What the BBA certificate actually requires
Every multifoil product on the UK market that carries a BBA certificate has installation conditions attached to it. The certificate is not a generic stamp of approval — it is product-specific, and the performance figures (U-value, air permeability) only apply when the product is installed exactly as the certificate describes.
- A minimum sealed air gap on each side of the multifoil — typically 25mm, sometimes more depending on the product
- Fully sealed joints using the manufacturer's own foil tape (not generic tape)
- A continuous vapour control layer with no penetrations breaking continuity
- A specific combined insulation buildup — multifoil + a stated thickness of rigid or mineral wool insulation — to achieve the claimed U-value
- Correct orientation, fixing centres, and overlaps
Common installation failures
The most frequent issue found on inspection is the multifoil installed tight to the rafter underside or felt with no air gap. Without the air gap, the product is essentially performing as a reflective layer with no thermal benefit, and condensation risk increases sharply. Other common failures: unsealed joints, missing vapour control continuity, and the product sold as a standalone solution where the certificate clearly requires a combined buildup.
Why multifoil is often mis-sold
Multifoil is sometimes sold as a single-product solution that 'meets building regulations on its own'. For most domestic roof applications, the BBA certificate is explicit that it does not. A combined buildup is needed. Selling it as a standalone solution, particularly under ECO4 or GBIS, is a documented basis for a Section 75 or FOS complaint.
Where the multifoil was installed after spray foam removal — sometimes as an upsell from the same removal contractor — independent inspection becomes particularly important. The contractor has a financial interest in confirming their own work meets standard.
What the inspection produces
- Identification of the multifoil product where possible
- Cross-reference against the relevant BBA certificate conditions
- Air gap measurement on both sides of the multifoil
- Joint sealing and tape continuity check
- Vapour control layer assessment
- Combined insulation buildup vs claimed U-value
- Photographic evidence and a clear plain-English summary