用在自住房上，需要通過home mortgage interest deduction
用在出租房上，可以抵消這個出租房的出租收入。
如果用在度假，大學學費，醫療費，律師費，不能抵消任何利息。
When a taxpayer borrows funds, the deductibility of the related interest expense is determined by how the loan proceeds are ultimately used and not how the loan proceeds are collateralized.
https://www.pkfod.com/insights/federal-interest-tracing-rules?
