How thick should wall insulation be to meet UK Building Regulations?

For a typical cavity wall to achieve the Part L target of 0.30 W/m²K, you typically need around 90-100mm of mineral wool or 50-60mm of PIR board. The exact thickness depends on your existing wall construction. Solid walls generally need more insulation than cavity walls.

What insulation thickness do I need for a loft?

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.

Is PIR board better than mineral wool?

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.

What are the Building Regs targets for insulation?

UK Building Regulations Approved Document L sets maximum U-values: walls 0.30 W/m²K, pitched roofs 0.16 W/m²K, flat roofs 0.18 W/m²K, and ground floors 0.25 W/m²K. These apply to new builds and extensions. Renovation work may have different requirements depending on the scope.

What is thermal conductivity and why does it matter?

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 (/insulation/u-value-calculator/) to model full wall build-ups.

Insulation Thickness Calculator — How Much Insulation Do I Need UK

How We Calculate This

This calculator determines the insulation thickness required to achieve a target U-value for your building element. It uses the standard thermal resistance calculation method from BS EN ISO 6946 and references UK Building Regulations Approved Document L.

The formula

Required insulation R = (1 / target U-value) - Rsi - Rse - existing construction R

Insulation thickness = R × thermal conductivity × 1000 (mm)

For example, to achieve 0.30 W/m²K on a cavity wall (base R = 0.45) with mineral wool (0.035 W/mK): Required R = (1/0.30) - 0.13 - 0.04 - 0.45 = 2.71 m²K/W. Thickness = 2.71 × 0.035 × 1000 = 95mm.

Building Regulations Part L targets

External walls: 0.30 W/m²K
Pitched roof: 0.16 W/m²K
Flat roof: 0.18 W/m²K
Ground floor: 0.25 W/m²K

The thickness is rounded up to the nearest 5mm to match standard product sizes. Always check manufacturer datasheets for exact lambda values, as these can vary slightly between products.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Insulation Thickness Calculator \u2014 How Much Insulation Do I Need?