UK authorities are promoting energy efficiency schemes to improve the performance of buildings as a result of the high levels of energy consumption and consequent CO2 emissions. A quarter of these emissions are due to requirements for space heating. Installation of insulation is one of the most common alternatives to thermally improve buildings, especially on buildings built of solid masonry (~20 percent of the housing stock). However, the thermal improvement of buildings located in conservation areas, listed buildings, decorative façades, or traditional buildings could be only achieved through the use of internal wall insulation. Solid masonry walls with high surface water absorption coefficients have a higher dependence on external climate conditions (e.g. rain, solar radiation), which are likely to affect the performance of internal wall insulation. This paper examines the effect of walls orientation on the hygrothermal behaviour of an internally insulated 16th century building. External walls have been insulated with a capillary active system that allows moisture movement towards indoor environments. Sensors to monitor relative humidity and temperature between the existing brick wall and the insulation were installed in the north-facing and south-facing walls of the building. Both walls are exposed to the same internal environmental conditions (teaching area). The study showed that drying of the south-facing wall occurred faster than drying of the north-facing wall and that drying of the south-facing wall was enhanced by the effect of direct solar radiation.