During the mortgage crisis, the vast majority of jumbos were adjustable-rate mortgages and hybrid adjustable-rate mortgages, which started at low fixed rates and switched to adjustable interest rates at the end of a set time period typically of five, seven or 10 years, said Keith Gumbinger, vice president at HSH.com, a mortgage-information website.

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By contrast, the national average rate for a home-equity line of credit is 5.15% and for a 30-year "private" jumbo mortgage it is 4.08%, according to HSH.com, which tracks the data.

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