Worked Example · External Wall
Cavity Wall U-Value Worked Example
A typical insulated masonry cavity wall for a new-build dwelling in England, checked against the Approved Document L (2021) limit of 0.18 W/m²K for external walls.
Build-up layers
| # | Layer | Thickness (mm) | λ (W/m·K) | R (m²K/W) |
|---|---|---|---|---|
| — | External surface resistance (Rse) | — | — | 0.040 |
| 1 | Outer leaf brickwork | 102 | 0.770 | 0.132 |
| 2 | Clear cavity (air space)(air space) | 50 | — | 0.160 |
| 3 | PIR insulation board | 120 | 0.022 | 5.455 |
| 4 | Inner leaf aerated block | 100 | 0.190 | 0.526 |
| 5 | Plaster | 13 | 0.500 | 0.026 |
| — | Internal surface resistance (Rsi) | — | — | 0.130 |
Calculated U-value
0.155 W/m²K
Total resistance R = 6.469 m²K/W
ADL 2021 limit
0.18 W/m²K
England — Approved Document L (2021), new dwellings
PASS
The calculated U-value of 0.155 W/m²K is at or below the 0.18 W/m²K limit — compliant with ADL 2021.
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