We consider the perturbative construction, proposed in [37], for a thermal state Ω β,λV {f } for the theory of a real scalar Klein-Gordon field φ with interacting potential V {f }. Here f is a spacetime cut-off of the interaction V and λ is a perturbative parameter. We assume that V is quadratic in the field φ and we compute the adiabatic limit f → 1 of the state Ω β,λV {f } . The limit is shown to exist, moreover, the perturbative series in λ sums up to the thermal state for the corresponding (free) theory with potential V . In addition, we exploit the same methods to address a similar computation for the non-equilibrium steady state (NESS) [59] recently constructed in [25].