Insulation Thickness Calculator — How Much Insulation Do I Need?

It depends on which Part L target applies. For an existing cavity wall filled by cavity insulation, the AD L 2021 renovation target (Table 4.3) is 0.55 W/m²K — typically met by the standard cavity fill alone. For a wall upgraded by internal or external insulation the renovation target is 0.30 W/m²K, needing roughly 90-100mm of mineral wool or 50-60mm of PIR board. New-build and extension walls must reach the tighter limiting value of 0.26 W/m²K (Table 4.1), and new walls in extensions 0.18 W/m²K (Table 4.2), which need more. Solid walls generally need more insulation than cavity walls, and the exact thickness depends on your existing fabric R-value.

To meet the current UK Building Regulations target of 0.16 W/m²K for a roof, you typically need 270mm of mineral wool insulation at ceiling level. This is usually installed as two layers: 100mm between the joists and 170mm laid across the top at 90 degrees.

PIR (polyisocyanurate) boards like Celotex and Kingspan have a lower thermal conductivity (0.022 W/mK vs 0.035 W/mK for mineral wool), so you need less thickness to achieve the same U-value. However, mineral wool is cheaper per m² and provides better sound insulation. The choice depends on space constraints and budget.

UK Building Regulations Part L 2021 (Approved Document L Volume 1, 2021 edition incorporating 2023 amendments) sets different limiting U-values depending on the work. New dwellings (Table 4.1): walls 0.26 W/m²K, all roof types 0.16 W/m²K (a single roof value, no pitched/flat split), floors 0.18 W/m²K, party walls 0.20 W/m²K, windows 1.6 W/m²K. New thermal elements in an existing dwelling or extension (Table 4.2): walls 0.18 W/m²K, roofs 0.15 W/m²K (a single roof value), floors 0.18 W/m²K, windows 1.4 W/m²K. Renovation of a retained (existing) element (Table 4.3): walls 0.30 W/m²K by internal/external insulation or 0.55 W/m²K by cavity fill, roofs 0.16 W/m²K, floors 0.25 W/m²K. Extensions use the tighter Table 4.2 values, not the renovation ones — always confirm which case applies to your project.

Thermal conductivity (lambda or λ value) measured in W/mK indicates how easily heat passes through a material. Lower values mean better insulation: PIR board is 0.022 W/mK, mineral wool 0.035 W/mK, and brick 0.77 W/mK. Choosing a material with lower conductivity means you need less thickness. See our U-Value Calculator to model full wall build-ups.