The two types of flue systems frequently used, are single wall (non-insulated) and double wall (insulated) flues.
The single wall flue is a straightforward stainless steel tube. When hot air moves through the non-insulated flue, its external surface becomes tremendously hot. This is not ideal if the flue needs to run close to flammable materials such as wood trusses or ceiling boards. However they are excellent for providing further warmth to the area.
The double wall insulated flues are made of two single wall flues with 50mm difference in diameter. The space between the two ducts is filled with insulating rock wool material. This prevents the external surface of the flue from becoming tremendously hot, thus making these flues appropriate for installation near flammable materials. The insulation furthermore keeps the flue air hotter for longer.
Normally we would utilize single wall flue piping from your fireplace to the ceiling, and then use double wall flue piping in the ceiling, where there are flammable materials, which are at risk to catch a light from the heat generated from a single wall flue.