Why the U-value matters
Anyone insulating a house, renovating a roof or planning a heat pump sooner or later comes across a number with the unusual unit W/(m²K): the U-value. It describes how much heat flows through one square metre of a building element when it is one degree warmer inside than outside. An uninsulated brick wall with a U-value of 1.8 therefore loses around 54 watts per square metre at a 30-degree temperature difference, while a well-insulated wall with 0.2 loses only 6 watts. The difference decides heating costs, comfort and whether a heat pump can work with a low flow temperature.
At the same time, the U-value is the figure that law and subsidies are built on. In Germany, anyone renewing an external wall must meet a legal maximum, and anyone wanting a subsidy for insulation must reach an even stricter value. This guide explains how to calculate the U-value yourself, which values apply today and where the limits of your own calculation lie. You can reproduce every number with your own build-up in the U-value calculator.
The formula: from λ-value to U-value
The calculation follows the European standard EN ISO 6946. It starts with the thermal conductivity λ (lambda) of each material in W/(m·K). It tells you how well a material conducts heat: concrete is around 2.3, solid brick 0.68 to 0.81, insulation 0.024 to 0.045. The lower λ, the better the material insulates.
From λ and the layer thickness d in metres you get the thermal resistance of that layer: R = d / λ. A 14 cm insulation layer with λ 0.035 therefore has R = 0.14 / 0.035 = 4.0 m²K/W. The resistances of all layers are simply added, because heat has to pass through them one after another.
Two surface resistances are added: Rsi on the inner surface and Rse on the outer surface. They reflect the fact that the transfer from room air into the wall and from the wall into outside air also slows heat down. The U-value is the reciprocal of the total:
U = 1 / (Rsi + d₁/λ₁ + d₂/λ₂ + … + Rse)
Rsi and Rse: the direction of heat flow counts
Surface resistances depend on the direction in which heat flows. Warm air rises, so heat transfer through a ceiling with upward heat flow is easier than through a floor where heat escapes downwards. EN ISO 6946 gives these values:
| Heat flow | Typical element | Rsi [m²K/W] | Rse [m²K/W] |
|---|---|---|---|
| horizontal | external wall | 0.13 | 0.04 |
| upwards | roof, flat roof | 0.10 | 0.04 |
| downwards | floor above outside air | 0.17 | 0.04 |
If an element does not border outside air but an unheated space such as a basement or attic, the internal value is used on the outer side as well. For elements in contact with the ground, Rse is dropped; the thermal resistance of the soil is calculated under a separate standard, EN ISO 13370. The calculator applies these rules automatically as soon as you choose the element.
Worked example: old brick wall with insulation
Take a typical wall of a German house from the 1950s or 1960s: 1.5 cm interior plaster, 24 cm solid brick, 14 cm mineral wool with λ 0.035 added outside, and 1.5 cm exterior render. The calculation looks like this:
| Layer | d [m] | λ [W/(mK)] | R = d/λ [m²K/W] |
|---|---|---|---|
| Internal surface (Rsi) | 0.130 | ||
| Lime plaster inside | 0.015 | 1.0 | 0.015 |
| Solid brick | 0.24 | 0.68 | 0.353 |
| Mineral wool | 0.14 | 0.035 | 4.000 |
| Lime render outside | 0.015 | 1.0 | 0.015 |
| External surface (Rse) | 0.040 | ||
| Total | 0.41 | 4.553 |
The U-value is therefore 1 / 4.553 = 0.22 W/(m²K). For comparison, without insulation the same wall would have a total resistance of only 0.553 m²K/W and thus a U-value of about 1.81. The insulation cuts heat loss to one eighth. It is also striking how little the brick contributes: 24 cm of masonry provide less resistance than 1.5 cm of insulation.
Which values apply in 2026?
For changes to existing buildings, Annex 7 of Germany's Building Modernisation Act (Gebäudemodernisierungsgesetz) sets maximum values. Until the end of July 2026 the law was called the Building Energy Act (GEG); the U-value table did not change with the new name. For residential buildings the values include:
| Element | Legal maximum | BEG subsidy |
|---|---|---|
| External wall | 0.24 | 0.20 |
| Pitched roof, top floor ceiling | 0.24 | 0.14 |
| Flat roof (with waterproofing) | 0.20 | 0.14 |
| Basement ceiling, walls against ground | 0.30 | 0.25 |
| Windows | 1.3 | 0.95 |
All values in W/(m²K). The example wall with 0.22 therefore meets the legal requirement but narrowly misses the subsidy threshold of 0.20. About two centimetres more insulation, i.e. 16 instead of 14 cm, would be enough. The calculator shows exactly this figure as "missing insulation". Exceptions and subsidy details are covered in U-value requirements 2026.
What is a good U-value?
As a rough guide for external walls: above 1.0 is uninsulated and noticeably cold on the surface in winter. Many houses from the 1980s and 1990s lie between 0.3 and 0.5. Values around 0.2 correspond to a modern renovation, and passive house elements reach 0.10 to 0.15. For roofs a low value is usually easier to achieve because there is plenty of room for insulation between and above the rafters.
A good U-value also pays off in comfort. The better a wall insulates, the closer its inner surface is to the room temperature. In our example it stays just above 19 °C at −11 °C outside, while the uninsulated wall only reaches about 13 °C. Cold surfaces feel draughty and increase the risk of mould in corners.
Typical mistakes in your own calculation
The formula is simple, yet errors creep in quickly. The most common ones:
- Centimetres instead of metres: thickness must be entered in metres. 14 instead of 0.14 gives a resistance a hundred times too large.
- Inhomogeneous layers: if insulation sits between rafters or studs, the timber forms thermal bridges. Depending on the timber fraction, the U-value worsens by 10 to 20 percent. The standard handles such elements with upper and lower limits.
- Ventilated layers: with rain-screen facades and ventilated roofs, layers outside the air gap do not count. Roof tiles and cladding then stay out of the calculation.
- Wrong λ-value: use the design value from the product documentation, not a figure from any data sheet. Typical values are listed in the post on [thermal conductivity of materials](/en/u-value-calculator/blog/thermal-conductivity-lambda-materials).
From U-value to heat load
The U-value of a single element is one building block for the question of how much heating power a house needs overall. Multiply U-value, area and temperature difference and you get the heat loss of that element in watts. Add up all elements, thermal bridges and ventilation and you get the heat load. The second part of our calculator makes exactly this rough estimate, either by construction year or via the envelope areas. It is explained in detail in Estimating heat load.
Which outdoor temperature to use is set by the design outdoor temperature. In Germany it ranges from about −7 °C in the Rhineland to below −16 °C in the Allgäu. A table for larger cities is in the post on the design outdoor temperature.
Limits of the calculation
A U-value calculation to EN ISO 6946 for homogeneous layers is a solid tool for comparing options and checking quotes. It does not replace a formal certificate. For a subsidy application, an accredited energy efficiency expert confirms the values. Internal insulation, timber elements and flat roofs also need a moisture check so that no condensation forms inside the construction. You can roughly estimate the cost of an insulation project with the renovation cost calculator.
Conclusion
The U-value is the key figure for the thermal protection of a building element. You calculate it by dividing thickness by λ for each layer, adding everything up, including the surface resistances, and taking the reciprocal. In 2026, German law sets a maximum of 0.24 W/(m²K) for external walls, and the subsidy requires 0.20. The U-value calculator shows immediately whether your planned build-up meets both limits and how many centimetres may still be missing.