How to Calculate U-Values
U-value (thermal transmittance) measures the rate of heat transfer through a building element. Lower values indicate better insulation. This guide explains how to calculate U-values following UK standards.
The Basic Formula
The U-value is calculated as the reciprocal of total thermal resistance:
U = 1 / Rtotal
Where Rtotal is the sum of all thermal resistances:
Rtotal = Rsi + R1 + R2 + ... + Rn + Rse
Step-by-Step Calculation
Step 1: Identify All Layers
List every layer in your construction from inside to outside, including:
- Internal surface resistance (Rsi)
- All material layers with their thicknesses
- Any air cavities
- External surface resistance (Rse)
Step 2: Calculate Each Layer's R-Value
For solid materials:
R = thickness (m) / thermal conductivity (W/mK)
Step 3: Surface Resistances
Standard surface resistances from BR 443:
| Element | Rsi (m²K/W) | Rse (m²K/W) |
|---|---|---|
| Walls | 0.13 | 0.04 |
| Roofs | 0.10 | 0.04 |
| Floors | 0.17 | 0.04 |
Step 4: Sum and Invert
Add all R-values together, then take the reciprocal to get the U-value in W/m²K.
Example: Cavity Wall Calculation
A typical cavity wall with:
- 102mm brick outer leaf (λ = 0.77 W/mK)
- 100mm mineral wool insulation (λ = 0.035 W/mK)
- 100mm concrete block inner leaf (λ = 0.51 W/mK)
- 13mm plasterboard (λ = 0.21 W/mK)
| Layer | Calculation | R-value (m²K/W) |
|---|---|---|
| Rsi | Standard | 0.13 |
| Plasterboard | 0.013 / 0.21 | 0.06 |
| Block | 0.100 / 0.51 | 0.20 |
| Insulation | 0.100 / 0.035 | 2.86 |
| Brick | 0.102 / 0.77 | 0.13 |
| Rse | Standard | 0.04 |
| Total | 3.42 |
U-value = 1 / 3.42 = 0.29 W/m²K
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