The following tables give examples of how to achieve Best Practice in refurbishment and home extension projects using appropriate levels of insulation.

Table 7: Insulation of walls

(a) Solid Walls

For an existing 220mm brick and internal plaster wall (U-value 2.1 W/m2K), either:

- apply external wall insulation with an R-value of 2.3 m2K/W and apply protective render (final U-value of 0.35 W/m2K), or

- apply internal plasterboard/insulation laminate with R-value of

1.8 m2K/W (giving a final U-value of 0.45 W/m2K)

(b) Cavity Walls

For a cavity wall with two brick leaves of 105mm, internal plaster and a cavity of 65mm (U-value 1.44 W/m2K):

- fill the cavity with suitable insulation material to give a final

U-value of 0.54 W/m2K (based on insulation with a thermal conductivity of 0.040 W/mK) 

Achieving the required R-value (R = d/λ)

Insulation conductivity (λ) Thickness (d) required (W/mK) (mm)

External Wall Insulation / Internal Insulation

0.020 / 48 / 35

0.025 / 60 / 44

0.030 / 72 / 52

0.035 / 82 / 61

0.040 / 95 / 70