Transient line source measurements have been employed in various industries, e.g., plastics, foodstuffs, and refractory bricks, to measure thermal conductivity and sometimes thermal diffusivity. Measurements have traditionally been carried out in carefully controlled thermal environments. In pursuance of better data to inform energy efficiency calculations for building envelopes, a transient line source using thermal probe technology is assessed for in situ measurements, where materials may be subject to varied moisture content under diverse environmental conditions. A robust stand alone field apparatus has been developed. Laboratory based measurements have been undertaken, and results are reported for agar immobilized water and polytetrafluoroethylene, as well as aerated concrete and oak at specific moisture contents. The field apparatus was used to measure in situ the thermal properties of walls in real buildings, one of aerated concrete and one of mass earth construction. A new and simplified method of identifying appropriate analysis time windows has been developed, which allows an assessment of confidence levels in thermal conductivity results. Thermal diffusivity results were discounted as the effects of contact resistance between the probe and sample could not be differentiated from the heat capacity effects of the sample material. Valid thermal conductivity results were achieved for reference materials containing moisture and in situ measurements. The in situ thermal conductivity values were shown to be at significant variance with design values. Consistent thermal conductivity results were achieved for common building materials above 0.15 W⋅m −1⋅K−1, and indicative results were achieved for values below this despite measurements being taken under varying environmental conditions. The principal cause of error in the measurement of materials with higher thermal resistance was identified.