The U-Value: The Key Metric for Thermal Insulation
The U-value (formerly k-value) is the most important metric for evaluating the thermal insulation quality of a building component. It indicates how much heat per second flows through one square metre of component surface when the temperature difference is one Kelvin.
What Does the U-Value Mean?
Definition: The U-value indicates the heat flow in watts that flows through 1 m² of component area when the temperature difference between inside and outside is 1 Kelvin (= 1°C).
Unit: W/(m²·K) – Watts per square metre and Kelvin
The Rule of Thumb
- Low U-value = Little heat loss = Good insulation
- High U-value = Much heat loss = Poor insulation
| U-value | Rating | Example |
|---|---|---|
| 0.1–0.2 W/m²K | Very good | Passivhaus wall |
| 0.2–0.3 W/m²K | Good | New build standard |
| 0.3–0.5 W/m²K | Medium | Refurbished old building |
| 0.5–1.0 W/m²K | Moderate | Partially insulated |
| > 1.0 W/m²K | Poor | Uninsulated old building |
Worked Example: What Does the U-Value Mean in Practice?
An external wall with:
- Area: 10 m²
- U-value: 0.24 W/m²K
- Indoor temperature: 20°C
- Outdoor temperature: 0°C
Calculation: Heat flow = U × A × ΔT = 0.24 × 10 × 20 = 48 Watts
At -10°C outside: 0.24 × 10 × 30 = 72 Watts
For comparison: An uninsulated wall with U = 1.5 W/m²K:
Heat flow = 1.5 × 10 × 30 = 450 Watts – more than 6 times as much!
How is the U-Value Calculated?
The U-value is derived from the thermal resistances of all layers:
Formula: U = 1 / RT
with RT = Rsi + R1 + R2 + ... + Rn + Rse
- RT = Total thermal resistance (m²K/W)
- Rsi = Internal surface resistance
- R1, R2... = Thermal resistances of each layer
- Rse = External surface resistance
The Resistance of a Layer
Each material layer has a thermal resistance:
Formula: R = d / λ
- R = Thermal resistance (m²K/W)
- d = Layer thickness (m)
- λ = Thermal conductivity (W/mK)
Thermal Conductivity λ
The thermal conductivity λ (lambda) is a material property:
| Material | λ (W/mK) | Rating |
|---|---|---|
| Copper | 380 | Extremely conductive |
| Steel | 50 | Very conductive |
| Concrete | 2.1 | Conductive |
| Solid brick | 0.8 | Moderately conductive |
| Wood | 0.13 | Slightly conductive |
| Mineral wool | 0.035 | Insulation material |
| EPS (polystyrene) | 0.035 | Insulation material |
| PUR/PIR | 0.024 | High-performance insulation |
| Still air | 0.025 | Theoretically optimal |
The lower λ, the better the material insulates. Insulation materials have λ-values below 0.1 W/mK – typically 0.03–0.04 W/mK.
Surface Resistances
At surfaces, heat transfer occurs between air and the component:
| Situation | Rsi (internal) | Rse (external) |
|---|---|---|
| Upward (ceiling) | 0.10 m²K/W | 0.04 m²K/W |
| Horizontal (wall) | 0.13 m²K/W | 0.04 m²K/W |
| Downward (floor) | 0.17 m²K/W | 0.04 m²K/W |
Complete Worked Example
An external wall with external wall insulation system (EWIS):
| Layer | d (cm) | λ (W/mK) | R (m²K/W) |
|---|---|---|---|
| Internal plaster | 1.5 | 0.70 | 0.02 |
| Masonry | 24 | 0.79 | 0.30 |
| EPS insulation | 14 | 0.035 | 4.00 |
| External render | 1.5 | 0.87 | 0.02 |
Calculation: RT = 0.13 + 0.02 + 0.30 + 4.00 + 0.02 + 0.04 = 4.51 m²K/W
U = 1 / 4.51 = 0.22 W/m²K
The 14 cm insulation (R = 4.00) accounts for over 88% of the total resistance!
U-Values of Different Components
External Walls by Construction Era
| Era | Construction | Typical U-value |
|---|---|---|
| pre-1918 | Solid brick 50cm | 1.2–1.5 W/m²K |
| 1919–1948 | Solid brick 38cm | 1.4–1.7 W/m²K |
| 1949–1968 | Hollow block 30cm | 1.0–1.4 W/m²K |
| 1969–1978 | Hollow block + 4cm insulation | 0.6–0.9 W/m²K |
| 1979–1994 | Aerated concrete/insulation | 0.4–0.6 W/m²K |
| 1995–2009 | 10–12cm insulation | 0.3–0.4 W/m²K |
| from 2010 | 14–20cm insulation | 0.2–0.28 W/m²K |
| Passivhaus | 30cm+ insulation | < 0.15 W/m²K |
Windows by Generation
| Generation | Glazing | Ug value | Uw value |
|---|---|---|---|
| pre-1978 | Single glazing | 5.8 | 5.0–5.5 |
| 1978–1995 | Double without low-e | 3.0 | 2.7–3.1 |
| 1995–2005 | Double with low-e | 1.1 | 1.3–1.6 |
| 2005–2015 | Triple standard | 0.7 | 1.0–1.2 |
| from 2015 | Triple premium | 0.5 | 0.8–0.9 |
Ug vs. Uw:
- Ug = U-value of glazing only (g = glazing)
- Uw = U-value of entire window including frame (w = window)
- The frame is often worse than the glazing!
