WASHINGTON – Long-term U.S. mortgage rates climbed upward this week, slightly worsening affordability for homebuyers. Mortgage buyer Freddie Mac said Thursday that the average rate on 30-year, fixed-rate mortgages rose to 4.47 percent from 4.42 percent last week. This benchmark rate averaged 3.97 percent a year ago. With the start of the traditional spring homebuying season, people shopping for homes are dealing with higher loan costs and fewer properties for sale.