Question 1

How is the U-value calculated?

Accepted Answer

For each layer, divide the thickness in metres by the thermal conductivity λ to get the thermal resistance R = d/λ. Add all R-values plus the internal (Rsi) and external (Rse) surface resistances. The U-value is the reciprocal of that total: U = 1/(Rsi + Σ d/λ + Rse). For an external wall, EN ISO 6946 uses Rsi = 0.13 and Rse = 0.04 m²K/W.

Question 2

What U-value must an external wall have in 2026?

Accepted Answer

If you renew or insulate an external wall and more than 10% of the wall area is affected, Annex 7 of Germany's Building Modernisation Act (called "GEG" until July 2026) allows a maximum U-value of 0.24 W/(m²K). For the BEG single-measure subsidy, the technical minimum requirements demand 0.20 W/(m²K). The values apply to residential buildings.

Question 3

What is a good U-value?

Accepted Answer

The lower, the better. An uninsulated 1960s brick wall is often around 1.2 to 1.8 W/(m²K), depending on thickness. Walls insulated to today's standard reach 0.15 to 0.24, passive house elements around 0.10 to 0.15. For roofs, 0.14 or less is a good target because there is usually plenty of space for insulation.

Question 4

Why does my result differ from the manufacturer's or energy consultant's value?

Accepted Answer

This calculator uses homogeneous layers. Rafters, studs or battens within the insulation layer noticeably worsen the U-value depending on the timber fraction; EN ISO 6946 handles such elements with upper and lower limits. Professionals also use the λ-value of the actual product, corrections for fixings and, for elements in contact with the ground, EN ISO 13370.

Question 5

How much insulation do I need for the BEG subsidy?

Accepted Answer

It depends on the existing build-up. The calculator shows how many centimetres of insulation with λ 0.035 W/(mK) are still missing. Example: a 24 cm solid brick wall with plaster and 14 cm of insulation reaches about 0.22 W/(m²K); for 0.20 you need roughly 2 cm more, i.e. 16 cm. An uninsulated wall often needs 16 to 18 cm.

Question 6

What is the heat load and how does it differ from heat demand?

Accepted Answer

The heat load is the power in kilowatts your heating system must deliver on the coldest design day to keep the rooms warm. Heat demand is the amount of energy in kilowatt-hours over the whole year. The heat load determines the size of the heat pump or boiler, the heat demand determines the heating costs.

Question 7

What is the design outdoor temperature?

Accepted Answer

The design outdoor temperature is the low outdoor temperature a heating system is designed for. Since 2020, Germany uses DIN/TS 12831-1 with values per postcode. It is, for example, −7.0 °C in Cologne, −11.1 °C in Berlin and −16.7 °C in Oberstdorf. It is not the coldest temperature ever recorded but a statistical design value.

Question 8

Is the estimated heat load enough to size a heat pump?

Accepted Answer

No. The estimate shows the order of magnitude and helps you sanity-check quotes. Sizing, hydronic balancing and, as a rule, the subsidy require a room-by-room heat load calculation to EN 12831-1. It accounts for every room, every thermal bridge and the actual ventilation.

Guide: U-Value Calculator

Calculating the U-Value: Formula, Worked Example and What Counts as Good

U-Value Requirements 2026: What German Law Demands and What the Subsidy Requires

Thermal Conductivity λ: Building and Insulation Materials Compared

Estimating Heat Load: Rule of Thumb, Envelope Method and EN 12831

Design Outdoor Temperature: Values for German Cities and What They Mean

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