Thermal resistance is a physical quantity that expresses the ability of a material, or a component, to oppose the passage of heat by conduction in a steady state.
More intuitively, it represents the “brake” that a layer of material opposes to the heat flow between two surfaces at different temperatures (one hot and one cold): as the R-value increases, for the same temperature difference, the heat flow through it decreases.
It is one of the fundamental parameters in the thermo-physical characterization of building materials and building envelope components. Its determination is the basis of numerous applications, including CE marking, verification of building energy requirements, and material qualification within sustainability protocols.
For a homogeneous layer, thermal resistance is calculated as:
R = d / λ
where:
d = layer thickness [m]
λ = thermal conductivity of the material [W/(m·K)]
The unit of measurement for thermal resistance in the International System is m²K/W.
From the formula, the inverse relationship with thermal conductivity (λ) clearly emerges: a material with a low λ value - typically an insulator - will offer high thermal resistance even with limited thicknesses. Conversely, highly conductive materials such as steel or aluminum have very low R values.
Thermal resistance is part of a system of three interconnected quantities that describe the thermal behavior of a material or a building element. It is essential to distinguish them, as they are often confused:
For a multi-layer structure, the total thermal resistance is the sum of the resistances of the individual layers, to which are added the internal (Rₛᴵ) and external (Rₛᵉ) surface resistances, which account for convective and radiative exchanges between the surface and the environment.
Thermal transmittance U is defined as the reciprocal of the total thermal resistance (Rₜ) and represents the reference parameter in regulatory contexts for describing the thermal performance of walls, roofs, floors, and windows. Knowing R therefore allows determining U, and vice versa.
The experimental determination of thermal resistance is carried out using standardized methods at European and international levels. The most common method for insulating products and building materials is the guarded hot plate method and the heat flow meter (HFM) method, regulated by the standard:
EN 12667:2001
Thermal performance of building materials and products - Determination of thermal resistance by means of guarded hot plate and heat flow meter methods - Products of high and medium thermal resistance
In this method, the sample is placed between two temperature-controlled plates. A heat flux meter measures the heat flux density (q) passing through the sample under steady-state conditions, while temperature sensors detect the temperature difference (ΔT) between the two faces of the sample.
Thermal resistance is calculated as:
R = ΔT / q
where:
q is the heat flux density [W/m²]
ΔT is the temperature difference between the faces [K]
The guard system surrounding the measurement area aims to reduce lateral heat losses, ensuring one-dimensional heat flow conditions and an accurate measurement of thermal resistance. The test conditions (temperature, duration, and stabilization criteria) are defined by the standard according to the type of material analyzed.
For materials with very high thermal resistance (e.g., high-performance insulators, such as PIR/PUR panels), very low resistance, or subject to significant thermal gradients, alternative methods or complementary standards are used, including:
EN ISO 8990 (climatic chamber method)
EN ISO 22007 (transient methods on small samples)
Thermal resistance is a central parameter in numerous application contexts. The main ones include:
+ Insulating materials for construction
Thermal resistance R is a mandatory parameter declared in the CE marking of insulating products (e.g., rock wool, expanded polystyrene, polyurethane, wood fiber). The manufacturer provides the declared value of thermal conductivity (λ) and thickness, from which R is derived, in accordance with product standards of the EN 13162–EN 13171 series.
+ Verification of minimum legal requirements
Italian D.M. 26 June 2015 (Minimum Requirements Decree), updated by D.M. 28 October 2025 (effective from 3 June 2026), sets the limit values for thermal transmittance U – and therefore the minimum R values – for building envelope components in new buildings and renovations. The experimental measurement of the R of the materials used is the tool to demonstrate compliance.
+ Qualification of products for environmental certifications
Protocols such as LEED, BREEAM, and ITACA require documentation of the thermal performance of the materials used, including R values certified by an accredited laboratory.
+ Industrial and plant engineering sector
Thermal resistance is also relevant in the qualification of gaskets, refractory materials, components for high or low-temperature systems, materials for automotive and aerospace, where heat flow control is critical for system performance.
+ R&D and development of new materials
In the development phase of a new product, the experimental measurement of R allows comparing different formulations, validating predictive models, and optimizing the composition or structure of the material before industrialization.
To highlight the impact of thermal resistance, three materials commonly used in construction are considered, with the same thickness (10 cm).
Note: the values reported are purely illustrative and represent typical indicative data. Actual performance may vary depending on the specific characteristics of the material, test conditions, and manufacturer's declarations.
For the same thickness, EPS has a thermal resistance approximately 9 times higher than that of perforated brick and up to about 70 times higher than reinforced concrete. This highlights the decisive role of thermal insulation in the energy performance of buildings.
The measurement of thermal resistance is part of a complex regulatory system:
Certimac laboratory supports companies both in testing activities and in application phases, such as CE marking and the drafting of the Declaration of Performance (DoP/DoPC), as well as in the development and optimization of new products through comparative tests and thermo-physical analyses.
The activities cover a wide range of materials, from traditional insulating products to innovative and composite materials, with the possibility of performing tests on both laboratory samples and samples taken on-site.
Measurements are carried out in accordance with reference technical standards (including EN 12667 and applicable product standards), using calibrated instrumentation and operating according to ISO 17025 accreditation requirements. This ensures reliable, reproducible, and nationally and European recognized results.
The outcome of the tests is a Test Report, which can be used for:
To learn more about thermal analysis services and evaluate the most suitable solutions for your company, you can consult the dedicated thermal analysis page or contact the Certimac laboratory directly by filling out the form at the bottom of the page or writing to materiali@certimac.it.
