The VELUX BDX 2011E is used for Coupled installations with a 100mm gap. It includes an Insulation Collar which reduces heat loss by insulating around the roof window frame and eliminates cold bridges. An underfelt collar is included in this package. It ensures a weather tight seal between the roof window and the roof and it includes a transverse drainage gutter which leads water away from the head of the window if a tile or slate is broken above the window. Easy installation and comes in a variety of sizes. The BDX 2011E will give you extra protection and peace of mind for a low cost.