Insulating element for insulating a structural element (12, 14) in a motor vehicle, the insulating element comprising: a carrier with a first wall and a second wall, wherein the walls (2, 3) are designed and disposed so as to be substantially congruent, and wherein the walls (2, 3) each have opposite first edges and second edges, and with a first rib and a second rib, wherein the ribs (6, 7) are disposed between the walls (2, 3), and wherein the ribs (6, 7) each extend between the first edge and the second edge, wherein the ribs (6, 7) each are at least half as long as a total distance between the first edges and the second edges in the direction of the ribs (6, 7); an expandable material which is disposed between the walls (2, 3) and which is in each case disposed on both sides of the ribs (6, 7); and a fixing element for fixing the insulating element in the structural element (12, 14), wherein the fixing element is disposed in a region of the first edges of the walls (2, 3); wherein the ribs (6, 7) subdivide a volume between the two walls (2, 3) in such a manner that between 35% and 65% of this volume is disposed between the ribs (6, 7); and wherein the ribs extend substantially up to the first edges of the walls (2, 3), but are spaced apart from the second edges of the walls (2, 3) by a spacing, wherein this spacing is at least 15% of a length of the ribs (6, 7), characterized in that the expandable material has an expansion rate of 1000% to 4000%; and in that one region of the walls (2, 3) directly on the second edges is in each case free of expandable material. a carrier with a first wall and a second wall, wherein the walls (2, 3) are designed and disposed so as to be substantially congruent, and wherein the walls (2, 3) each have opposite first edges and second edges, and with a first rib and a second rib, wherein the ribs (6, 7) are disposed between the walls (2, 3), and wherein the ribs (6, 7) each extend between the first edge and the second edge, wherein the ribs (6, 7) each are at least half as long as a total distance between the first edges and the second edges in the direction of the ribs (6, 7); wherein the ribs (6, 7) are disposed between the walls (2, 3), and wherein the ribs (6, 7) each extend between the first edge and the second edge, wherein the ribs (6, 7) each are at least half as long as a total distance between the first edges and the second edges in the direction of the ribs (6, 7); an expandable material which is disposed between the walls (2, 3) and which is in each case disposed on both sides of the ribs (6, 7); and a fixing element for fixing the insulating element in the structural element (12, 14), wherein the fixing element is disposed in a region of the first edges of the walls (2, 3); wherein the ribs (6, 7) subdivide a volume between the two walls (2, 3) in such a manner that between 35% and 65% of this volume is disposed between the ribs (6, 7); and wherein the ribs extend substantially up to the first edges of the walls (2, 3), but are spaced apart from the second edges of the walls (2, 3) by a spacing, wherein this spacing is at least 15% of a length of the ribs (6, 7), characterized in that the expandable material has an expansion rate of 1000% to 4000%; and in that one region of the walls (2, 3) directly on the second edges is in each case free of expandable material.