Thermal liner is basically a thin material that is used to help maintain heat in a room. It is made from wood fibres interwoven with durable textile fibres. This gives a very strong, flexible material that when attached to the walls is said to increase the energy efficiency of the room. Air trapped between material layers forms the main thermal protection, because fibres conduct heat 10-20 times faster than still air. This is the determining principle in the construction of a thermal liner which should prevent the transfer of heat from the environment to the body by slowing down the passage of heat from the outside to the inside of the garment. The products used for firefighters' clothing can consist of a spun laced, nonwoven felt or batting quilted or laminated to a woven lining fabric. It can also be a knitted fabric between the outer shell and the lining to give the highest insulation against heat, but at the same time allowing the escape of moisture due to perspiration. The thermal liner is normally made of inherently flame-retardant fabrics or their blends A similar fibre content of the thermal liner and outer shell fabric make the laundering of the garment easier.