The in situ thermal transmittance (U-value) of a building element is typically measured using a heat flow meter at a single point, as well as an internal and external temperature sensor. This paper aims to assess the affect temperature sensor location had on the measured U-value. To do so, the U-value of a replica Victorian solid wall was measured under controlled conditions, with measurements taken at varying distances from both the internal and external surface. Over the course of a 3-week monitoring programme, a variety of steady state and dynamic temperature profiles were applied to both the internal and external environment. The U-value was then calculated using combinations of internal and external temperatures to assess the impact of temperature sensor placement on in-situ U-value. It was found that in both dynamic and steady state conditions, there was no significant change in U-value when the internal temperature was measured at distances ranging from 50 to 300 mm from the wall surface, suggesting that the internal air temperature should be measured no closer than 50 mm from the wall. However, a ⁓4% difference was observed between the U-value calculated using the temperature measured at 100 mm from the external surface and using the “environmental” temperature measured 1 m from the wall under dynamic external conditions. Similar results could be seen under steady state external conditions, with the U-value decreasing as external temperature sensors’ distance from the wall increased. Although this is a small variation, with further research required to give statistical significance to the observed trend, the finding suggests that a localised external temperature measurement should be taken at a distance of 100 mm from the wall surface.