Retrofitting existing buildings to meet and even exceed current high efficiency building standards is a critical approach to reducing the overall existing building energy consumption and associated carbon emissions. Considering the significant role of the envelope in retrofit design, it is important to quantify its insulation performance for assessing the pre-retrofit condition and to verify its post-retrofit performance. However, the envelope monitoring challenges include wiring requirements and power supply, data communication, sensor localization, monitoring discontinuity, and high costs. To overcome these challenges, our study developed sensor packages equipped with a communication network and Power over Ethernet technology to monitor the envelope’s pre-retrofit conditions. This innovative approach not only ensures effective sensor localization but also addresses power supply and data communication issues. The sensor packages were deployed in an occupied demonstration building on Syracuse University’s campus, focusing on different wall assemblies including a pre-cast concrete wall and a wood-framed wall. The sensor packages measured heat flux, surface temperature, relative humidity, and air temperature on both the interior and exterior of the existing wall assemblies. This study collected and analyzed hygrothermal data from August to November 2023. The analysis revealed a higher thermal mass effect for the concrete assembly. Furthermore, the study interpreted moisture transfer through the walls using the daily relative humidity fluctuations percentage to evaluate the buffering capacity of the walls. The application of networked sensors in this study demonstrates their effectiveness in overcoming monitoring challenges and for evaluating exterior energy retrofitting approaches.