The sides of cathedral ceilings have equal slopes, reach to the highest peak of the room, and attach to the roof trusses, whereas vaulted ceilings have unequal sides meeting at a room's high point. Vaulted ceilings are usually the result of walls of unequal height in the same room.
Cathedral ceilings are not considered energy friendly because of the amount of space they add to a room and the difficulty with insulating them. Vaulted ceilings are considered more energy efficient, though they also add significant space to a room. The main difference is the closeness to the roof of each type of ceiling, because heat penetrates more easily through cathedral ceilings.