  We consider a voltage-biased Normal metal-Insulator-Superconductor (NIS)
tunnel junction, connected to a high-temperature external electromagnetic
environment. This model system features the commonly observed subgap leakage
current in NIS junctions through photon-assisted tunneling which is detrimental
for applications. We first consider a NIS junction directly coupled to the
environment and analyze the subgap leakage current both analytically and
numerically; we discuss the link with the phenomenological Dynes parameter.
Then we focus on a circuit where a low-temperature lossy transmission line is
inserted between the NIS junction and the environment. We show that the subgap
leakage current is exponentially suppressed as the length, $\ell$, and the
resistance per unit length, $R_0$, of the line are increased. We finally
discuss our results in view of the performance of NIS junctions in
applications.
