An additive manufacturing system includes a substantially moisture-impermeable barrier comprising a guide tube assembly for supplying filament from a filament supply to a print head in an extrusion-based additive manufacturing system, where the print head melts the filament and extrudes the melted filament to form a 3D part. The guide tube assembly includes an inner tube permeable to moisture and an outer tube that s substantially moisture impermeable. The inner tube has an interior passageway configured to receive the filament, and has a relatively low coefficient of friction to minimize drag force as the filament travels through it. The outer tube surrounds the inner tube and provides a substantially moisture-impermeable barrier around the inner tube.