Home generalCalculate U-value - Definition + table for windows / doors and walls

Calculate U-value - Definition + table for windows / doors and walls

  • What is the U-value "> Calculate the insulation value
    • example
  • Use calculator correctly
  • Weak point doors and windows
    • frame
    • front door

Germany is in twilight. The state is giving considerable incentives to make homeowners taste for the thermal renovation of their homes. However, many people wanting to relax want to shoot beyond the goal: the thermal insulation of facades and roofs has physical limitations. These can not be overcome even with so much insulating material. Therefore, an exact knowledge of the U-value is indispensable. Find out in this text everything you need to know about calculating insulation materials for your home.

What is the U-value?

The U value is the "heat transfer coefficient". It is the reciprocal of "lambda" - the thermal conductivity. Both are values ​​that are specific to a specific material. Roughly speaking, one can assume the following limits:

  • Anything that has a lambda value of 0.004 to 0.07 W / (m²K) is considered an "insulating material"
  • Anything that has a lambda value of 0.1 to 2.3 W / (m²K) is considered "building material"
temperature gradient

Would you like to learn more about Inner wall insulation "> Interior wall insulation

From Lambda and the thickness of the component and the direction of the heat flow can first determine the R value. The R value is the "heat transfer resistance". It is calculated by dividing the thickness of a building material or insulating material by its physico-technically determined lambda value. The formula for the R value is:

  • R = d / λ (lambda)
  • The unit for R is according to [m²K / W]

U is now purely mathematically the reciprocal of R. He does not indicate the heat transfer resistance, but makes an indication of how well a component passes heat through it. The value is also called "heat transfer value" or "insulation value". Its unit of measure is W / (m²K). W stands for "watt" and denotes a radiated energy. m² means square meters and refers to the illuminated area. K is a temperature unit, here Kelvin. The unit of measurement Kelvin was chosen because of its superior precision over the Celsius.

What sounds so awfully complicated is quite simple to use: the lower the heat transfer value, the better the insulating properties of the material and vice versa.


A solid brick has a heat transfer value of approx. 1.5 W / (m²K). This corresponds to a hard foam plate made of polystyrene of about two centimeters.

Calculate the insulation value

First of all, it is not possible to calculate the insulation value of a material or component yourself with home remedies. One must rely in this point on the information provided by the manufacturer. However, as the producers of building materials have to bring certificates of their products and the test methods used, the information on the packaging is quite believable.

The somewhat complicated introduction to the insulation value calculation becomes clearer if one considers the heat flow. This is part of the calculation of a ETICS. The associated R value is assumed as follows:

Direction in which the heat flow flows

  • upwards: 0.10 (m²K / W)
  • horizontal: 0.13 (m²K / W)
  • downwards: 0.17 (m²K / W)

In addition, a Rse value of 0.04 (m²K / W) is assumed.

But what you can do is to find the right insulating material for your application based on the manufacturer's information. The formula for calculating a composite thermal insulation system is:

U = (Rsi + Rse + 1 / λ1 + 1 / λ2 + 1 / λ3 ...) ^ - 1

Now you only need to know the thickness and the characteristic values ​​of the individual components, you can already calculate the U-value.


A building with an exterior wall has the following structure:

  • Inside: Normal lime-cement plaster with a thickness of 1.5 cm and a λ of 0.7
  • Load-bearing outer wall: perforated bricks with a thickness of 37.5 cm and a λ of 0.25
  • Exterior: Insulating plaster with polystyrene addition and a thickness of 3 cm and a λ of 0.13

The wall therefore has a total heat transfer resistance of
R ges = 0.13 (Rsi) + 0.04 (Rse) + 0.015m / 0.7 + 0.375m / 0.25 + 0.03m / 0.13 = 1.92 m²K / W

The inverse of this gives U = 0.52 W / (m²K)

Even if the calculation may be a bit difficult, this formula makes one thing very clear: not much helps much. It does not help to apply metering-thick insulation material to a wall. The damping effect tends towards zero after a short distance. No matter how much insulation is applied in addition, not a single additional Watt of energy is saved.

