Anytime you read about health-savings accounts (HSAs), there's the obligatory windup about how quickly they're growing. And the uptake has been notable, mainly due to the growth in high-deductible health plans (HDHPs), which workers must be covered by in order to contribute to an HSA. A survey by America's Health Insurance Plans showed that the number of Americans covered by HDHPs--and who are, in turn, eligible for HSAs--rose by two million between 2014 and 2015, a more than 10% growth rate.

But plenty of workers who are eligible to contribute to HSAs don't take full advantage of them. Even though HSAs offer the best tax treatment of any savings vehicle--pretax contributions, tax-free compounding, and tax-free withdrawals for qualified healthcare expenses--only 4% of HSA holders choose to invest their funds, according to research from HelloWallet, a Morningstar company.