Whether you are an architect, contractor or developer, you may need to compare the thermal performance of different materials for an upcoming construction project. Thermal performance is a key element in determining the energy efficiency of buildings.

Values for measuring thermal performance

Although several values are used to measure different aspects of the thermal performance of materials, such as C-value (thermal conductance) and K-value (heat transfer between the outside and inside of a building), the insulation performance of a product is typically measured using three thermal values: the coefficient of thermal conductivity (λ), thermal resistance (R) and thermal transmittance (U).

What is the thermal conductivity coefficient (λ) value?

The coefficient of thermal conductivity (λ) is the intrinsic property of materials and is related to the amount of heat that is transmitted between the two faces of a flat surface of a material, the lower this value the better thermal insulator the material is. It is expressed in units of W/(mK).

What is the thermal resistance (R) value?

The R-value indicates the thermal resistance of a material or its ability to resist heat flow. Expressed as m²K/W, the R-value depends on the thickness of the material used, unlike the lambda value. The higher the R-value of a product, the higher the thermal resistance and therefore the more energy savings it provides.

What is the thermal transmittance (U) value?

Often mistakenly considered the inverse of the R-value, the U-value actually measures thermal transmittance, or how fast heat moves through a given product, whether it is composed of a single material or multiple materials. Like the R-value, the U-value of a product also depends on its thickness, although conversely, lower U-values indicate better insulation.

To summarise:

d = material thickness (m)

(λ) : thermal conductivity - (W/mK) - The lower the value, the better the insulation performance

R = d/λ : thermal resistance - (m2K/W) - The higher the value, the better the insulation performance

U = 1/ summation R : heat loss value - (W/m2K) - The lower the value, the better the insulation performance

Polyurethane as insulation for energy-efficient buildings

As the demand for energy-efficient buildings continues to grow, building professionals are increasingly turning to smarter insulation solutions based on hybrid technologies.

Polyurethane foam, which is generally considered one of the best products for insulation, has a lambda value that can range from 0.018 for panels to 0.042 for low-density open-cell. As insulation technologies continue to evolve, we can expect to see even greater improvements in the thermal values of products, resulting in even more impressive energy efficient buildings.

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