28 Composite panels
A pitched roof with the insulation at the rafters from the tool's Metal roofs group. With the layers below it has a U-value of 0.19 W/m²K, is 121 mm thick, and has about 47 kgCO₂e/m² of embodied carbon.
- U-value
- 0.19W/m²K
- Overall thickness
- 121mm
- Embodied carbon
- 47kgCO₂e/m²
- Fire
- Worst class E
Against the Building Regulations
| New home in | U-value limit | This build-up, 0.19 |
|---|---|---|
| England | 0.16 W/m²K | Misses by 0.03 |
| Wales | 0.13 W/m²K | Misses by 0.06 |
| Scotland | 0.12 W/m²K | Misses by 0.07 |
These are the limits for a single element in a new home. In England a new home usually needs better figures to pass the whole-house target. All UK U-value limits.
Section, drawn to scale
Layers, outside to inside
| # | Layer | mm | λ, W/mK | R, m²K/W | kgCO₂e/m² | Fire class |
|---|---|---|---|---|---|---|
| 1 | Profiled steel sheet or panel facing | 0.5 | 50 | 0.000 | 10.8 | A1 |
| 2 | Composite panel core, PIR | 120 | 0.022 | 5.455 | 16.7 | E |
| 3 | Steel liner sheet or tray | 0.5 | 50 | 0.000 | 10.8 | A1 |
| Total, with surface resistances | 121 | 5.595 | 46.7 |
U-value worked out to BS EN ISO 6946 from the total resistance, plus 0.008 W/m²K of corrections for fixings, gaps or brackets. Carbon is stages A1 to A3 for the layers, including 8.3 kgCO₂e/m² for steel rails or purlins.
Moisture and rain
Condenses in winter, dries by summer Up to 20 g/m² at between profiled steel sheet or panel facing and composite panel core, pir (part 1 of 22), all of it drying out over the year. Surface factor 0.95 (needs over 0.72 and at least 0.75).
Rain: Not checked for exposure zone 2 (moderate).
Other metal roofs
Figures come from the Build-up Atlas's own calculation, with generalised values for typical products, for an England new home with default settings. They are indicative, for design and teaching: check the manufacturer's declared values and the current regulations for your project. Rules and standards last checked 21 October 2026.