The Net-Zero Energy Residential Test Facility (NZERTF) was constructed at the National Institute of Standards and Technology (NIST) to support the development and adoption of cost-effective net-zero energy designs and technologies. The 250 m² two-story, unoccupied NZERTF, built in 2012, had among its design goals an airtight and highly insulated building enclosure designed for heat, air, and moisture control. The airtightness goal was achieved through detailed envelope design and careful construction as well as during and after construction commissioning. When it was built, the NZERTF was one of the tightest residential buildings in North America with a whole building pressurization test result of roughly 0.6 h⁻¹ at 50 Pa measured per ASTM E779-19, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization. No special attention was given to the airtightness of the interior floors and other interior partitions. To support airflow modeling efforts, this interior leakage was quantified through a series of interzonal pressurization tests. Both the basement and attic were considered to be conditioned spaces because the thermal and air-moisture barriers encompass the basement walls and attic roof. Transfer grilles and other openings linked the living space to these two zones. A series of fan and partition configurations were used to quantify the leakage values of the various interzone airflow paths. Test results showed that the interior floors were more than 10 times leakier than the exterior building envelope and that the leakage associated with the transfer grilles between levels was less than the floor leakage. This paper describes the design of the interzonal tests and the challenges in performing them, which included isolating zones, controlling multiple blower doors, and access for installing pressurization fans. The results of these tests were inputs to a multizone airflow (CONTAM) model of the building for use in evaluating the effects of different ventilation strategies and other airflow-related technologies on energy consumption and indoor air quality.