Abstract: An insulation system for the fuselage walls of an aircraft. The insulation is a formed-in-place, sprayed-on foam that is applied against a barrier material that is draped against the inside of the exterior skin of the fuselage and over any inwardly projecting structural members attached to the exterior skin. No substantial portion of the barrier material passes underneath any laterally projecting portions of the structural members, so that the subsequently solidified, semi-flexible foam (i.e. insulation) can be removed at a later time for necessary inspections of the aircraft, without being trapped behind these lateral projections. The foam is preferably fire retardant and hydrophobic. An additional protective layer may also be sprayed on to the outside surface of the insulating foam.