The unexpected decline in mortgage rates this year has moved a significant portion of the agency MBS market into the zone where it’s worthwhile for borrowers to refinance, according to a new analysis of agency MBS data by Inside MBS & ABS. As of the end of December, some 24.4 percent of loans backing Fannie Mae, Freddie Mac and Ginnie Mae MBS had mortgage rates ranging from 4.01 percent to 4.50 percent. Altogether, $1.500 trillion of existing single-family mortgages were in that bucket. According to Inside Mortgage Finance, the average offering rate for 30-year fixed-rate conventional mortgages this week was...[Includes one data table]