Thermal conductivity

Thermal conductivity (λ - lambda) is an intrinsic physical property of materials that describes their ability to transmit heat. In rigorous terms, it represents the heat flow that passes, per unit of time, through a unit surface of material of unit thickness, when a temperature difference of 1 K is applied between the two opposite faces.

In the technical field, thermal conductivity is the fundamental parameter for distinguishing an insulating material from a conductor: the lower the λ value, the higher the insulating power of the material. Unlike thermal resistance (R), which depends on thickness, conductivity is a specific property of the material, independent of its geometry.

Formula and unit of measurement

In the International System, thermal conductivity is measured in W/(m·K).

The link between conductivity (λ), thickness (d), and thermal resistance (R) is expressed by the relationship:

λ = d / R

From this relationship, it can be deduced that to obtain high thermal resistance (R) values with limited thicknesses (d), it is necessary to use materials with low thermal conductivity.

Conductivity, Resistance, and Transmittance: the thermal performance system

For correct thermo-physical characterization, it is essential to understand how conductivity interacts with the other quantities that describe the building envelope or industrial components:

While λ describes the material "in its pure state," transmittance U is the value that designers use to verify compliance with legal limits (e.g., Minimum Requirements Decree).

How thermal conductivity is measured in the laboratory

The experimental determination of λ requires high-precision instrumentation and controlled environmental conditions. At Certimac, measurement is primarily performed through two standardized methodologies:

1. Guarded Hot Plate Method (GHP): considered the absolute method with high accuracy, it uses a guard ring to eliminate lateral heat dispersions, ensuring a perfectly one-dimensional heat flow. It is ideal for materials with medium-low conductivity. 
2. Heat Flow Meter Method (HFM): regulated by the EN 12667 standard, this method involves measuring the heat flow through a sample placed between two plates maintained at different temperatures.

For special applications or innovative materials subject to thermal transients, advanced techniques such as Light Flash Analysis (LFA) are used, capable of operating up to 1250 °C, or hot wire/plane methods for rapid measurements.

Declared Lambda (λD) and Design Lambda (λcalc)

In the regulatory field, it is necessary to distinguish between:

• λD (declared)
  A statistical value (90/90 fractile) that the manufacturer guarantees for at least 90% of production, with a 90% confidence level, referring to standard conditions (typically 10 °C and dry material). 
• λcalc (design)
  The value of thermal conductivity corrected according to actual operating conditions (temperature, humidity, aging), determined according to UNI EN ISO 10456.

Why thermal conductivity is a key parameter

Thermal conductivity is a parameter used in various application fields. In the construction sector, it is the basis for the CE marking of insulating materials and contributes to the determination of the energy performance of buildings. It is also used in regulatory verifications and environmental certifications for evaluating the thermal behavior of materials.

Outside of construction, conductivity finds application in the industrial field, where heat flow control is relevant for the operation of components and systems. In research and development activities, it allows for comparison between materials and supports the optimization of performance before the industrialization phase.

Practical example: the value of λ

By comparing the conductivity of common materials, the importance of technological choice is understood.

Note: the values reported are indicative and illustrative and may vary depending on the composition of the material, test conditions, and product specifications.

Reference regulatory framework

Thermal conductivity is a parameter regulated by a complex regulatory framework, constantly evolving to meet energy efficiency and sustainability needs. Key references include:

• EN 12667:2001: specifies methods for determining thermal resistance and thermal conductivity using the guarded hot plate (GHP) and heat flow meter (HFM), for materials with high thermal resistance.
  
• EN 12664:2001: specifies methods for determining thermal resistance using the guarded hot plate (GHP) and heat flow meter (HFM), applied to dry and moist products with medium and low thermal resistance.
• UNI EN ISO 10456:2008: this standard establishes procedures for determining declared and design thermal values for building materials and products. It provides rules for converting laboratory-measured values (at standard conditions) into values usable for calculating the energy performance of buildings, taking into account factors such as temperature and humidity.
  
• D.M. 28 October 2025 (New Minimum Requirements Decree): operational from 3 June 2026, this decree updates and replaces the previous D.M. 26 June 2015. It transposes the European EPBD (Energy Performance of Buildings Directive) directives and sets limit values for thermal transmittance (and consequently minimum conductivity requirements) for building envelope components of new buildings and major renovations. The goal is to improve the energy efficiency of the Italian real estate stock.
• Regulation (EU) 2024/3110 (New CPR - Construction Products Regulation): published on 18 December 2024 and fully applicable from 8 January 2026, this regulation replaces the previous (EU) 305/2011. It introduces the new Declaration of Performance and Conformity (DoPC), which not only requires the declaration of technical performance (such as λ and R) but also integrates environmental sustainability criteria throughout the product's entire life cycle (Life Cycle Assessment - LCA). This imposes greater transparency and environmental responsibility on manufacturers.

Support from Certimac

Certimac positions itself as a strategic partner for companies, offering comprehensive support that goes beyond the simple measurement of thermal conductivity.

As a Notified Body for the Construction Products Regulation (CPR) and an ISO 17025 Accredited Laboratory by ACCREDIA, we guarantee reliable test results recognized at national and European levels.

Our services include:

• Accredited laboratory tests: we perform precise measurements of thermal conductivity (λ) and thermal resistance (R) according to EN 12667 and other industry specifications, using HFM instrumentation.
• Regulatory and technical consultancy: we assist companies in interpreting the complex regulatory framework (CPR, Minimum Requirements Decrees, UNI EN ISO standards) to ensure product compliance and support the drafting of the Declaration of Performance (DoP/DoPC).
  
• Support for CE marking: we guide manufacturers through the entire CE Marking process, from initial product characterization to final documentation, ensuring access to the European market.
  
• Product optimization and development: through comparative tests and in-depth thermo-physical analyses, we help companies optimize existing formulations and develop new innovative materials with superior thermal performance, reducing R&D time and costs.

To learn more about testing methodologies, request personalized consultancy, or discover how Certimac can support your company in achieving energy efficiency and sustainability goals, visit our dedicated thermal analysis page, contact our experts directly by filling out the form at the bottom of the page, or write to materiali@certimac.it.