Molecular-scale insulating and dielectric materials rely on the exponential attenuation of tunneling with increasing length, presenting a potential problem of increased leakage current as the dimensions of the device decrease. An alternative design strategy is to use molecules with destructive quantum interference in the electronic transmission. However, a small organic molecule where all tunneling paths are fully cancelled has not been realized because contributions to the tunneling transmission from both the sigma and pi-orbital systems must be simultaneously suppressed. Here, I will report on a fully saturated molecule, a functionalized bicyclo[2.2.2]-octasilane moiety, where destructive sigma-interference decreases its conductance dramatically to create an effective single-molecule insulator. I will demonstrate, through a combination of conductance measurements and ab-initio calculations, that the functional moiety in this sub-nanometer fully saturated molecule is a better insulator than the vacuum it occupies. I will also show that it has a record thermopower (0.97 mV/K), providing an experimental signature of destructive interference where all tunneling paths are significantly suppressed.