A backer rod adapted for insertion into a construction joint having a polymeric foam body of closed-cell construction and a foraminous skin with both open and closed cells. The backer rod, when compressively inserted into a construction joint, is impervious to hot and cold sealants alike and maintains the seal integrity during the extremes of expansion and contraction. The backer rod is usable over the temperature range of -70 to 450 degrees Fahrenheit. The backer rod hereof is a continuously extruded closed-cell foam body that is degassed by a secondary extrusion. The skin portion is formed by heat treating the exterior of the polymeric foam body so as to create microperforations therein.