A research study was undertaken to evaluate an in situ test method for determining the existing moisture content of brick masonry for use in hygrothermal models. Currently, ASTM E3069, Standard Guide for Evaluation and Rehabilitation of Mass Masonry Walls for Changes to Thermal and Moisture Properties of the Wall [1], is one of the only industry references providing guidance for evaluating the existing moisture content of masonry materials in buildings when the addition of insulation or other changes to the wall assembly are being considered. When interior insulation or a continuous air barrier is considered for addition to mass masonry walls to meet new energy standards, understanding the properties of the brick, the condition of the wall, and the existing moisture content of the brick are imperative for evaluating the hygrothermal performance of the wall assembly. The assumptions made for the initial moisture content of materials in the hygrothermal models for a mass masonry wall can change the design and alteration approach and, in some cases, could lead to a repair that would have detrimental long-term effects. In the laboratory, the interstitial relative humidity within the brick was measured utilizing temperature and humidity probes embedded within the brick body. The moisture content of the brick was measured at various humidity levels and then compared to various published isotherm curves. In addition to the laboratory testing, field-testing was conducted at five historic buildings with mass masonry wall assemblies to assess the moisture content of the masonry utilizing the same methods as were used for the laboratory testing. When within the hygroscopic range, these methods could be used as minimally invasive and direct techniques to measure the existing moisture content of the brick to develop more accurate and situationally specific hygrothermal models when evaluating existing mass masonry walls prior to a renovation.