A combination of ab initio, atomistic, and finite element methods (FEM) was used to investigate fundamental properties of grain boundaries and grain boundary networks and their impact on lattice thermal conductivity in the ultra high-temperature ceramic ZrB2. The structure, energetics, and lattice thermal conductance of certain low energy grain boundaries were studied. Atomic models of these boundaries were relaxed using density functional theory. Information about bonding across the interfaces was determined from the electron localization function. Interfacial thermal conductances were computed using nonequilibrium molecular dynamics. Microstructural models were used to determine the reduction in lattice thermal conductivity due grain boundary networks where FEM meshes were constructed on top of microstructural images.