R-value vs U-value

R-value describes the thermal resistance of one layer or a full set of layers: the higher it is, the more the assembly resists heat flow. U-value describes heat transfer through the complete assembly: the lower it is, the better the final insulation performance. They are connected, but they are not interchangeable.

The simple relationship is R = d / lambda for a layer and U = 1 / total R after surface resistances and all layers are included. Adding insulation raises R and lowers U. In real assemblies the improvement is not endlessly linear because other layers and thermal bridges become more significant.

For example, 100 mm of a material with lambda 0.040 W/mK gives about R 2.50 m²K/W. The U-value of a wall, roof, or floor will not simply be 1 / 2.50 if other layers and surface resistances are present.

A common mistake is comparing the R-value of a single layer with the U-value of a complete building element or ignoring the units. R helps judge the contribution of a material; U helps check the final assembly against a requirement. A practical decision needs both views.

After comparing materials, move to the U-value calculator and test the complete assembly. The comparison table shows material differences, but the full build-up reveals the effect of render, cladding, air spaces, and surface resistances.

If two results are close, do not choose from one number alone. Check moisture behaviour, fire performance, available board formats, system requirements, and whether the layer can be installed continuously without open joints.

For renovation work, also check substrate condition, condensation risk, and details around windows, ring beams, and balconies. A strong lambda value will not fix a layer interrupted at the most important junctions.

The final step should be comparison with the current manufacturer data sheet. The database helps narrow the direction, but the documentation for the exact product matters most for ordering and construction.

In practice, make a short decision table: thermal result, installation difficulty, moisture behaviour, fire behaviour, acoustics, and system availability. That reduces the risk of choosing a material only because one number looks best.

If the comparison is used in a supplier discussion, record the data-check date and assumed thickness. It becomes easier to separate a price or product change from a real change in thermal performance.

A useful comparison includes at least three variants: current state, economical option, and option with margin. That shows whether extra thickness still delivers a meaningful improvement.

If the comparison is tied to legal requirements, do not rely on an internet average. Use the local threshold, the correct calculation method, and data for the product that will actually be installed.