As the impacts of climate change and urban densification continue to escalate, existing buildings face increasing pressure to adapt to current standards for energy performance, occupant comfort, building resiliency and durability. Increasing effort has been put into researching sound retrofit strategies for older masonry structures that consider hygrothermal impacts in addition to energy improvements and structural upgrades. However, existing masonry buildings have a wide variability of material characteristics. Therefore, calibration of hygrothermal computer models for masonry buildings is challenging without extensive material testing or acquiring long-term measurements of in-situ conditions. As an alternative, material testing in combination with guarded hot box (GHB) testing was used in this project to investigate the hygrothermal performance of different interior retrofit options for a historic double-wythe brick building typology. Samples of the brick material used in the GHB test wall were tested as per ASTM standards to determine density, water absorption, thermal conductivity, sorption isotherms, and vapour permeability. The results from the material testing have informed the analysis of the GHB testing results and will be used in subsequent work to calibrate hygrothermal computer models to facilitate analysis of a wider array of retrofit solutions.