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Insulation U-Value Calculator

Calculate thermal performance & check Part L compliance

Professional U-Value Calculator

Thermal performance estimator for UK building regulations with accurate lambda values and surface resistances

New builds have stricter targets than extensions

Material affects total thermal resistance calculation

λ (lambda) = thermal conductivity in W/mK - Lower = better insulation

Affects thermal resistance calculation

Typical: 50-300mm depending on material

For material cost estimation

Calculate Insulation for Your Specific Project

Select your project type for tailored calculations, tips, and guidance:

Loft Insulation

270mm target, mineral wool rolls

Cavity Wall

Blown or batt insulation

Solid Wall (Internal)

IWI insulated dry lining

Solid Wall (External)

EWI render systems

Floor Insulation

Suspended and solid floors

Room in Roof

Loft conversion insulation

Flat Roof

Warm deck insulation

New Build Walls

Part L 2021 compliant

Part L 2021 Compliance Standards

  • Walls: 0.18 W/m²K (new build) | 0.28 W/m²K (extension)
  • Roofs: 0.15 W/m²K (new build) | 0.16 W/m²K (extension)
  • Floors: 0.18 W/m²K (new build) | 0.22 W/m²K (extension)
  • Requirement: 31% CO2 reduction vs 2013 standards
  • Non-compliance: Failed Building Control inspection, remedial costs, warranty issues
  • Surface Resistances: Walls Rsi=0.13/Rso=0.04, Roofs Rsi=0.10/Rso=0.04, Floors Rsi=0.17/Rso=0.04

Understanding U-Values & Thermal Performance

U-value measures thermal transmittance in W/m²K - how much heat passes through a building element. Lower U-values mean better insulation and less heat loss. The calculation includes surface film resistances (Rsi internal, Rso external), construction material layers, insulation layer, and any air gaps. Building Regulations Part L 2021 sets maximum U-values for different construction types.

Core Formula: U-value = 1 / (Rsi + R construction + R insulation + R air gap + Rso). Rsi and Rso vary by element type and heat flow direction. This calculator uses correct values for walls, roofs, and floors.

What is Thermal Resistance (R-value)?

R-value (m²K/W) measures insulation effectiveness. Higher R = better insulation. Calculated as thickness / thermal conductivity (λ). Total R-value includes all layers: surface resistances, construction materials, insulation, and air gaps.

  • U-value = 1 / Total R-value (inverse relationship)
  • Building regulations use U-values
  • Each material layer contributes to total R
  • SAP calculations require accurate U-values

Insulation Material Lambda Values

Thermal conductivity (λ-value) in W/mK determines insulation quality. Lower λ = better insulation = thinner needed. Premium materials cost more but achieve targets with less thickness, saving space.

  • Phenolic: λ 0.020 (best, thinnest)
  • PIR: λ 0.023 (premium performance)
  • EPS/Wood: λ 0.038 (good value)
  • Cellulose/Hemp: λ 0.040 (natural)
  • Mineral Wool: λ 0.044 (basic cost)

Construction Material & Air Gap Calculations

U-value calculations account for all layers in the construction. Different construction materials (brick, block, plaster) have different R-values that affect total thermal resistance. Air gaps (cavities) also contribute to thermal resistance, with unventilated cavities having R=0.18 and ventilated cavities R=0.00.

Construction Material R-Values

  • 100mm Brick: R = 0.13 m²K/W (λ 0.77)
  • 215mm Brick: R = 0.28 m²K/W (λ 0.77)
  • 100mm Aerated Block: R = 0.91 m²K/W (λ 0.11)
  • 100mm Dense Block: R = 0.33 m²K/W (λ 0.30)
  • 12.5mm Plasterboard: R = 0.05 m²K/W
  • 50mm Timber: R = 0.15 m²K/W (λ 0.13)

Surface Resistance Values (m²K/W)

Vary by element type and heat flow direction:

  • Walls (Horizontal): Rsi 0.13, Rso 0.04
  • Roofs (Upward): Rsi 0.10, Rso 0.04
  • Floors (Downward): Rsi 0.17, Rso 0.04
  • Unventilated Air Gap: R = 0.18
  • Ventilated Air Gap: R = 0.00

Choosing Insulation & Calculating Requirements

Material Selection Guide

  • PIR/Phenolic: Best performance, premium cost, thinnest application
  • EPS: Good all-rounder, moisture-resistant, moderate cost
  • Mineral Wool: Fire-resistant, acoustic benefits, cost-effective, thicker needed
  • Natural Materials: Hemp/wood breathable, sustainable, higher cost, thicker needed
  • Cellulose: Recycled content, cost-effective, moisture sensitive

Thickness Calculation Formula

Required thickness (mm) = (Target R-value - existing R) x λ-value x 1000

This calculator automatically determines recommended thickness for your target U-value. Typical applications:

  • Cavity walls: 75-150mm (stone), 50-100mm (premium)
  • Loft insulation: 150-300mm depending on material
  • Solid walls: 100-200mm internal or external
  • Suspended floors: 100-200mm between joists

Frequently Asked Questions

Q: How accurate is this calculator for Building Control?

This calculator provides simplified estimates for planning purposes only. Building Control submissions require detailed SAP calculations from accredited energy assessors. Professional calculations account for thermal bridging, air tightness, and complex constructions. Use this for initial design; obtain professional SAP report before Building Control submission to avoid rejection.

Q: What is thermal bridging and does it affect U-values?

Thermal bridging occurs where materials with poor insulation cross insulation layers (studs, joists, beams). Reduces overall U-value performance per BS EN ISO 6946. Professional calculations use linear thermal transmittance (ψ-values) and correction factors (ΔU) to account for thermal bridges. This simplified calculator does not account for bridging - actual performance may be worse than calculated.

Q: Do I need SAP calculations for my project?

Required for: New dwellings (Building Regulations compliance), EPC assessments, mortgage valuations, renewable energy grants. Recommended for: Extensions over 25% of wall area, major renovations. Simplified calculations sufficient for: Minor work, self-build planning, material selection. Obtain professional assessment if uncertain.

Q: Why are surface resistances different for walls, roofs, and floors?

Surface resistances (Rsi/Rso) depend on heat flow direction and exposure. Walls have horizontal heat flow (Rsi 0.13). Roofs have upward heat flow - internal resistance lower (Rsi 0.10) because warm air rises. Floors have downward heat flow - internal resistance higher (Rsi 0.17) because warm air does not support downward movement. These values are per BS EN ISO 6946.

Q: What is the difference between new build and extension targets?

New buildings (Part L 2021) have stricter targets: walls 0.18, roofs 0.15, floors 0.18 W/m²K. Extensions have higher allowed U-values: walls 0.28, roofs 0.16, floors 0.22. Retrofit/upgrade work follows extension standards unless replacing entire element. New build standards require thicker insulation or premium materials.

For Planning Purposes Only - Professional Assessment Recommended

This calculator provides simplified estimates using correct lambda values and surface resistances per BS EN ISO 6946. Complex constructions, thermal bridging corrections, air tightness, and renewable energy systems require professional SAP calculations. Building Control submissions must include accredited energy assessor reports. Non-compliance with Part L can result in failed inspections, remedial work costs, warranty voidance, and prosecution.

Need Help or Have Questions?

Got a specific calculation requirement or want a custom tool for your trade? Fill out the form below.

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