Abstract: An insulation-filled jacket has removably fastened sleeves, and retaining means for attaching the sleeves, when removed, to the inside of the jacket thereby increasing the depth of insulation over substantial surface areas of the jacket. The retaining means are positioned to hold the sleeves so as to maximize the surface area of the sleeves presented, and the depth of insulation afforded to the wearer at critical heat loss locations of his body, in order to minimize critical heat loss and to retard hypothermia. The retaining means may include fastening components constructed integrally with the removable sleeves.