Inside of an exterior hollow core door with the sun shining on it

 Inside of an exterior hollow core door with the sun shining on it. The interior of a hollow core door is made up of little more than a few interlocking strips of cardboard that forms a honeycomb type pattern. The cardboard is there only to maintain a consistent spacing between the inner and outer door panels. These doors provide almost no thermal, weather or security protection. While it creates an interesting thermal image (you can clearly see the cells created by the cardboard separator), you do not want a hollow core door on the exterior of your home.