The purpose of this paper is to show the differences in thermal insulation of a given set of clothing under various environmental conditions and to discuss the necessity of making a clear distinction between several kinds of clothing insulation: the basic clothing insulation Ic1 influenced by posture and air velocity; the effective clothing insulation Icle function of Icl and the fc1 coefficient which is also influenced by the nature (radiation or convection) of tie cold or heat stress and by the sweat content of the garment; the resultant clothing insulation Ic1r function of Icle and the pumping phenomenon which is also influenced by the differences, file tie kind of movement done by the subjects and the fullness of the garment.