Method of assembling a wall insulation system (10; 100) for a double shell exterior wall comprising the steps in the following order of I) providing an inner shell (1: 101) extending in a first plane with mounting anchors (8; 108) extending substantially perpendicularly to the first plane and being fastened to the inner shell (1; 101) with lateral offsets, and in rows with uniform elevation offsets from one another, II) providing a plurality of insulation elements (41, 51; 141, 151) having a width in the direction between two adjacent rows of the mounting anchors (8; 108) and a length extending perpendicularly to the width, wherein an insulation element (41, 51; 141, 151) comprises a main portion (42, 52) having a back surface, a top and a bottom flange (43, 44, 53, 54) protruding from the main portion (42, 52) and extending along the length of the insulation element (41, 51; 141, 151), III) guiding a first insulation element (41; 141) in a direction towards the inner shell (1; 101) between two adjacent rows of mounting anchors (8; 108) wherein the first insulation element (41; 141) is guided in such a way that the back surface faces the inner shell (1;101) and at least one mounting anchor (8; 108) penetrates at least the bottom flange (44, 54) of the first insulation element (41; 141) said method being characterised by the following subsequent steps: IV) guiding a second insulation element (51; 151) in a direction towards the inner shell (1; 101) above the first insulation element (41; 141), such that the top flange of the first insulation element (41; 141) abuts the bottom flange of the second insulation element (51; 151), thereby locking the first and/or the second insulation element (41, 51; 141, 151), V) attaching securing means (6; 106) to the mounting anchor(s) (8; 108) to fasten the insulation elements (41, 51; 141, 151) to the inner shell (1; 101), repeating steps III) to V) so that a plurality of insulation elements (41, 51; 141, 151) in an assembled state define at least one first insulation layer (4; 104) of said wall insulation system (10; 100). I) providing an inner shell (1: 101) extending in a first plane with mounting anchors (8; 108) extending substantially perpendicularly to the first plane and being fastened to the inner shell (1; 101) with lateral offsets, and in rows with uniform elevation offsets from one another, II) providing a plurality of insulation elements (41, 51; 141, 151) having a width in the direction between two adjacent rows of the mounting anchors (8; 108) and a length extending perpendicularly to the width, wherein an insulation element (41, 51; 141, 151) comprises a main portion (42, 52) having a back surface, a top and a bottom flange (43, 44, 53, 54) protruding from the main portion (42, 52) and extending along the length of the insulation element (41, 51; 141, 151), III) guiding a first insulation element (41; 141) in a direction towards the inner shell (1; 101) between two adjacent rows of mounting anchors (8; 108) wherein the first insulation element (41; 141) is guided in such a way that the back surface faces the inner shell (1;101) and at least one mounting anchor (8; 108) penetrates at least the bottom flange (44, 54) of the first insulation element (41; 141) said method being characterised by the following subsequent steps: IV) guiding a second insulation element (51; 151) in a direction towards the inner shell (1; 101) above the first insulation element (41; 141), such that the top flange of the first insulation element (41; 141) abuts the bottom flange of the second insulation element (51; 151), thereby locking the first and/or the second insulation element (41, 51; 141, 151), V) attaching securing means (6; 106) to the mounting anchor(s) (8; 108) to fasten the insulation elements (41, 51; 141, 151) to the inner shell (1; 101),