How and where to use guide
VELUX Vapour Barrier 78x98cm BBX MK04 0000
Where to use this product
Install the vapour barrier collar in any roof space where a VELUX roof window or Sun Tunnel is being fitted. The collar bridges the gap between the window frame and the internal ceiling or plasterboard finish, creating a continuous vapour barrier across the roof plane. It is designed to work with typical UK roof construction including timber rafters, insulation layers, and internal finishes. Use it in both new build installations and when upgrading or replacing existing VELUX systems in refurbishment projects.
- New build pitched roof installations with VELUX roof windows or Sun Tunnels
- Refurbishment and replacement projects in existing properties
- Any application where a weather-tight internal seal is required around the window frame
- Installations where vapour control and thermal performance are priorities
How to install the vapour barrier collar
The collar must be positioned and secured during the internal finishing stage, after the roof window frame is in place but before final plasterboard or ceiling boarding. Ensure the roof window frame is correctly installed and fixed according to VELUX instructions. Position the collar around the frame perimeter, ensuring it sits flat and makes full contact with both the window frame and the surrounding insulation or structural elements. Secure it using appropriate fixings as specified in the VELUX installation documentation, typically adhesive tape or mechanical fixings. Seal all joints and overlaps with a suitable vapour-tight tape or sealant to create a continuous barrier. The collar must be installed before any internal finishes are applied.
- Install after the roof window frame is fixed but before internal boarding
- Ensure full contact between collar and frame perimeter
- Seal all joints and overlaps with vapour-tight tape or sealant
- Follow VELUX fixing recommendations for your specific window model
- Check that the collar sits flat with no wrinkles or gaps
Key installation considerations
The vapour barrier collar works as part of a complete VELUX system and must be compatible with your specific roof window or Sun Tunnel model. The MK04 size (780×980mm) is designed for standard VELUX installations; verify compatibility with your window code before ordering. Proper installation is critical to prevent condensation and moisture ingress into the roof structure. The collar must form a continuous, unbroken vapour barrier, so take care not to puncture or tear it during handling and installation. In refurbishment work, ensure existing insulation and roof structure are in good condition before fitting the new collar.
- Verify the MK04 collar is compatible with your VELUX window or Sun Tunnel model
- Handle carefully to avoid tears or punctures that would compromise the vapour barrier
- Ensure the roof structure and insulation are sound before installation
- Do not install if the roof window frame is damaged or not properly secured
- Allow adequate access and working space for proper sealing and finishing
Maintenance and on-site tips
Once installed and sealed, the vapour barrier collar requires no routine maintenance. However, during the construction phase, protect it from damage by avoiding contact with sharp tools, wet trades, or heavy materials. If the collar is damaged before final sealing, it must be repaired or replaced to maintain the integrity of the vapour barrier. After installation, ensure that internal finishes (plasterboard, paint, or other materials) are applied neatly around the collar to complete the professional appearance and maintain the seal. In properties with high moisture environments such as kitchens or bathrooms, ensure adequate ventilation is provided through the roof window or other means to prevent excessive condensation.
- Protect the collar from damage during construction and wet trades
- Repair or replace any damaged sections before sealing
- Apply internal finishes neatly to maintain the professional appearance and seal
- Ensure adequate ventilation in high-moisture areas to prevent condensation issues
- Do not paint or seal over the collar in a way that prevents it from functioning as a vapour barrier