A vapour controlling insulation structure (1) for a flat or low slope warm roof with a load supporting structure (2), comprises a first insulation layer (3), a vapour controlling membrane (4), and a second insulation layer (5), whereby said vapour controlling membrane (4) is situated between the first insulation layer (3) and second insulation layer (5), and the second insulation layer (5) has a top surface (6) for mounting of a roofing membrane.
The first insulation layer (3) has a thickness (t) and is constituted by one or more insulation boards (8), and said second insulation layer (5) is constituted by two stacked sub-layers (9) both with substantially the same thickness (t) as the first insulation layer (3).