Allocation of Interest(p159)

If you use the proceeds of a loan for more than one purpose (for example, personal and business), you must allocate the interest on the loan to each use. However, you do not have to allocate home mortgage interest if it is fully deductible, regardless of how the funds are used.

You allocate interest (other than fully deductible home mortgage interest) on a loan in the same way as the loan itself is allocated. You do this by tracing disbursements of the debt proceeds to specific uses. For details on how to do this, see chapter 4 of Publication 535.