We took out a HELOC against our rental property to help pay for improvements on our primary residence. Do I apply the interest deduction to the rental property or our primary residence?
If the loan is secured by your rental property, the mortgage interest is reported as a Rental Expense.
Note that if any portion of the loan proceeds are used for something other than the rental property, the portion of interest allocable to loan proceeds not related to rental use generally cannot be deducted as a rental expense. Chapter 4 of Pub. 535 explains mortgage interest in detail: https://www.irs.gov/publications/p527/ch01.html