on 30-year fixed-rate mortgages falling below 5% to a four-month low, according to Freddie Mac's (FRE) weekly survey of mortgage rates.

Meanwhile, the 15-year fixed rate dropped to 4.36%, the lowest figure since Freddie began tracking such mortgages in 1991. The average rate was 4.46% last week and 5.78% last year. Five-year Treasury-indexed hybrid adjustable-rate mortgages also hit a record low, dropping to 4.42% from 4.51% and 6%, respectively.

After yields on Treasurys rebounded from the multi-decade lows they hit earlier this year, they have since retraced some - taking mortgage rates along with them.

According to the Commerce Department's data for August, new home sales rose for the fifth time but existing home sales fell unexpectedly, breaking four months of improvement. And while the S&P Case-Shiller indexes showed home prices increased again in July from the prior month, Dallas Federal Reserve President Richard W. Fisher warned on Tuesday that housing, though stabilizing, was still "on life support."

The 30-year, fixed-rate mortgage averaged 4.94% for the week ended Thursday, down from last week's 5.04% average and 6.1% a year ago. The last time the 30-year fixed rate mortgage was below 5% was the week ending May 28, when it averaged 4.91%.

Meanwhile, one-year Treasury-indexed ARMs were at 4.49%, down from 4.52% last week and 5.12% last year.

To obtain the rates, the 30-year fixed-rate mortgage required payment of an average 0.7 point, while the 15-year fixed and five-year adjustable required an average 0.6 point. The one-year carried an average 0.5 point. A point is 1% of the mortgage amount, charged as prepaid interest.

-By Mike Barris and Joan E. Solsman, Dow Jones Newswires; 212-416-2330; mike.barris@dowjones.com