The security for the debt has applicability only with respect to qualified residence debt, which interest might otherwise be deductible on Schedule A (hence, the reason that Schedule continues to be raised in the rules, regs, and publications).
The security for the debt is irrelevant if the proceeds are used for business or investment purposes.
@anon30 You marked the wrong answer as a solution.
If you choose to treat the debt as not secured by your home, you can deduct the interest paid to the extent you use the proceeds of the loan for business purposes (including rental properties).
See https://www.irs.gov/publications/p936#en_US_2019_publink1000229898
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