How to Calculate Insulation Thickness — Step-by-Step UK Guide

Key Concepts

Step-by-Step Calculation

Step 1: Identify the Target U-Value

Check Approved Document Part L for your specific situation — new build, extension, conversion or renovation. Note the maximum permitted U-value for the building element you are insulating (wall, roof, floor). For building control compliance, you may need to achieve a lower U-value than the maximum if other elements under-perform.

Step 2: Calculate the Total Required Thermal Resistance

Convert the U-value target to total thermal resistance: R_total = 1 / U-value. For example, for a wall with a target U-value of 0.26 W/m²K: R_total = 1 / 0.26 = 3.846 m²K/W.

Step 3: Add Surface Resistances

Every building element has internal and external surface resistances (Rsi and Rse). Standard values from BS EN ISO 6946 are: Rsi = 0.13 m²K/W (internal surface, horizontal heat flow) and Rse = 0.04 m²K/W (external surface). For floors: Rsi = 0.17 m²K/W. For ceilings/roofs with upward heat flow: Rsi = 0.10 m²K/W.

Step 4: Calculate Resistance of Existing Layers

For each existing layer of the building element (brick, block, plaster, air cavity, etc.), calculate its thermal resistance: R = thickness (m) / lambda (W/mK). For example, a 102.5mm brick with λ = 0.77 gives R = 0.1025 / 0.77 = 0.133 m²K/W. Add up all existing layers including any air gaps (a 50mm unventilated cavity ≈ 0.18 m²K/W).

Step 5: Calculate the Insulation Resistance Needed

Subtract the surface resistances and existing layer resistances from the total required resistance: R_insulation = R_total - Rsi - Rse - R_{existing layers}. This gives you the thermal resistance that the insulation alone must provide.

Step 6: Calculate the Insulation Thickness

Multiply the required insulation resistance by the lambda value of your chosen insulation: Thickness (m) = R_insulation × λ. Convert to millimetres and round up to the nearest available board thickness (typically 25, 30, 40, 50, 60, 75, 80, 90, 100, 120, or 150mm).

Step 7: Verify the Final U-Value

Recalculate the U-value using the actual insulation thickness you will use (rounded up): U = 1 / (Rsi + R_{all layers including insulation} + Rse). The final U-value should be equal to or lower than your target.

Worked Example: Cavity Wall

Common Mistakes to Avoid

• Ignoring thermal bridging: Timber studs and joists conduct heat faster than insulation. Between-joist insulation performs worse than the calculation suggests unless you account for this using the proportional area method.
• Using incorrect lambda values: Always use the lambda value declared by the manufacturer for the specific product, not a generic figure. Some products have different lambda values at different thicknesses.
• Forgetting surface resistances: Rsi and Rse add up and affect the calculation. Always include them.
• Confusing R-value and U-value: R-value is resistance (higher = better). U-value is transmittance (lower = better). They are reciprocals of each other for the whole element.
• Not rounding up: Always round up to the next available insulation thickness. Rounding down will fail to meet the target U-value.

Use our Insulation Thickness Calculator and U-Value Calculator to run these calculations automatically for your project.