Germany is in twilight fever. The state is giving stately incentives to make thermal renovation of their houses palatable to homeowners. However, many people who want to insulate want to overshoot the mark: there are physical limits to the thermal insulation of facades and roofs. These cannot be overcome, no matter how much insulation material. That is why precise knowledge of the U-value is essential. In this text, you will learn everything you need to know about calculating the insulation materials for your house.
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 apply specifically to a very specific material. Roughly speaking, the following limit values can be assumed:
- Everything that has a lambda value of 0.004 to 0.07 W/(m²K) is considered “insulating material”.
- Everything that has a lambda value of 0.1 to 2.3 W/(m²K) is considered a “building material”.
Would you like to learn more about interior wall insulation? Here you can find all the important information: Interior wall insulation
The R-value can first be determined from the lambda and the thickness of the component as well as the direction of the heat flow. The R-value is the “thermal resistance”. It is calculated by dividing the thickness of building material or insulating material by its physically and technically determined lambda value. The formula for the R-value is accordingly:
- R = d/ λ (lambda)
- The unit for R is accordingly [m²K/W]
U is now purely mathematically speaking the reciprocal value of R. It does not indicate the thermal resistance, but gives an indication of how well a component conducts heat. The value is also called “heat transfer value” or “insulation value”. Its unit of measurement is W/(m²K). W stands for “Watt” and denotes irradiated energy. m² means square meter and refers to the illuminated area. K is a unit of temperature, here Kelvin. The Kelvin unit of measurement was chosen because of its superior precision to Celsius.
What sounds so terribly complicated is actually very easy 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 sheet made of polystyrene of around two centimeters.
Calculate the insulation value
First of all: It is not possible to calculate the insulation value of a material or a component yourself using home remedies. You have to rely on the manufacturer’s information on this point. However, since the producers of building materials have to provide certificates for their products and the test methods used, the information on the packaging is quite credible.
The somewhat complicated entry into the insulation value calculation becomes clearer when the heat flow is taken into account. This is part of the calculation of an ETICS. The associated R-value is assumed as follows:
The direction in which the heat flow is flowing
- upwards: 0.10 (m²K/W)
- horizontal: 0.13 (m²K/W)
- downwards: 0.17 (m²K/W)
In addition, an Rse value of 0.04 (m²K/W) is assumed.
But what you can do is find the right insulating material for your application based on the information provided by the manufacturer. The formula for calculating a thermal insulation composite 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 before you can 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 brick with a thickness of 37.5 cm and a λ of 0.25
- Outside: insulating plaster with polystyrene additive and a thickness of 3 cm and a λ of 0.13
The wall, therefore, has a total thermal resistance of
R tot = 0.13 (Rsi) + 0.04 (Rse) + 0.015m/0.7 + 0.375m/0.25 + 0.03m/0.13 = 1.92 m²K/W
The reciprocal of this gives U = 0.52 W/(m²K)
Even if the calculation may be a bit difficult, this formula makes one thing very clear: a lot doesn’t help much. It’s no use laying meters of insulating material on a wall. The insulating effect tends towards zero after a short distance. No matter how much additional insulation is then added, not a single additional watt of energy is saved.
Consequences if you don’t calculate the U-value
The exact calculation of the U-value has another major advantage: it prevents the dew point from being moved into the wall due to an incorrect combination of insulation materials. That would have fatal consequences: the insulation would become permanently damp. This not only lowers the entire insulating effect. It also promotes the formation of mould, both on the facade and in the apartment.
use the calculator correctly
Although the online calculators also do nothing but apply the named formula, using these tools also has its advantages. You only need to enter “U-value calculator” in a search engine and you will get numerous suggestions. Only the thickness and the characteristic value of the material are given in the calculator, and you get a correct result. If you love a challenge, you can easily program such a calculator yourself with the help of an Excel spreadsheet. It is only important to get the correct lambda values. We have compiled the most common building and insulation materials for you in the table below.
|λ – values in (W/mk)|
|plasterGypsum plaster without supplement 0.35Lime gypsum plaster 0.70 – 0.87Cement plaster 1.4Mineral light plaster 0.31|
|load-bearing wallNormal concrete according to DIN 1045 2.1Lightweight concrete according to DIN 4219, 80 kg/m³ 0.390 – 1.60Exterior wall lightweight brickwork 0.160 – 0.27Perforated brick masonry 0.330 – 0.45Solid brick masonry 0.5 – 0.81Weatherproof facing clinker 0.68 – 0.96Sand-lime solid brick masonry 0.7 – 1.1Aerated concrete masonry 0.24 – 0.29|
|insulating materialsStyrofoam and Styrodur 0.025 – 0.04Glass wool or rock wool 0.035 – 0.045|
Weak point doors and windows
The openings in a façade are of course always a challenge in terms of insulation. They interrupt the ETICS and are therefore always a potential point at which the expensive heat can escape to the outside. However, a lot of research has been done on these building elements, so that doors and windows can sometimes surpass the insulating effect of facades.
In contrast to insulating and building materials, the U-value is usually given for windows and doors.
See the table below for typical values of windows and window frames available today:
- Insulating glass, double, 24 mm gap, argon filling: 1.1 W/(m²K)
- Insulating glass, double, 36 millimeter wide gap, argon filling: 0.7 W/(m²K)
- Insulating glass, triple, 44 millimeter wide gap, argon filling: 0.6 W/(m²K)
- Insulating glass, triple, 36 millimeter wide gap, argon filling: 0.5 W/(m²K)
Here you can see that a good window in the above example does not cause any interruption of the insulating surface. The U-value of the 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) given in the table is currently the top of what is technically feasible for a window. For comparison: A single-glazed window has a U-value of an incredible 5.5 W/(m²×K). From a thermal point of view, such a poor value hardly makes any difference whether you keep the window open or closed.
The window frames are now designed to match the window. Aluminum window frames also have an interesting U-value thanks to a sophisticated 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 a little open at the side, the best value from the table is of no use. The heat inevitably escapes to the outside.
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Front doors should securely close a building to the street. That is why the entrance doors are usually quite large, heavy and solid. From a thermal point of view, the high use of metal is not ideal. But there are definitely manufacturers who have managed to combine solidity, 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 an additional resistance value against the burglary of RC4. This is the second-best classification and is only surpassed by steel doors.
Use calculators and spreadsheets cleverly
You can see: The use of a calculator and table is worthwhile in any case. With a detective’s instincts and the desire to save, you can achieve a lot in the course of planning a new building or energy-efficient renovation. Because you have to be clear about one thing: Good and sustainable insulation achieves much more than the use of expensive and complicated heating technology.