Short answer
A U-value describes the whole element, not a product. Multifoil certificates give a system resistance with specified air spaces; the roof's U-value adds every other layer and the timber bridging. Quilt alone typically lands around 0.3 W/m²K; with 50mm PIR it can reach the current 0.18 target.
- U-value is for the element
- R-value and lambda are for layers and materials
- Product value ≠ roof U-value
- Certificate figures only
- Bridging counts
U-value, R-value, lambda and thermal resistance
Lambda (λ, W/mK) is a material's conductivity. R-value (m²K/W) is the resistance of a layer: thickness divided by lambda for a homogeneous material, or a tested system value for a multifoil with its air spaces. U-value (W/m²K) describes the whole element. A product's declared thermal value is never the completed roof's U-value.
Thermal resistance (R, m²K/W) adds up layer by layer. U-value (W/m²K) is 1 divided by the total resistance including surface resistances. Lower U is better. Under the current edition of Approved Document L, Volume 1 (as amended), the target for an upgraded pitched roof insulated at rafter level in England is 0.18 W/m²K; new elements, extensions and loft conversions have their own figures, and transitional provisions determine which edition applies to a project. Building control confirms the applicable target.
What a multifoil certificate gives you
Third-party certificates (BBA, LABC and similar) state a thermal resistance for the product in a defined build-up — typically the quilt plus a 25mm still air gap on each face. Values in the region of R 2.5–3.5 m²K/W for the quilt-plus-gaps system are common for full-thickness multifoils; thin bubble foils are far lower. The exact figure is on the certificate for the exact product, and only applies if the gaps are built.
Worked example — rafter-level upgrade
| Layer | Thickness | R (m²K/W), indicative |
|---|---|---|
| Internal surface | — | 0.10 |
| Plasterboard | 12.5mm | 0.06 |
| Multifoil + two 25mm gaps (certified system) | ~90mm | 2.5 – 3.5 |
| PIR between rafters (bridged by timber) | 50mm | 1.6 – 1.9 |
| Ventilated void and covering | — | 0.20 (cap) |
| External surface | — | 0.04 |
| Total | — | 4.5 – 5.8 |
| U-value | — | ≈ 0.17 – 0.22 W/m²K |
The example shows why multifoil is so often combined with a PIR layer: quilt alone typically lands around 0.3 W/m²K, which improves a roof enormously but may not meet a regulatory target. Timber bridging through rafters is included in a proper calculation and is why real figures are worse than the simple sum.
Why packaging equivalence claims mislead
“Equivalent to 200mm of glass wool” compares certified system resistance with a nominal wool thickness, ignores bridging and gaps, and assumes a perfect installation. It is not a U-value and building control will not accept it. Ask for the certificate and the calculation.
Who calculates it
Installers or building control using the certificate values and BR 443 conventions. On a specified InsulationMatch job the target and the calculated U-value appear on the specification. Building regulations guide.
A survey measures the rafters and existing layers so the U-value can be calculated properly, not guessed.
Questions homeowners ask
What U-value does multifoil achieve?
What is the Building Regulations U-value for a roof?
Is R-value the same as U-value?
- GOV.UK — Approved Document L, Volume 1: Dwellings (current edition, as amended) — checked 2 September 2026
- BRE — U-value calculation guidance (BR 443) — checked 2 September 2026
- BBA — Agrément certificates search — checked 2 September 2026
- LABC — Registered details and product assessment — checked 2 September 2026