Today's mortgage rates are very low. There is also now a big difference between the mortgage rate of a 30-year fixed and a 15-year fixed. For the first time in history, the 15-year fixed rate mortgage is almost a full percentage point cheaper than the 30-year fixed rate mortgage. To be exact, the average 15-year is 0.92% lower than the 30-year. That amounts to 65% less in mortgage interest over the life of the loan.

It's no surprise then that almost half of the refinances from April-June 2014 were those that ditched the 30-year and financed into a 15 or 20-year. It's so much less expensive to have a shorter loan period that the raise in monthly payments is completely worth it. By historical standards, the discount on a 15-year loan is twice the size it has always been.

The 15-year fixed is most popular among HARP loans. This program is available to those who are "underwater" meaning that they have lost equity in their homes. When equity is lost in a home, those who refinance have to take on new private mortgage insurance (PMI). The HARP loan allows people to bypass this traditional rule. Those who put down 20% down at the time of purchase are able to skip the new PMI.