  We theoretically compute the interface thermal resistance between crossing
single walled carbon nanotubes of various chiralities, using an atomistic
Green's function approach with semi-empirical potentials. The results are then
used to model the thermal 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.
