The wide availability of power distribution cables provides an interesting no-new-wires communication channel. However, its electrical characteristics make it a harsh environment for the data transmission purpose and prevent the deployment of services with high reliability requirements. This paper proposes and implements an OSI-Layer2 network coding-based cooperative scheme with the aim of improving communication reliability in indoor narrowband powerline channels. The proposed scheme uses random linear network coding with a sliding window and relaying. We use network coding to replace the retransmissions triggered by legacy subsequent repeat request (ARQ) schemes. We evaluate the performance of our approach in terms of throughput and delay. Regarding the throughput achieved in harsh environments, we show that our scheme often more than doubles the throughput of existing legacy ARQ schemes. At the same time, and even under the large variation of traffic characteristics, it is shown that the delay is likely to be upper bounded by a few seconds, a bound that cannot be guaranteed in other existing transmission techniques.