We theoretically compute the interface thermal resistance between crossing single walled carbon nanotubes of various chiralities, using an atomistic Greens function approach with semi-empirical potentials. The results are then used to model the therm
al conductivity of three dimensional nanotube pellets in vacuum. For an average nanotube length of 1 $mu$m, the model yields an upper bound for the thermal conductivity of densely compacted pellets, of the order of a few W/m-K. This is in striking contrast with the ultra-high thermal conductivity reported on individually suspended nanotubes. The results suggest that nanotube pellets might have an application as thermal insulators.
