What is thermal conductivity?

General

Understanding thermal conductivity is important in many industries, including construction, manufacturing, and engineering. It helps determine how materials transfer heat, which can affect product performance, energy efficiency, and safety.

In our blog, we’ll explain what thermal conductivity means, its significance, how it is measured, and examples of materials with high and low thermal conductivity.

What is thermal conductivity?

Thermal conductivity refers to a material’s ability to conduct heat. It tells us how quickly heat moves through a material when there is a temperature difference on either side. This is important because it influences how materials respond to heat, impacting insulation, heat sinks, and many other applications.

Thermal conductivity is usually represented by the symbol k or λ and measured in watts per meter kelvin (W/m·K). A higher value indicates that heat passes through the material quickly, while a lower value means the material resists heat flow.

Heat transfer basics

Heat moves in three main ways: conduction, convection, and radiation. Thermal conductivity relates to conduction, the direct transfer of heat through a material without the material itself moving.

Conduction occurs when particles in a material vibrate and pass energy to adjacent particles.
Materials with tightly packed atoms usually have higher thermal conductivity because vibrations transfer energy easily.
Materials with loosely packed atoms or air pockets tend to have low thermal conductivity since they block heat flow.

Understanding conduction and thermal conductivity helps engineers choose materials for insulation or heat dissipation.

High thermal conductivity meaning

When we say a material has high thermal conductivity, it means heat moves through it rapidly. Such materials are often metals like copper or aluminium. High thermal conductivity is desirable in applications needing efficient heat transfer. For example, heat sinks in electronics rely on materials with high thermal conductivity to pull heat away from components and prevent overheating.

Why does thermal conductivity vary?

Thermal conductivity depends on several factors:

Material composition: Metals generally have free electrons that carry heat quickly, while non-metals rely on lattice vibrations, which are tiny movements of atoms in a solid that help carry heat through the material.
Temperature: Thermal conductivity can change as temperature rises or falls. For example, with metals, the conductivity decreases as temperature increases because atomic vibrations scatter free electrons.
Material structure: Crystalline materials conduct heat differently from amorphous (glass, plastic, rubber) or porous materials (sponge, foam, some types of concrete like lightweight, aerated and pervious).
Moisture content: Wet materials like damp wood or wet concrete often conduct heat better than dry ones because water transfers heat efficiently.

Examples of materials with high thermal conductivity

Here are some common materials with high thermal conductivity values (approximate). These materials are excellent at quickly transferring heat, making them ideal for applications where heat flow is needed:

Copper: About 400 W/m·K. Copper is widely used in electrical wiring and heat exchangers because it transfers heat and electricity very well.
Aluminium: Around 237 W/m·K. Aluminium is lighter than copper and is often used in heat sinks because it quickly absorbs and spreads heat.
Silver: Roughly 429 W/m·K. Silver has the highest thermal conductivity of all metals, but it is expensive, so it’s used less frequently.
Gold: About 318 W/m·K. Gold is excellent at conducting heat, but is mainly used in electronics like connectors and circuit board contacts for corrosion resistance.
Diamond: Exceeds 1000 W/m·K. Diamond is a non-metal with extremely high thermal conductivity, useful in high-performance electronics like heat spreaders for CPUs and cutting tools such as industrial drill bits and saw blades.

These materials are chosen for applications that require fast and efficient heat transfer.

Low thermal conductivity and its importance

Materials with low thermal conductivity resist heat flow. These materials act as insulators and are essential in construction, refrigeration, and clothing industries. Low thermal conductivity helps keep buildings warm or cool and reduces energy consumption.

Why is low thermal conductivity useful?

Low thermal conductivity is valuable because it helps materials slow down heat flow, making certain materials ideal for keeping environments warm or cool, whether that’s protecting people from extreme temperatures or preserving temperature-sensitive goods:

Thermal insulation: Prevents heat loss in buildings and can keep buildings cool, reducing heating and cooling costs.
Protective clothing: Keeps workers safe from extreme temperatures.
Packaging: Maintains the temperature of sensitive goods during transport.

By slowing heat flow, these materials make environments more comfortable and processes more energy-efficient.

Examples of materials with low thermal conductivity

Materials known for their insulating properties are designed to resist heat flow and are often used where maintaining temperature is important, like building insulation, packaging and clothing. Some common examples include:

Polystyrene foam: Around 0.03 W/m·K. This foam is lightweight and commonly used for thermal insulation in walls and packaging.
Fibreglass: Approximately 0.04 W/m·K. Fibreglass is a popular insulation material in construction due to its effectiveness and affordability.
Wood: Roughly 0.12 W/m·K. Wood is a natural insulator often used in building structures.
Concrete: Around 1.7 W/m·K, though this varies with density and moisture content. Concrete provides moderate insulation but is mainly valued for structural strength.
Air: About 0.024 W/m·K. Air itself is one of the best natural insulators, which is why materials that trap air (like foam or fibreglass) have low thermal conductivity.

