A packed installation kit for use in the installation of a roof window system comprising two window units (11, 12) and configured to be built in side by side in an inclined roof structure such that the roof window system defines a common height and a combined width in the mounted condition, said installation kit comprising a number of installation assemblies (3, 4, 5, 6, 7, 8), each including one or more components having a length (L3, L5, L6, L8), and optionally auxiliary equipment (96, 96A, 97, 98), the components of a first set of installation assemblies (4, 5, 6, 7, 8) of the installation kit being accommodated in a first packaging comprising a box shaped as a rectangular prism, preferably of corrugated cardboard, having a length in a longitudinal direction, a width in a width direction and a height in a height direction to provide a supply condition of the installation kit, characterised in that the components of the first set of installation assemblies (4, 5, 6, 7, 8) of the installation kit are accommodated in the first packaging either as they are or partially or entirely packed in a sub-packaging (104, 1063; 1063', 1064, 107, 108) of cardboard or paper material, that the length of the first packaging in the longitudinal direction is selected in accordance with a length of a longest component(s) of the first set of installation assemblies, that a first installation assembly of said first set of installation assemblies comprises the longest component(s) (52, 53) having a first length, that the components of a second installation assembly (6; 6') of said first set of installation assemblies have a second length which is shorter than the first length of the longest component(s) (52, 53) of the first set of installation assemblies, and that the components of the second installation assembly (6; 6') of said first set of installation assemblies are partially packed in a sub-packaging comprising at least one open cardboard box and/or paper ribbon (1063; 1063', 1064) located at or near longitudinal ends of the components of the second installation assembly (6; 6') and temporarily fixated to the first packaging to allow the components of the second installation assembly (6; 6') and/or the sub-packagings (1063; 1063', 1064) to function as positioning elements relative to components of at least one other installation assembly (5, 8) of said first set of installation assemblies in the longitudinal, width and/or height direction of the first packaging. the components of the first set of installation assemblies (4, 5, 6, 7, 8) of the installation kit are accommodated in the first packaging either as they are or partially or entirely packed in a sub-packaging (104, 1063; 1063', 1064, 107, 108) of cardboard or paper material, that the length of the first packaging in the longitudinal direction is selected in accordance with a length of a longest component(s) of the first set of installation assemblies, that a first installation assembly of said first set of installation assemblies comprises the longest component(s) (52, 53) having a first length, that the components of a second installation assembly (6; 6') of said first set of installation assemblies have a second length which is shorter than the first length of the longest component(s) (52, 53) of the first set of installation assemblies, and that the components of the second installation assembly (6; 6') of said first set of installation assemblies are partially packed in a sub-packaging comprising at least one open cardboard box and/or paper ribbon (1063; 1063', 1064) located at or near longitudinal ends of the components of the second installation assembly (6; 6') and temporarily fixated to the first packaging to allow the components of the second installation assembly (6; 6') and/or the sub-packagings (1063; 1063', 1064) to function as positioning elements relative to components of at least one other installation assembly (5, 8) of said first set of installation assemblies in the longitudinal, width and/or height direction of the first packaging.