Calculating U-Values: Insulation Rules Under Germany's GModG
What the U-value means, how to calculate it and which maximum values Germany's Building Modernisation Act (formerly GEG) sets for renovations and retrofits.
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Anyone who insulates a facade, re-roofs a house or replaces windows in Germany has to meet a maximum value for heat loss. The yardstick is the U-value. This guide explains how it is calculated, which values the law prescribes and when owners must insulate even without renovating.
Key takeaways
- The U-value states how much heat passes through one square metre of a component per degree of temperature difference – the lower, the better.
- Germany's Building Energy Act was renamed the Building Modernisation Act (GModG) on 29 July 2026; the maximum values in Annex 7 stayed the same.
- When an exterior wall or a roof is renewed, the limit is usually U ≤ 0.24 W/(m²·K); for a flat roof with a waterproofing membrane it is U ≤ 0.20.
- If the change affects no more than 10 percent of the area of a component group, the maximum values do not apply (Section 36 GModG).
- Uninsulated top floor ceilings must be retrofitted (Section 35 GModG), with exceptions for owner-occupied houses with one or two dwellings.
What the U-value tells you
The thermal transmittance U is given in W/(m²·K). A U-value of 0.24 means that 0.24 watts of heat flow through one square metre of wall per degree of difference between inside and outside. It is the reciprocal of the total thermal resistance – every layer with thickness d and thermal conductivity λ adds d/λ.
U = 1 / (Rsi + d₁/λ₁ + d₂/λ₂ + … + Rse)
Rsi and Rse are the surface resistances on the inside and outside. Under DIN EN ISO 6946, which Section 37 GModG refers to for opaque components, an exterior wall uses Rsi = 0.13, a roof or ceiling (heat flowing upwards) uses Rsi = 0.10, and the outside uses Rse = 0.04 m²·K/W in both cases.
The existing structure counts
Under Section 37(1) GModG, the U-value of a modified component is calculated from the new layers together with the existing ones. An old brick wall may therefore be included – its U-value is often stated in the energy performance certificate or the building documents.
Maximum values for renovations (Annex 7)
Whoever renews, replaces or installs for the first time exterior components of heated rooms must meet the maximum values of Annex 7 (Section 36 sentence 1 GModG). For residential buildings:
| Component | No. in Annex 7 | Maximum |
|---|---|---|
| Exterior wall (replacement, insulation, new render) | 1a, 1b | U = 0.24 W/(m²·K) |
| Pitched roof, top floor ceiling, knee wall | 5a, 5b | U = 0.24 W/(m²·K) |
| Roof with waterproofing membrane (flat roof) | 5c | U = 0.20 W/(m²·K) |
| Basement ceiling, wall against soil or unheated rooms | 6a, 6b | U = 0.30 W/(m²·K) |
| Windows and French windows | 2a | Uw = 1.30 W/(m²·K) |
| Roof windows | 2b | Uw = 1.40 W/(m²·K) |
| New exterior door | 4 | U = 1.80 W/(m²·K) |
De minimis rule: if the change affects no more than 10 percent of the total area of the component group, the maximum value does not have to be met (Section 36 sentence 2 GModG).
If the insulation thickness is limited for technical reasons – between rafters, for example – the greatest possible thickness is sufficient, using an insulation material with λ no higher than 0.035 W/(m·K); for blown-in or renewable materials λ = 0.045 W/(m·K) (Annex 7, footnote 1).
Mandatory insulation of the top floor ceiling
Regardless of any renovation, Section 35 GModG requires top floor ceilings above heated rooms that do not meet the minimum thermal protection of DIN 4108-2 to be insulated to U ≤ 0.24 W/(m²·K). Alternatively, the roof above may be insulated. The obligation applies to residential buildings and to non-residential buildings heated to at least 19 °C for at least four months a year.
Exception for owner-occupiers
In a house with no more than two dwellings whose owner lived there on 1 February 2002, only the next owner has to retrofit – within two years of the change of ownership (Section 35(3)). If the insulation of an owner-occupied one- or two-family house cannot pay for itself within a reasonable period, the obligation lapses (Section 35(4)).
How thick does the insulation need to be?
The table shows which insulation thickness alone – without counting the load-bearing layer – reaches the maximum value. This is the safe upper bound; with an existing wall it will be less.
| Component | Maximum | Thickness at λ = 0.035 | Thickness at λ = 0.045 |
|---|---|---|---|
| Exterior wall (replacement, insulation, new render) | 0.24 | 14 cm | 18 cm |
| Pitched roof, top floor ceiling, knee wall | 0.24 | 14.1 cm | 18.1 cm |
| Roof with waterproofing membrane (flat roof) | 0.20 | 17 cm | 21.9 cm |
| Basement ceiling, wall against soil or unheated rooms | 0.30 | 10.5 cm | 13.5 cm |
Example: according to its energy certificate, an exterior wall has U = 1.4 W/(m²·K). With insulation of λ = 0.035, it needs 12.1 cm of additional insulation to reach U ≤ 0.24. Boards come in fixed thicknesses, so choose the next size up.
Calculate insulation thickness and U-value
Enter component, insulation material and existing structure – the calculator shows the U-value and whether the maximum is met.
Estimate heating costs
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Frequently asked questions
Does the GEG still apply?
Which U-value applies to an exterior wall during renovation?
Do I have to insulate my roof?
What does λ (lambda) on the insulation board mean?
Sources
Every figure in this article comes from these primary sources or from the linked calculators, which are based on the same sources.
- Section 35 GModG – retrofitting an existing building (accessed September 30, 2026)
- Section 36 GModG – requirements when components are changed (accessed September 30, 2026)
- Section 37 GModG – calculating the U-value (accessed September 30, 2026)
- Annex 7 GModG – maximum U-values for renovations (accessed September 30, 2026)
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