A thermally insulating member includes a plane sheet having two ends spaced in a length direction and two sides spaced in a thickness direction perpendicular to the length direction. The plane sheet includes a plurality of rows of slits between the ends of the plane sheet. Each row of slits extends from one of the sides through the other side of the plane sheet. A first spacing between two adjacent rows of slits in the length direction is larger than a spacing between two adjacent slits in the same row of slits in a width direction perpendicular to the length and width directions by a second spacing. A thermally insulating strip is defined between two adjacent rows of slits. A stretchable rib is formed between two adjacent slits in the same row of slits. Each end of the plane sheet is a coupling portion free of the slits.