The average interest rate on 30-year mortgages slipped to 8.27% this week, down from 8.31% the week before, Freddie Mac said. Two weeks ago, the rate on 30-year mortgages hit a 3 1/2-year high of 8.38%. That was the highest since a 8.42% rate for the week ending July 12, 1996. Fifteen-year mortgages, a popular option for refinancing, also fell this week to an average rate of 7.84%, down from 7.92% last week. On one-year adjustable-rate mortgages, lenders were asking an average initial rate of 6.68% this week, down from 6.73% the previous week. The rates do not include add-on fees known as points, which averaged at or just over 1% of the loan amount for all three types of mortgages.