We have applied Umklapp phonon-phonon and phonon-defect scattering to calculate the thermal conductivity of unalloyed as well as Fe- and La-alloyed RuO2 (P42/mnm). These models are computationally efficient and parameter free as they are supported by density functional theory. We predict an order of magnitude drop in the thermal conductivity upon alloying, which is beneficial for thermoelectric applications as it increases the figure of merit. Thermal conductivity data obtained by thermoreflectance on magnetron sputtered thin films are consistent with the calculations. The here employed research strategy may also be beneficial for designing phases that require manipulation of entangled properties.