Hollow-core doors are constructed with a thin layer of wood or fiberboard applied over a core of honeycombed cardboard or plastic. Hollow-core doors are cost-saving products often used for the many interior passage doors found in a house.
Most mid-range production houses built today receive hollow-core interior doors as a matter of course. Inexpensive and lightweight, these doors are easy to install and can save thousands of dollars on construction costs, since a typical house may have a dozen or more interior doors.
To call these doors hollow is somewhat misleading since they do have a honeycombed core placed within a solid wood outer frame, over which the surface veneer is glued. The frame and honeycomb core provides some rigidity to the door, as well as minimal sound-blocking ability. The significant amount of empty space lends the word hollow to the door and also helps make the door light so that it is easy to hang and easy to swing.