✦ BS EN ISO 6946 Standard

U-Value Calculator

Calculate the thermal transmittance (U-value) of composite multi-layer walls, roofs, and floors. Build custom assemblies with real thermal conductivity values (λ), compute total thermal resistance (Rtot), and visualize temperature gradients.

Assembly Type:

🧱 Multi-Layer Construction Stack (From Inside to Outside)

Material Layer Thickness (mm) Conductivity λ (W/mK) Resistance R

🌡️ Cross-Sectional Thermal Drop Profile

← Indoor Heated Space (21°C) Outdoor Environment (−4°C) →
Composite Thermal Transmittance
0.28 W/m²K
Overall U-Value · 0.049 BTU/hr·ft²·°F
Thermal Resistance (BS EN ISO 6946)
Surface Resistances (Rsi + Rse): 0.17 m²K/W
Material Layers Resistance (ΣR): 3.40 m²K/W
Total Resistance (R_total): 3.57 m²K/W (R-20.3)
✓ Passes UK Part L 2021 (Target ≤ 0.26 W/m²K)
This wall assembly achieves excellent compliance with modern energy efficiency standards.
🧱 Material Physics

Material Thermal Conductivity (λ) & Thickness Matcher

Compare how much physical thickness is required across different materials to hit a target thermal resistance of R-5.0 m²K/W (U = 0.20 W/m²K).

Required Physical Thickness
110 mm (4.3 in)
Equivalent Assembly U-Value: 0.20 W/m²K
Space Saving: PIR is 3.5× more space-efficient than brickwork.
📉 Law of Diminishing Returns

Insulation Thickness vs. U-Value Diminishing Returns Simulator

Slide insulation thickness to observe how the first 50mm delivers the overwhelming majority of energy savings.

Resulting U-Value 0.21 W/m²K
Heat Loss Reduction 90.0% Cut
Wall Conduction (20m² wall) 105 Watts
📋 Regulatory Standards

UK Building Regulations Part L (2021) Limiting U-Values

Statutory maximum U-values for new residential dwellings and major thermal renovations in England & Wales.

Building Element Target U-Value (W/m²K) Limiting Maximum (W/m²K) Passivhaus Benchmark
External Walls 0.18 W/m²K 0.26 W/m²K ≤ 0.15 W/m²K
Pitched Roof (Insulated at Ceiling) 0.11 W/m²K 0.16 W/m²K ≤ 0.12 W/m²K
Flat Roof / Rafter Slope 0.11 W/m²K 0.16 W/m²K ≤ 0.12 W/m²K
Ground Floor 0.13 W/m²K 0.18 W/m²K ≤ 0.15 W/m²K
Windows & Glazed Doors 1.20 W/m²K 1.60 W/m²K ≤ 0.80 W/m²K
⚠️ Calculation Traps

5 Common U-Value Calculation Mistakes

Avoid these frequent errors when modeling multi-layer composite structures.

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1. Forgetting Surface Air Films (R_si & R_se)

Omitting the internal (0.13 m²K/W) and external (0.04 m²K/W) boundary air films causes a 5%–15% error in uninsulated and thin assembly calculations.

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2. Ignoring Timber Stud Repeating Thermal Bridges

In timber stud walls, softwood studs represent 15% of the wall area. Because timber has a higher thermal conductivity (λ=0.13) than mineral wool (λ=0.038), it creates a bridging penalty.

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3. Adding U-Values Directly

You cannot add U-values together (1/U_total ≠ U1 + U2). You must sum thermal resistances (R_total = R1 + R2 + ...), and then invert the total: U = 1 / R_total.

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4. Using Nominal Instead of Declared Lambda

Always use aged declared thermal conductivity values (λ_90/90) that account for long-term gas diffusion in foam insulations.

Thermal Envelope Tools

Related Building Physics Tools

Explore imperial R-value conversions, single-room conduction calculations, and convective air permeability modeling.

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Single Room

Room Heat Loss Calculator

Calculate precise heat loss for individual rooms to size replacement radiators.

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4-Way Converter

R-Value to U-Value Converter

Instantly convert between imperial R-values (hr·ft²·°F/BTU) and metric U-values.

Open Dedicated Tool →
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Blower Door & MVHR

Air Infiltration / ACH Calculator

Estimate ventilation and air leakage heat loss based on air changes per hour.

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U-Value & Thermal Assembly FAQs

Expert answers on multi-layer thermal resistance, surface air film coefficients, and Part L building regulation compliance.

U-value (thermal transmittance) measures the rate of heat conduction through one square metre of a building element for every degree of temperature difference across it. In SI metric units, it is expressed as Watts per square metre Kelvin (W/m²K). A lower U-value signifies superior thermal insulation.
Under BS EN ISO 6946, U-value is calculated by adding the thermal resistances (R = thickness / thermal conductivity) of every individual material layer, plus internal (R_si = 0.13) and external (R_se = 0.04) surface air film resistances. The composite U-value is the reciprocal of the total thermal resistance: U = 1 / R_total.
For new residential dwellings in England & Wales: External Walls must achieve U ≤ 0.18 W/m²K (limiting threshold 0.26), Roofs U ≤ 0.11 W/m²K (limiting 0.16), Ground Floors U ≤ 0.13 W/m²K (limiting 0.18), and Windows/Glazed Doors U ≤ 1.2 W/m²K (limiting 1.6).
Because heat loss is inversely proportional to thermal resistance (Q ∝ 1/R). Upgrading an uninsulated solid brick wall (U=2.1) with just 50mm of PIR insulation drops its U-value to 0.38 (an 82% reduction in heat loss). Adding another 50mm (100mm total) drops it to 0.21 (saving only an additional 8%).
Thermal conductivity (λ, in W/mK) is an intrinsic property of a material measuring how easily heat passes through 1 metre thickness. Thermal resistance (R, in m²K/W) measures how much a specific thickness (d) of that material resists heat flow: R = d / λ.
Still air on the interior surface of a wall forms a boundary layer that provides thermal resistance (R_si = 0.13 m²K/W for walls, 0.10 for roofs, 0.17 for floors). Moving air on the exterior surface forms a thinner boundary (R_se = 0.04 m²K/W).