As substitution on the 𝜅 latt . Wu and Mingo used similar calculations to study fully-filled BaCo4Sb12 and YbFe4Sb12, and were able to establish the mechanisms by which these fillers suppress the heat transport, and also to explain the higher 𝜅 latt of IrSb3 relative to CoSb3. There are also a small number of modelling studies based on force-field techniques, for example using non-equilibrium molecular-dynamics simulations to explore the impact of Sb vacancies on the thermal transport in CoSb3. More systematic modelling studies could provide valuable insight into the underlying microscopic mechanisms by which doping and filling reduce the 𝜅 latt and thereby direct future work to identify and optimise novel high-performance skutterudite TEs. In this study, we present a systematic set of calculations on CoSb3 and XCo8Sb24 skutterudites filled with the noble gases X = He, Ne, Ar, Kr and Xe and explore the links between the size and mass of the filler atoms and their impact the thermal conductivity. By selecting chemically-inert filler atoms, we are able to isolate the effects of the fillers from the impact of changes to the electronic structure -Page 4 -of the host framework through the redox reactions that can occur with most other fillers. We propose and apply a variety of analysis techniques to elucidate the microscopic mechanisms by which the fillers suppress the heat transport through the CoSb3 framework, and show that an approximate rattling frequency, determined from the atomic mass and harmonic force constants, is a good predictor of the effect of a filler on the 𝜅 latt . The general conclusions and modelling techniques from this study are readily applicable to other filled skutterudites, and can help to improve our fundamental understanding of the heat transport in cage compounds and identify promising research directions towards future high-performance TEs.