Three of these conservation options (the FTP and FTW) come complete as a kit with flashing and insulation collars. There are two choices when selecting a kit: ‘V’ and ‘J’. These indicate varying levels at which the roof window can be installed within the roof structure. For example, when choosing a flashing to lift the window by 3cm from the standard ‘V’ level, the N variations will be suitable. On the other hand, ‘V’ is a standard installation level whereas ‘J’ is recessed – 3cm below the ‘N’ level.