The APU mounting rail forms, together with the APU base edge profile W66 and W67-2, a wall insulation's bottom termination.The rail gets fixed to the wall. It is available in 3 projection lengths (50 mm, 100 mm and 160 mm) and can thus be flexibly used in combination with the APU base edge profile W66 and the APU extension rail Z23 up to an insulating material depth of 300 mm. Where the insulating material thickness is 150 mm or more, perimeter insulation is required. Finally, the base edge profile (W66) is pushed in between insulation and mounting rail and plastered over. At the end of the insulation work, the profiles should overlap by at least 20 mm.