Lambda, R and U Value Guide

28 March 2019 Cem Sungar

Here's a quick guide to explain how thermal insulation performance is measured

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Whether you’re an architect, contractor, or a developer, you may find yourself needing to compare the thermal performance of different materials for an upcoming construction project, as thermal performance is a key element in determining the energy-efficiency of buildings. Although there are a number of values used to measure different aspects of thermal performance in materials, such as the C-value and the K-value, the insulation performance of a product is commonly measured using three thermal values: the lambda value, the R-value, and the U-value.
 
The lambda value (λ) of a product indicates its insulating capacity in terms of the material’s level of thermal conductivity. Expressed as watts per meter-Kelvin (W/mK), the lambda value is one factor used to calculate thermal efficiency of buildings, with lower lambda values indicating a higher level of insulation for any given product.
 
The R-value, on the other hand, indicates a material’s thermal resistance, or its ability to resist the flow of heat. Expressed as m²K/W, the R-value is dependent on the thickness of the material used, unlike the lambda value. The higher the R-value of a product, the greater thermal resistance – and therefore more energy savings – it provides.
 
Often mistakenly regarded as the inverse of the R-value, the U-value actually measures thermal transmittance, or how fast heat moves through a certain product, whether it’s comprised of a single material or is a composite. Expressed as W/m²K, the U-value is often used to indicate the thermal transmittance of doors and windows. Similar to the R-value, the U-value of a product is also dependent on its thickness, although, conversely, lower U-values indicate better insulation.

To summarize:

d = material thickness (mm)
(λ) : thermal conductivity  - (W/mk) - Lower the value, better the insulation performance
R = d/λ : thermal resistance - (m2k/W) - Higher the value, better the insulation performance
U = 1/R : Heat Loss Value (W/m2k) - Lower the value, better the insulation performance
 

As the demand for energy-efficient buildings continues to grow, construction professionals are increasingly turning to smarter insulation solutions that rely on hybrid technologies. For builders, it’s no longer enough to find the ideal combination of thermal values for a specific building type or climate – they also demand that these insulation systems are as lightweight as possible to save on both labour and transport costs during the initial setup.
 
Take for example Kingspan’s popular QuadCoreTM insulation core, which offers an industry-leading lambda value of 0.018 W/mK and the potential to reach U-values as low as 0.08 W/m2K. By contrast, polyurethane foam – which is generally considered to be among the best products for insulation – has a lambda value that ranges from 0.02 to 0.025 W/mK, as well as an average U-value of 0.30 W/m2K. As insulation technologies continue to evolve, we can expect to see even greater improvements in thermal values for products – which translates into even more impressive energy-efficient buildings.