How is thermal conductivity measured?

Measuring thermal conductivity requires precise methods to ensure accurate results. Different techniques exist depending on the material type and intended use. Common methods include steady-state techniques, which keep heat flow constant, and transient methods, which measure how heat spreads over time. Choosing the right test depends on the material’s form and the conditions it will face.

Common measurement methods

There are several ways to test how well a material conducts heat. Each method suits different materials and applications.

Guarded hot plate method: A sample is placed between a heated plate and a cooled plate. The heat flow and temperature difference are recorded to calculate conductivity. This method is often used for solid insulating materials.
Heat flow meter: Similar to the guarded hot plate method but designed for quicker testing, often applied to insulation products.
Laser flash method: A short laser pulse heats one side of a thin sample, and the temperature change on the opposite side is recorded. This is common for metals and ceramics as they conduct heat well, meaning the heat pulse from the laser can pass through quickly and be measured accurately.
Transient plane source method: A sensor generates heat and measures the temperature response in the material. It works for a wide range of materials, including powders and liquids.

Factors affecting measurement accuracy

Accurate thermal conductivity testing depends on more than just the method. Various factors can affect the final results, and controlling these variables is important so you’re able to get reliable and consistent measurements.

Sample uniformity: Variations in thickness or composition can cause uneven heat flow, resulting in inaccurate or inconsistent thermal conductivity readings.
Environmental conditions: Changes in temperature and humidity during testing can affect the material’s properties and the way it transfers heat, so controlling these factors makes sure you get stable and repeatable results.
Contact resistance: Gaps between the sensor and sample can create resistance to heat flow, which could affect heat flow readings.
Calibration: Regular calibration of equipment ensures reliable and consistent measurements. Without it, the equipment could provide inaccurate readings, which means you’re not getting accurate test results.

Thermal conductivity in construction and industrial applications

Thermal conductivity influences many building and industrial products, and by selecting materials with the appropriate thermal properties, you can boost energy efficiency, ensure structural durability and maintain safe and comfortable environments.

In this section, we’ll explore how thermal conductivity impacts concrete, insulation materials, admixtures, and formwork in construction and industrial applications.

Concrete and insulation materials

Concrete has moderate thermal conductivity that varies based on its density and moisture content. Denser or wetter concrete conducts heat more effectively. Understanding concrete’s thermal behaviour helps in designing energy-efficient buildings, where insulation is also critical.

Insulation materials like foams, fibreglass, and mineral wool have low thermal conductivity. They reduce heat transfer through walls and roofs, helping maintain consistent indoor temperatures and reducing energy costs.

Admixtures and concrete performance

Certain concrete admixtures can affect thermal conductivity by changing the concrete’s density, porosity, or moisture retention:

Air-entraining admixtures introduce tiny air pockets in concrete, which lowers thermal conductivity and improves insulation.
Lightweight aggregates reduce the concrete's density, which also decreases thermal conductivity.
Water-reducing admixtures influence the hydration and curing process, which can indirectly affect heat transfer by changing moisture content.

Using the right admixtures helps optimise concrete for both strength and thermal performance.

Formwork and thermal effects

Formwork materials affect heat transfer during concrete curing. Metal formwork conducts heat rapidly, which can speed up or disrupt curing. Timber or insulated formwork slows heat transfer, helping maintain a stable temperature that encourages even curing.

Choosing the right formwork type helps control the thermal environment during construction, improving concrete quality.

Why understanding thermal conductivity matters

Knowing the thermal conductivity of materials helps engineers and architects make informed decisions for design and material selection. It affects:

Energy efficiency in buildings: Materials with appropriate thermal conductivity help keep heat inside during winter and outside during summer, reducing heating and cooling costs.

Safety in high-temperature processes: Using materials that manage heat effectively prevents overheating, reducing the risk of fire or equipment failure.

Product durability and performance: Proper thermal management minimises thermal stress and damage, extending the lifespan and reliability of products.

Cost-effectiveness in material use: Selecting materials with the right thermal properties avoids over-engineering and energy waste, saving money in the long run.

Proper knowledge reduces the risk of product failure, energy waste, and safety hazards.

Practical tips

Applying the right techniques and precautions can improve the accuracy and reliability of thermal conductivity measurements. Here are some practical tips to consider.

Always check the thermal conductivity of materials before use.
Consider environmental conditions like temperature and humidity.
Use products designed for specific thermal requirements.
Combine materials strategically for better insulation or heat transfer.

We’re here with the construction products that you need

Understanding what thermal conductivity is and how it affects materials like concrete, insulation, and formwork can help you make better choices for your projects. Whether you’re working on improving energy efficiency, ensuring durable concrete structures, or selecting the right protective coatings, knowing the thermal performance of each product is essential.

If you’d like advice on choosing the right concrete admixtures, formwork products, or any other products we stock for your project, please get in touch with us today. Our team is ready to help you find reliable solutions.

By Katie Taylor

What is thermal conductivity?