Working as a residential electrician back in the late 1990’s, I remember learning of a new electrical box that we were asked to use on all exterior walls and in the ceilings of unconditioned attic spaces. At the time, they were called “vapor tight” boxes. These boxes were designed to reduce air movement through wall or ceiling cavities by sealing the electrical box to the wall or ceiling air barrier and also to seal the electrical wires where they enter the box. Air will contain at least some water vapor, by air sealing the electrical box, we were also reducing the amount of water vapor that could potentially enter a wall or ceiling. The name “vapor tight” was at least partially correct.