The Velux Insulation Ck04/C04 is specifically designed for new construction projects with roof windows. Manufactured by VELUX, performance and energy efficiency in your roofing system. VELUX BDX 2000 Insulation and Underfelt Collar:Â Elevate the energy efficiency and weather resistance of your roof window installation with the VELUX BDX 2000 Insulation and Underfelt Collar. Designed to provide a robust seal between your roof window and the roofing material, this essential accessory helps prevent heat loss and water ingress, ensuring a comfortable and protected indoor environment.