PIR between and under rafters
A pitched roof with the insulation at the rafters from the tool's Typical UK roofs group. With the layers below it has a U-value of 0.14 W/m²K, is 252.5 mm thick, and has about 36 kgCO₂e/m² of embodied carbon.
- U-value
- 0.14W/m²K
- Overall thickness
- 252.5mm
- Embodied carbon
- 36kgCO₂e/m²
- Fire
- Worst class E
Against the Building Regulations
| New home in | U-value limit | This build-up, 0.14 |
|---|---|---|
| England | 0.16 W/m²K | Meets it |
| Wales | 0.13 W/m²K | Misses by 0.01 |
| Scotland | 0.12 W/m²K | Misses by 0.02 |
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 | Roof tiles, clay outside the ventilated cavity: not counted | 15 | 1 | – | 13.7 | A1 |
| 2 | Ventilated cavity behind cladding outside the ventilated cavity: not counted | 25 | – | – | 0 | – |
| 3 | Unventilated cavity, foil-faced one side | 30 | – | 0.339 | 0 | – |
| 4 | PIR board bridged by joists at 600 mm | 120 | 0.022 | 3.783 | 13.5 | E |
| 5 | PIR board | 50 | 0.022 | 2.273 | 5.6 | E |
| 6 | Plasterboard | 12.5 | 0.21 | 0.060 | 3.5 | A2-s1,d0 |
| Total, with surface resistances | 252.5 | 7.086 | 36.3 |
U-value worked out to BS EN ISO 6946 from the total resistance. Carbon is stages A1 to A3 for the layers.
Moisture and rain
Traces only Under 10 g/m² at most, which the tool treats as negligible. Surface factor 0.96 (needs over 0.72 and at least 0.75).
Rain: Not checked for exposure zone 2 (moderate).
Other typical UK 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.