To improve the energy efficiency of buildings, the thermal performance of opaque walls has traditionally been laboratory-tested samples under controlled conditions. However, these methods often fail in accurately reflecting real-world conditions, highlighting a need for the development of in situ measurement techniques. This study presents a novel device designed to measure the thermal transmission coefficient of building envelopes using an active thermal excitation method. The approach addresses critical challenges, including the impact of environmental factors, especially moisture, and the behavior of bio-based insulation materials. These materials are particularly sensitive to moisture variations, making it essential to assess their thermal conductivity under actual operating conditions. Thermal resistance remains crucial for improving energy efficiency throughout a building's lifecycle, from construction to renovation. Existing methods, whether conducted in laboratories or experimental settings, often lack universal applicability across different wall types and fail to adequately consider moisture effects. The developed in situ device combines active thermal techniques with advanced data processing to account for environmental influences. Validation involves multiple strategies, including laboratory characterizations using a guarded hot plate, tests in climate-controlled chambers, and a prototype incorporating heating elements and conventional sensors. This paper details the device’s development and its real-world applications, with a focus on bio-based materials and the critical role of moisture in thermal conductivity.