Thermal properties of a series of type II clathrates of the formula Na x Si 136 with 0< x< 24 and Na guests occupying the Si cages have been investigated over the temperature range from 2 to 300 K. Heat capacity and thermal conductivity results show that the structure is remarkably responsive to the loading of Na guests. The response is phononic: the host lattice expands in a non-monotonic way, and first stiffens, then relaxes at low loading into the larger Si 28 cages (x< 9), then stiffens again as the Na concentration increases further. The response is also electronic, through changes in electronic properties as additional Na is loaded into the smaller Si 20 cages at high loading (x> 9). In total, the influence of the guest loading illustrates the complexities of structure–property relations in a guest–host system.