The VELUX BDX 2011 is used for twin installations with an 18mm gap. It includes an Insulation Collar which reduces heat loss by insulating around the roof window frame and eliminates cold bridges. It ensures a weather tight seal and is made from diffusion open material which minimises risk of condensation. It includes a transverse drainage gutter too 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 2011 will give you extra protection and peace of mind for a low cost.