U-value and insulation: formulas and GEG requirements
From λ and d to the U-value – and how much insulation is enough
The U-value (thermal transmittance) is the key metric for the thermal insulation quality of a building component. It describes how much heat energy passes through 1 m² of area per second at a temperature difference of 1 Kelvin. The calculation per DIN EN ISO 6946: U = 1 / (Rsi + d/λ + Rse), where λ is the thermal conductivity of the insulation material in W/(m·K), d is the thickness in metres and Rsi/Rse are the surface resistances (external wall: 0.13 and 0.04 m²K/W). For 10 cm of insulation with λ = 0.035: R = 0.10/0.035 = 2.86 → U = 1/(0.13 + 2.86 + 0.04) ≈ 0.33 W/(m²·K).
GEG 2024 and funding standards: Germany's Buildings Energy Act (GEG) requires U ≤ 0.24 W/(m²·K) for external walls when altering external components (renovation, Annex 7 GEG). New builds have no fixed per-component limits – compliance is verified via the reference building and primary energy demand. The KfW efficiency house funding standard usually requires U ≤ 0.20 W/(m²·K) for external walls; passive houses achieve U ≤ 0.15 W/(m²·K) or better. These values are readily achievable with 16–18 cm of EPS or mineral wool.
Thermal resistance R: The R-value is the reciprocal of the U-value: R = 1/U. It is additive: in multi-layer components, the individual resistances add up. A complete component build-up (e.g. an external wall) consists of the internal surface resistance (Rsi = 0.13 m²K/W), all material layers (render, masonry, insulation), and the external surface resistance (Rse = 0.04 m²K/W). This calculator includes the insulation layer plus surface resistances; optionally, the U-value of the existing component can be included (R supplement 1/U). For formal GEG verification, consult an energy consultant.
Cost-effectiveness of insulation: The payback period for insulation measures is typically 8–20 years, depending on energy prices, the existing condition of the building and the insulation thickness. Rule of thumb: doubling the insulation thickness halves the U-value, but saves proportionally less energy (diminishing returns). So the question is not "as much as possible" but "what is the economically optimal insulation thickness" – usually around 14–20 cm for mineral wool and 12–16 cm for PUR.