What U-value do I need to achieve?

Under Approved Document L Volume 1 (2021 edition incorporating 2023 amendments, England), the limiting U-values for new dwellings are: walls 0.26 W/m²K, floors 0.18, roof 0.16 (one value for all roof types). For new elements in an extension or existing dwelling (Table 4.2) they are stricter: walls 0.18, floors 0.18, roof 0.15, windows 1.4. Renovating a retained element (Table 4.3): cavity wall 0.55, internally or externally insulated wall 0.30, floor 0.25, roof 0.16. Scotland and Wales have slightly different requirements.

What is lambda value?

Lambda (λ) is the thermal conductivity of an insulation material, measured in W/mK. The lower the lambda value, the better the insulator. For example, PIR board (Celotex/Kingspan) has a lambda of 0.022 W/mK, mineral wool is 0.034-0.044 W/mK, and EPS is 0.030-0.038 W/mK. A lower lambda means you need less thickness to achieve the same U-value.

How do I calculate insulation thickness?

The simplified formula is: Thickness (m) = Lambda (W/mK) × (1/U-value target - Rsi - Rse - R existing layers). In practice, you subtract the thermal resistance of all existing building layers, surface resistances, and any air gaps from the total thermal resistance needed (1/U-value), then multiply the remaining resistance needed by the lambda value of your chosen insulation.

What is the difference between R-value and U-value?

R-value is thermal resistance — the higher the better. It measures how well a layer resists heat flow. U-value is the overall thermal transmittance of the complete building element — the lower the better. U-value = 1 / (Rsi + R1 + R2 + ... + Rn + Rse), where Rsi and Rse are internal and external surface resistances.

Do I need to account for thermal bridging?

Yes. Timber studs, joists, mortar joints and metal fixings create thermal bridges that reduce the overall thermal performance. For between-joist insulation, the U-value calculation must use the proportional area method (BS EN ISO 6946) to account for the thermal bridging effect of the timber. Continuous insulation over the top eliminates this issue.

How to Calculate Insulation Thickness — Step-by-Step UK G...

Key Concepts

U-Value Targets (Part L 2021, England)

New-build walls: 0.26 W/m²K
New-build floors: 0.18 W/m²K
New-build roof (all types): 0.16 W/m²K
Extension walls: 0.18 W/m²K
Extension floors: 0.18 W/m²K
Extension roofs: 0.15 W/m²K
Windows: 1.6 W/m²K new-build limiting; 1.4 in existing dwellings (Table 4.2)

Common Lambda (λ) Values

PIR board (Celotex/Kingspan): 0.022 W/mK
Phenolic board: 0.020 W/mK
EPS (expanded polystyrene): 0.030-0.038 W/mK
XPS (extruded polystyrene): 0.033-0.036 W/mK
Mineral wool (slab): 0.034-0.038 W/mK
Mineral wool (roll): 0.040-0.044 W/mK
Spray foam (closed cell): 0.025-0.028 W/mK
Wood fibre board: 0.038-0.043 W/mK

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

Target: U-value = 0.26 W/m²K (new-build wall)

R_total = 1 / 0.26 = 3.846 m²K/W

Existing layers:

Rsi = 0.13
Plasterboard 12.5mm (λ=0.25): 0.0125/0.25 = 0.050
Blockwork 100mm (λ=0.15): 0.100/0.15 = 0.667
Cavity 50mm (partially filled): residual cavity = 0.18
Brickwork 102.5mm (λ=0.77): 0.1025/0.77 = 0.133
Rse = 0.04

Total existing R = 0.13 + 0.050 + 0.667 + 0.18 + 0.133 + 0.04 = 1.200 m²K/W

R_{insulation needed} = 3.846 - 1.200 = 2.646 m²K/W

Using PIR (λ=0.022): Thickness = 2.646 × 0.022 = 0.058m = 58mm → use 60mm

Using mineral wool (λ=0.034): Thickness = 2.646 × 0.034 = 0.090m = 90mm → use 90mm

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.

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