How to read U and lambda when choosing a mineral wool wall?

In the calculation example, 0.10 m of material with λ = 0.040 W/(mK) gives R = 2.5 m²K/W. Doubling the thickness gives R = 5. These are assumed data to show the method. For ordering, use the properties declared for the selected product, not any value associated with mineral wool.

The core of the panel is part of a composite product. Therefore, when selecting a finished panel, its declared thermal transmittance in a specific configuration is the more direct information.

Pay attention to the units in the table. λ has the unit W/(mK), and U — W/(m²K). The resistance R is given in m²K/W. Swapping these designations can lead to an incorrect order, even though all the numbers used look credible.

In an example family of wall panels with mineral wool, the Uc values are approximately 0.49 W/(m²K) for 80 mm, 0.39 for 100 mm and 0.27 for 150 mm. The 200 mm variant has approximately 0.20 W/(m²K). This is a data set for a specific family. It should not be transferred to all products with a mineral core.

A lower value means less heat flow through the element under consideration at the same temperature difference. However, changing the thickness also affects weight, fixing lengths and flashings. Thermal selection should therefore take place before ordering the substructure and the details at windows.

Check whether the compared data refer to the same calculation method and assembly. Do not set the coefficient of the panel alone against the result for the entire wall of another building as if they were identical quantities.

For a simplified comparison, the relationship Q = U × A × temperature difference can be used. With an area of 80 m² and a difference of 20 K, a panel with U = 0.39 gives 624 W of heat flow. For U = 0.27, the result is 432 W. The difference under these conditions is 192 W.

This comparison covers two idealised wall areas. It does not include ventilation, door openings, thermal bridges or weather variations. It cannot be converted into annual monetary savings without additional data. Such an assessment requires heating operating hours, climatic conditions and the way the building is used.

The calculation does, however, help to show why the same material carries different weight in the balance of a small room and a large hall. Both the U-value and the area of the building envelope, as well as the temperature difference, matter.

The junction with the plinth, the window frame and the connection to the roof can create additional heat flow paths. In these places the geometry does not resemble a simple, flat panel area. An assessment of the details is needed, especially where there is a risk of low surface temperature on the inside.

Air leakage is another problem. Moist, warm air can penetrate through discontinuous joints. This phenomenon is not described by the lambda of the core alone. The choice of thickness must therefore go together with correct installation and a sealing solution.

When extending a hall, the junction between the old and new envelope must be checked in particular. Increasing the thickness of the new panels does not automatically eliminate a weak point at the connection between the two parts.

First define the target interior conditions and the project requirements. Then select a panel that meets them in the specific assembly. If, for example, a target wall U-value at a certain level has been assumed, the contribution of the details must be checked. It is not enough to choose a product with an identical figure in the Uc table.

In the enquiry, state the thickness, the parameter family, the facings and the lengths. Do not mix the properties of two variants. A lighter core may have different insulation performance, but also different mechanical data. The set should correspond to the solution actually available.

Finally, keep the selection result and the panel documents. If the supplier proposes a replacement, compare the same items. This avoids a situation in which a seemingly equivalent thickness changes the thermal balance of the entire investment.