Follow if you do not calculate the U-value

The exact calculation of the U value has another great advantage: It prevents you from laying the dew point in the wall by using the wrong combination of insulation materials. That would have fatal consequences: the insulation would be permanently wet. This not only lowers the whole insulation effect. It also promotes the formation of mold on the facade and in the home.

Use calculator correctly

Although the online calculators do nothing other than apply the named formula, the use of these tools also has its advantages. You only have to enter a "U-value calculator" in a search engine, you will already get numerous suggestions. In the calculator, only the thickness and the characteristic value of the material are given, already you get a correct result delivered. If you love the challenge, you can also program such a computer yourself with the help of an Excel spreadsheet. It is only important to get the correct lambda values. In the following table we have put together the most common construction and insulation materials for you.

λ - values ​​in (W / mk)
  • Gypsum plaster without surcharge 0, 35
  • Lime plaster plaster 0, 70 - 0, 87
  • Cement plaster 1.4
  • Mineral light plaster 0, 31
Carrying wall
  • Normal concrete according to DIN 1045 2.1
  • Lightweight concrete according to DIN 4219, 80 kg / m³ 0, 390 - 1, 60
  • Exterior wall Lightweight brickwork 0, 160 - 0, 27
  • High-hole brickwork 0.330 - 0.45
  • Solid brick masonry 0.5 - 0.81
  • Weatherproof facing bricks 0.68 - 0.96
  • Sand-lime brick masonry 0.7 - 1.1
  • Aerated concrete masonry 0.24 - 0.29
insulation materials
  • Styrofoam and Styrodur 0, 025 - 0, 04
  • Glass wool or rock wool 0, 035 - 0, 045

Weak point doors and windows

Of course, the openings in a facade are always a challenge in terms of insulation technology. They interrupt the ETICS and are therefore always a potential point where the expensive heat can escape to the outside. However, a great deal of research was done on these components, so that doors and windows can in places exceed the insulation effect of facades.

In contrast to insulating materials and building materials, windows and doors usually have the same U-value.

See the following table for typical values ​​of windows and window frames available today:

  • Insulating glass, double, 24 mm wide gap, argon filling: 1, 1 W / (m²K)
  • Insulating glass, double, 36 mm wide gap, argon filling: 0.7 W / (m²K)
  • Insulating glass, triple, 44 mm wide gap, argon filling: 0.6 W / (m²K)
  • Insulating glass, triple, 36 mm wide gap, argon filling: 0.5 W / (m²K)

From this one can see that a good window in the above example does not cause any interruption of the insulating surface. The U-value of wall and window is the same. This also reliably prevents the formation of dew points. However, the U-value of 0.5 W / (m² × K) indicated in the table is currently at the top of a technically feasible window. By comparison, a single-glazed window has a U-value of fabulous 5.5 W / (m² × K). From a thermal point of view, such a bad value hardly makes any difference whether the window is kept open or closed.


The window frames are designed today to match the window. Aluminum window frames also have an interesting U-value thanks to a clever system of hollow chambers. Plastic windows are quite good value for money. however, they should be of high quality. Cheap windows warp easily. If the window always remains ajar at the side, the best value from the table is of no use. The heat escapes inevitably to the outside.

front door

House doors are to close a building safely to the street. That is why the front doors are usually quite large, heavy and massive. Thermally, the high use of metal is not optimal. But there are certainly manufacturers who have managed to unite massiveness, security and insulation in the front door. The leader is currently a product from Steinhagen: A front door with a U-value of 0.45 and in addition a resistance value against burglary of RC4. This is the second best classification and is only surpassed by steel doors.

Use calculator and table in a clever way

One sees: The employment of a computer and table and is worthwhile in each case. With detective instinct and the desire to save, you can achieve a lot in the course of a planning of the new building or energetic refurbishment. After all, one thing must be kept in mind: good and sustainable insulation is much more than the use of expensive and complicated heating technology.

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