How and where to use guide
VELUX Vapour Barrier 78x140cm BBX MK08 0000
Where to use this product
Install this vapour barrier in roof window openings where moisture control is essential. The 78x140cm size is designed to fit standard VELUX roof window openings. The barrier works as part of a complete roof window system and should be used wherever a compatible VELUX roof window is being fitted, particularly in rooms prone to moisture generation such as bathrooms, kitchens, or loft conversions where condensation risk is higher.
- Roof window installations in domestic and light commercial properties
- Loft conversions and attic rooms with moisture-generating activities
- Any roof window opening where insulation protection is required
- Use only with compatible VELUX roof window systems
Installation and positioning
The vapour barrier must be positioned on the warm side of the insulation layer—this means on the interior face of your roof construction, facing into the heated room. This positioning is critical for effective moisture management. Install the barrier before or during the insulation installation process to ensure it sits correctly between the warm interior air and the insulation material. The membrane should be fitted carefully around the roof window frame to create a continuous barrier that prevents moisture-laden air from reaching the insulation.
- Position on the warm interior side of the insulation layer only
- Install before or during insulation placement
- Ensure the barrier sits flush against the window frame
- Seal any gaps or overlaps to maintain continuity
- Do not install on the cold exterior side of insulation
How it works and benefits
Water vapour naturally migrates from warm areas towards cold areas. Without a vapour barrier, moisture from indoor air can penetrate into insulation layers, reducing their thermal efficiency and creating conditions for condensation and potential mould growth. This barrier blocks that moisture migration, keeping your insulation dry and maintaining its R-value performance. By reducing condensation around the roof window, it also helps prevent water damage to the window frame and surrounding timber.
- Prevents moisture vapour penetrating insulation materials
- Reduces condensation formation on and around the window
- Maintains insulation thermal performance over time
- Protects timber frames and surrounding construction from moisture damage
- Contributes to a healthier internal environment
Maintenance and limitations
Once installed correctly, the vapour barrier requires no ongoing maintenance. However, it is not a waterproofing membrane and will not prevent liquid water ingress—your roof window's flashing and external weatherproofing must be properly installed to handle rain. The barrier works only as part of a complete roof window system; it cannot be used as a standalone product or with non-compatible window types. Ensure adequate ventilation in the room to manage any residual moisture, particularly in high-moisture areas like bathrooms.
- Not a waterproofing membrane—external flashing must be properly sealed
- Use only with compatible VELUX roof window systems
- Maintain adequate room ventilation, especially in bathrooms and kitchens
- Do not puncture or damage the membrane during or after installation
- Check that the barrier remains intact during subsequent building work