A balloon includes a pair of overlying valve film strips disposed between a pair of overlying balloon films, which include neck and body portions. The valve film strips extend at an angle to a central axis of the neck portions, and valve seals formed in the valve film strips direct incoming gas from an inlet adjacent the neck, to an outlet between the body portions. Methods of assembling the balloons by forming the valve seams and neck seams without moving the balloon films or valve film strips is also disclosed.