The VELUX BFX 1000 Underfelt Collar is used to ensure a weather tight seal between the Roof Window and the roof. It is made from a special diffusion open polypropylene to minimise the risk of condensation and it comes with a transverse drainage gutter to lead water away from the head of the window if a tile or slate is broken above the window. The BFX 1000 will give that added reassurance for a low cost. The BDX 2000 package comes with the BFX 1000 and you get an insulation collar as well.