The U-Value Explained - Summary
The U-value (formerly k-value) is the most important characteristic for evaluating the thermal insulation quality of a building component. It indicates how much heat per second flows through one square meter of component surface when the temperature difference is one Kelvin.
What Does the U-Value Mean?
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 meter 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 | Passive house wall |
| 0.2-0.3 W/m²K | Good | New construction standard |
| 0.3-0.5 W/m²K | Medium | Renovated old building |
| 0.5-1.0 W/m²K | Moderate | Partially insulated |
| > 1.0 W/m²K | Poor | Uninsulated old building |
How is the U-Value Calculated?
The U-value is calculated from the thermal resistances of all layers: U = 1 / RT