A simple and accurate expression for slip in the thermal creep problem is obtained via the use of the Roussopolous variational principle. The linearized Boltzmann equation is used and it is shown that, contrary to the results obtained by Yalamov, Ivchenko, and Deryagin, the Bhatnagar‐Gross‐Krook model, and the Maxwell molecules give essentially the same numerical values for the slip coefficient. Interestingly, the result for the rigid sphere molecules is about 12% lower than these values. Further, it is shown that some variational principles used previously in the kinetic theory are, in fact, only a special case of the Roussopolous principle.