Air flowing through a supply duct infiltrates perviously faced, porous, internal duct insulation, degrading its thermal performance. Encapsulating the insulation's air-facing surface with an impervious barrier prevents infiltration, increasing the capacity of the conditioned supply air to heat or cool the space to which it is delivered. This study determined the air-speed dependence of the thermal conductivity of fiberglass insulation by measuring the inlet-to-outlet temperature drop of heated air flowing through a long, insulated flexible duct. The conductivity of a flexible duct's low-density, internal, fiberglass-blanket insulation increased with the square of the duct air speed, rising by 140% as the duct air speed increased from 0 to 15 m s(-1). At air speeds recommended for branch ducts, the conductivity of such insulation would increase by 6% above its still-air value in a residential system and by 16% in a commercial system. Results partially agreed with those reported by an earlier study. Simulations indicate that encapsulating the air-stream surface of internal fiberglass duct insulation with an impervious barrier increases the effectiveness with which a duct delivers the thermal capacity of supply air by 0.15%-0.9% in typical duct systems.