R-value: definition and calculation

What is thermal resistance?

Thermal resistance, denoted as R, measures a material layer's ability to slow down heat transfer. It is expressed in m².K/W. The higher the value, the better the insulation.

It is calculated simply: thickness in meters divided by the material's lambda value. Ten centimeters of insulation with a lambda of 0.025 W/m.K therefore results in an R-value of 4 m².K/W.

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What is the difference between the R-value and the U-value?

R describes a single layer, while U describes the entire wall assembly. The resistances of all layers are added together, and the U-value is the inverse of this total. R is viewed as a gain to be maximized; U is viewed as a loss to be minimized.

The lambda value, on the other hand, depends only on the material. See our guide thermal conductivity : it is the thickness that turns a lambda value into thermal resistance.

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Why does the R-value matter for grants?

Regional floor insulation grants often base their eligibility criteria on the thermal resistance of the installed insulation. The Rd-value is required, where the "d" stands for the declared value—the value certified by the manufacturer for a specific thickness.

This is why the insulation's technical data sheet and the actual thickness installed must appear on the invoice: without them, the resistance cannot be verified. Details by region can be found in our article on floor insulation grants.

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How Davide Chape documents the R-value

During installation, the R-value depends on the thickness achieved at every point, not the average thickness. For sprayed PUR as well as forEPSpanels, we record the installed thickness and provide the product data sheet. See our floor insulation solutions.