Aspects of the disclosure relate to techniques for manufacturing a balloon envelope including a duct for high altitude balloons. In one example, a first sheet of material may be provided. A premade duct may be arranged at least partially on the first sheet of material. The duct may include a first substance on an internal surface. A second sheet of material may be arranged over at least a portion of the duct. A heat sealing device may be applied to the second sheet of material to heat seal the first sheet of material to the second sheet of material. The heat sealing device may be applied over at least a portion of the duct in order to seal external surfaces of the duct to each of the first and second sheets of material and form a balloon gore. The first substance may keep the duct from being sealed to itself.