Fixing and buying guide

VELUX Vapour Barrier 134x140cm BBX UK08 0000

What this product is for

This vapour barrier membrane is essential for loft conversions, roof insulation projects, and any roof space where you need to manage moisture. It works by creating a barrier on the warm side of insulation—typically between the rafters and internal plasterboard—to prevent water vapour from the heated interior reaching cold surfaces where it would condense. This protects the timber structure, maintains insulation effectiveness, and reduces heat loss.

Loft conversions and new roof insulation installations
Protecting timber rafters and joists from condensation damage
Maintaining thermal performance of insulation layers
Meeting building regulations for moisture control in roof spaces

Sizing and compatibility

This sheet measures 134×140cm and is specifically sized for UK08 VELUX window openings. It is compatible with standard rafter spacing found in typical UK roof construction. Before ordering, confirm your roof opening size matches UK08 dimensions and check your rafter spacing to ensure the membrane will fit your installation without excessive cutting or gaps.

Size: 134×140cm sheet
Designed for UK08 VELUX window openings
Suits standard UK rafter spacing
Measure your roof opening and rafter layout before purchase

Installation and positioning

The vapour barrier must be installed on the warm side of the insulation layer—between the rafters and the internal plasterboard or finish. This positioning is critical: placing it on the wrong side will trap moisture in the insulation and cause damage. Ensure the membrane is continuous across the roof space, with all seams sealed to prevent moisture bypassing the barrier. Work carefully to avoid tears or punctures during installation.

Install between rafters and internal plasterboard (warm side of insulation)
Never install on the cold (external) side of insulation
Seal all seams and overlaps to maintain continuity
Avoid tears, punctures, or gaps that allow moisture to pass through

Pre-installation checks

Before fitting, inspect your roof structure for existing damp, leaks, or water damage. The vapour barrier prevents internal moisture reaching cold surfaces, but it cannot remedy external water ingress or existing condensation problems. Ensure your roof is sound and dry. Check that your insulation layer is ready to receive the membrane and that you have adequate access and working space. Confirm the sheet size is correct for your UK08 opening and that you have any additional materials needed for sealing seams.

Check roof structure is dry and free from leaks
Verify UK08 opening size matches this product
Ensure insulation layer is in place and ready
Have sealing tape or adhesive available for seam joints
Plan installation sequence to avoid damaging the membrane