Roof / Loft Floor
| Condition | Typical U-value |
|---|---|
| Uninsulated | 2.0–3.5 W/m²K |
| 6cm insulation | 0.5–0.6 W/m²K |
| 12cm insulation | 0.25–0.30 W/m²K |
| 20cm insulation | 0.15–0.18 W/m²K |
| 30cm insulation | 0.10–0.12 W/m²K |
Ground Floor / Floor Slab
| Condition | Typical U-value |
|---|---|
| Uninsulated | 0.8–1.2 W/m²K |
| 6cm insulation | 0.4–0.5 W/m²K |
| 10cm insulation | 0.25–0.30 W/m²K |
| 16cm insulation | 0.15–0.20 W/m²K |
Building Regulations Requirements
The German Building Energy Act (GEG) and UK Building Regulations define minimum requirements:
When Renovating Individual Components
| Component | Max. U-value (GEG 2020) | UK Part L (2021) |
|---|---|---|
| External wall | 0.24 W/m²K | 0.18–0.26 W/m²K |
| Roof/loft floor | 0.24 W/m²K | 0.11–0.16 W/m²K |
| Ground floor | 0.30 W/m²K | 0.18–0.25 W/m²K |
| Windows | 1.30 W/m²K (Uw) | 1.2–1.4 W/m²K |
| External doors | 1.80 W/m²K | 1.0–1.4 W/m²K |
New Build (Reference Building)
| Component | Reference U-value |
|---|---|
| External wall | 0.28 W/m²K |
| Roof | 0.20 W/m²K |
| Floor slab | 0.35 W/m²K |
| Windows | 1.30 W/m²K |
Note: Building regulations set minimum requirements. For an economical heating system (especially heat pumps), better U-values are often sensible!
Sources of Error in U-Value Data
1. Laboratory Values vs. Reality
| Factor | Effect on U-value |
|---|---|
| Moisture in masonry | +10 to +30% |
| Incomplete insulation | +20 to +50% |
| Thermal bridges (not captured) | Additional 0.05–0.15 W/m²K |
2. Different Calculation Methods
- Simplified method: Tabulated values by construction era
- Detailed calculation: Layer-by-layer build-up
- Measurement: On-site with heat flux sensors
3. Confusion of Parameters
| Symbol | Meaning |
|---|---|
| U | Thermal transmittance (component) |
| Ug | U-value of glazing |
| Uw | U-value of window (complete) |
| Uf | U-value of window frame |
| λ | Thermal conductivity (material) |
| R | Thermal resistance |
U-Value Improvement Through Insulation
How much insulation achieves which U-value?
Example: External Wall (Starting U-value 1.4 W/m²K)
| Additional insulation | New U-value | Improvement |
|---|---|---|
| 4 cm (λ = 0.035) | 0.55 W/m²K | -61% |
| 8 cm | 0.34 W/m²K | -76% |
| 12 cm | 0.25 W/m²K | -82% |
| 16 cm | 0.20 W/m²K | -86% |
| 20 cm | 0.16 W/m²K | -89% |
The law of diminishing returns: The first centimetres of insulation bring the most benefit. Going from 0 to 8 cm is more effective than from 16 to 24 cm!
The Heating Load Calculator
Our heating load calculator works with U-values:
- Component catalogue with over 150 typical constructions
- Automatic U-value estimation by construction era
- Manual input for known U-values
- Renovation proposals with improved U-values
Calculate now: Determine your building's heating load and see how U-value improvements take effect – with our heating load calculator.
Further Reading
- Transmission Heat Losses – How U-values affect heating load
- Thermal Bridges – The hidden weak points
- Renovation Recommendations – Improving U-values
- What is Heating Load? – Back to the fundamentals
Sources
- DIN EN ISO 6946 – Building components – Thermal resistance and thermal transmittance
- DIN 4108-2 – Thermal protection and energy economy in buildings
- DIN EN ISO 10077-1 – Thermal performance of windows
- GEG 2020 – German Building Energy Act
- UK Building Regulations Part L