A method for insulating a roof window comprising a stationary rectangular frame with top, bottom, and side frame pieces surrounding an opening, each frame piece having a length, an inner side facing the opening and an outer side opposite the inner side, and an interior side and an exterior side opposite the interior side, said method comprising the steps of: - providing an insulating member having an elongate shape with a length corresponding substantially to the length of said top and bottom frame pieces (12, 13) or to the length of said side pieces, and a pre-defined cross-section, wherein said insulating member comprises: a first insulating element and a second insulating element which, in a supply condition, are interconnected at a predefined separation zone extending between a first side of the interconnected insulating member and a second side opposite to the first side and along the length of said insulating member, the first insulating element being adapted for insulating a first frame piece of the window by, in an installed condition, being arranged adjacent to the outer side of the first frame piece with an abutment side of said first insulating element facing the frame, and the second insulating element being adapted for insulating a second frame piece of the window, which is opposite to the first frame piece, by, in an installed condition, being arranged adjacent to the outer side of the second frame piece with an abutment side of said second insulating element facing the frame, - separating the interconnected first and second insulating elements at said predefined separation zone, and - arranging the first insulating element adjacent to the outer side of the first frame piece with the abutment side facing the first frame piece, and the second insulating element adjacent to the outer side of the second frame piece with the abutment side facing the second frame piece. - providing an insulating member having an elongate shape with a length corresponding substantially to the length of said top and bottom frame pieces (12, 13) or to the length of said side pieces, and a pre-defined cross-section, wherein said insulating member comprises: a first insulating element and a second insulating element which, in a supply condition, are interconnected at a predefined separation zone extending between a first side of the interconnected insulating member and a second side opposite to the first side and along the length of said insulating member, the first insulating element being adapted for insulating a first frame piece of the window by, in an installed condition, being arranged adjacent to the outer side of the first frame piece with an abutment side of said first insulating element facing the frame, and the second insulating element being adapted for insulating a second frame piece of the window, which is opposite to the first frame piece, by, in an installed condition, being arranged adjacent to the outer side of the second frame piece with an abutment side of said second insulating element facing the frame, a first insulating element and a second insulating element which, in a supply condition, are interconnected at a predefined separation zone extending between a first side of the interconnected insulating member and a second side opposite to the first side and along the length of said insulating member, the first insulating element being adapted for insulating a first frame piece of the window by, in an installed condition, being arranged adjacent to the outer side of the first frame piece with an abutment side of said first insulating element facing the frame, and the second insulating element being adapted for insulating a second frame piece of the window, which is opposite to the first frame piece, by, in an installed condition, being arranged adjacent to the outer side of the second frame piece with an abutment side of said second insulating element facing the frame, - separating the interconnected first and second insulating elements at said predefined separation zone, and - arranging the first insulating element adjacent to the outer side of the first frame piece with the abutment side facing the first frame piece, and the second insulating element adjacent to the outer side of the second frame piece with the abutment side facing the second frame piece.