Abstract This paper presents recent findings about the influence of oil-soaked mineral wool insulation material on the thermal heat loss of the insulated pipe. Thermal oil is a popular heat transfer fluid in high-temperature solar cooling applications using concentrating collectors, eg parabolic trough or linear Fresnel collectors. Leaks in thermal oil pipes or components thereof, eg. fittings, valves etc., result in thermal oil saturating the surrounding insulation layer. Mineral wool soaked with thermal oil has a higher heat conductivity than pure dry mineral wool. The influence of oil content on the heat loss has been investigated using a test stand with a heated section of insulated pipe. Simulated leaks have been applied and the temperature changes on the outside of the insulation have been observed. Different grades of saturation (0%, 33% and 50%) have been investigated in the work presented, as well as different temperature levels (175 °C/347 °F and 250 °C/482 °F) of the thermal oil within the pipe. Thermal conductivity and heat loss are calculated and presented for these. The results show that an oil content of 33% in mineral wool insulation increases thermal conductivity by a factor of 2.5 to 3, compared to dry, oil-free insulation. An oil content of 50% increases thermal conductivity by a factor of up to 3.3. It was observed that the difference in thermal conductivity between 175 °C/347 °F and 250 °C/482 °F of bulk oil temperature inside the pipe is rather negligible, both at 33% and 50% oil content.