  We study the nonlinear steady Boltzmann equation in the half space, with
phase transition and Dirichlet boundary condition. In particular, we study the
regularity of the solution to the half-space problem in the situation that the
gas is in contact with its condensed phase. We propose a novel kinetic weight
and establish a weighted $C^1$ estimate under the spatial domain $x\in
[0,\infty)$, which is unbounded and not strictly convex. Additionally, we prove
the $W^{1,p}$ estimate without any weight for $p<2$.
