  In (Commun Pure Appl Math 2(4):331-407, 1949), Grad proposed a Hermite series
expansion for approximating solutions to kinetic equations that have an
unbounded velocity space. However, for initial boundary value problems, poorly
imposed boundary conditions lead to instabilities in Grad's Hermite expansion,
which could result in non-converging solutions. For linear kinetic equations, a
method for posing stable boundary conditions was recently proposed for
(formally) arbitrary order Hermite approximations. In the present work, we
study $L^2$-convergence of these stable Hermite approximations, and prove
explicit convergence rates under suitable regularity assumptions on the exact
solution. We confirm the presented convergence rates through numerical
experiments involving the linearised-BGK equation of rarefied gas dynamics.
