WHAT IS THERMAL CONDUCTIVITY ? | FORMULA

Thermal conductivity, also known as the Lambda (λ) value, comes from from the Greek numeral system and has a value up to 30. It is measured in Watts per Meter Kelvin (W/mK). The lambda value is a characteristic of a material that indicates how efficiently it conducts heat, whether it is thermal insulation or glass. In other words, it represents the material's ability to transfer energy through conduction. The λ-value is crucial for calculating the R-value and U-value.

From a mathematical perspective, the lambda value signifies the rate of energy transmission through 1m² of material, 1m thick, with a temperature difference of 10°C on both sides.

Lambda values are typically used to compare different material properties, making this property essential in designing high-performance buildings and passive houses. In Europe, the lambda value [λ] of a material is required. In other words this is the information that manufacturers must include in the product's pdf data sheet.

Pic. Heat flow and thermal conductivity

Pic.credit:open.edu

WHAT IS A GOOD THERMAL CONDUCTIVITY VALUE FOR INSULATION?

In general, the lower the conductivity the better the level of insulation. To determine whether we are dealing with material that has good thermal conductivity, it is good to know its lambda value then compare it to other materials on the scale.
An effective insulation material aims for the lowest possible lambda value to minimise heat loss. To illustrate, glass fibre insulation boasts a thermal conductivity of 0.044 W/mK, whereas dense concrete has a thermal conductivity of around 1.5 W/mK. To emphasise further, copper has a considerably higher lambda value of 401 W/mK, explaining its common usage in applications such as plumbing and electrical systems.
Generalising, an insulation material with good thermal conductivity is one with a value no higher than 0.030W/mK. 
Values of thermal conductivity above 0.030W/mK would require using a thicker insulation layer, which may not always be possible.
 

THERMAL CONDUCTIVITY OF INSULATION MATERIALS | COMPARISON

Insulation material thermal conductivity is usually printed on the packaging, listed on the manufacturers website, alternatively on the product pdf datasheet.
Below, we present sample thermal conductivity values for conventional insulation materials from the highest (worse) to the lowest (better).

• Straw - 0.08W/mK
• Expanded polystyrene - 0.030-0.045W/mK
• Mineral or glass wool - 0.032-0.044W/mK
• Wood fibre - 0.038W/mK
• Hemp - 0.038W/mK
• Cellulose fibre - 0.04W/mK
• Cork - 0.04W/mK
• Sheep wool - 0.036W/mK
• PIR (polyisocyanurate foam) - 0.022W/mK
• Phenolic foam - 0.018W/mK
• Aerogel - 0.012-0.016W/mK

THERMAL CONDUCTIVITY OF BUILDING MATERIALS

Various materials, including insulation types and others, possess distinct thermal conductivity values that serve as indicators of their insulating capabilities. For instance, metals exhibit high thermal conductivities, allowing significant heat transmission even for minor temperature differentials. Elements like metal window frames, lintels, and insulation fixings may act as "thermal bridges" or "cold bridges," transmitting substantial heat despite their limited total area. In contrast, window glass, characterised by high conductivity, sees minimal impact on its overall U-value when using thicker glass. While structural building materials like brick and concrete have lower conductivities, the expansive surface areas of walls and roofs still pose the potential for considerable heat losses.

Below, we present the thermal conductivity values of common building materials:

• Insulation PIR board – 0.022W/mK
• Insulation mineral wool batt - 0.040W/mK
• Brick work - 0.77W/mK
• Concrete block - 1.93W/mK
• Aluminium - 220W/mK
• Glass - 0.57W/mK
• Plaster – 0.18W/mK
• Steel - 60W/mK
• Timber – 0.13W/mK
• Aerated concrete - 0.11W/mK

INSULATION LAMBDA CALCULATION

The thermal conductivity coefficient, lambda (λ), is measured by taking samples of a particular material under laboratory conditions - typically at a temperature of 10 degrees Celsius. The results obtained from these samples are then summed up and divided by their quantity. This process allows for the calculation of the average lambda coefficient. Often, achievable heat transfer coefficient values are also provided, representing the best result among the collected sample outcomes.

*Disclaimer

Insulationgo LTD strives to ensure that the content is accurate and up-to-date; however, we cannot be held responsible for any errors, omissions, or inaccuracies.
The information provided is for general informational purposes only and does not constitute professional or expert advice (No Professional Advice Disclaimer). It should not be considered a substitute for consultation with qualified professionals. Before making any purchasing decisions or taking action based on the information presented here, we strongly recommend contacting the relevant manufacturer or certified expert directly to verify product suitability for your specific needs.
Descriptions, illustrations, photographs, data, dimensions, weights, and measurements provided are subject to change without prior notice and do not represent guaranteed contractual product characteristics. Users are solely responsible for ensuring compliance with all applicable laws, regulations, and proprietary rights.
By using this page, you acknowledge and agree that Insulationgo LTD shall not be liable for any damages, losses, or inconveniences resulting from the use of or reliance on the information provided. This limitation of liability applies to all users, including visitors, readers, and subscribers.
This section is written in good faith and intended exclusively for neutral, informational, and comparative purposes. It is not intended to endorse, promote, or discredit any specific brand, manufacturer, or product (No Endorsement Clause). Therefore, Insulationgo LTD assumes no responsibility for any misunderstandings, disputes, or claims that may arise from the content. This article cannot serve as a legal basis for actions against any of the companies or brands mentioned. At the request of a manufacturer, we may edit or remove this section. All rights, including potential claims, are expressly waived by all parties involved.
All product names, logos, and brands are property of their respective owners. All company, product, and service names used are for identification purposes only. Use of these names, logos, or brands does not imply endorsement (Trademark Disclaimer).
Insulationgo LTD is not affiliated, associated, authorized, endorsed by, or in any way officially connected with any of the manufacturers or brands mentioned here.
Legal context: This disclaimer is grounded in general principles of civil and consumer protection law, including — but not limited to — the EU E-Commerce Directive (2000/31/EC), the UK Defamation Act 2013, and the UK Consumer Rights Act 2015. These legal frameworks support the limitation of liability for online publishers, the use of disclaimers where no expert advice is provided, and the protection of neutral comparative content. The use of No Professional Advice and No Endorsement disclaimers is a common legal and industry standard to maintain transparency and reduce legal risk.
Some links in this section may lead to external product pages. If a purchase is made after clicking such a link, Insulationgo LTD may receive revenue or commission from the sale. This does not influence the objectivity of the article, which remains neutral and written for informational purposes only. The presence of a link does not constitute an endorsement.
For more details, please refer to our Terms of Use and Privacy Policy. All trademarks and brand visuals belong to their respective owners.