This idea came to me while checking if a methane-filled dirigible could fly. I saw that heliox (a breathable mix of helium and oxygen) has a density at 1 bar of 0.5g/L compared to 1.25g/L for air, so it could be used to fill an airship where people would live inside the envelop.

The habitat and service modules have a rigid structure formed from geodesic-like elements supporting a transparent gas-tight membrane. Hydroponic gardens and lightweight houses can be found in the habitat modules, while service modules contains offices and stores. The gas exchangers use water as the gas carrier because helium is much less soluble than oxygen and carbon dioxide.