What are U-values, R-values, Lambda Values and Kappa Values?

An introduction to lambda values, R-values, U-values and Kappa values.

There are a number of useful 'values' when looking at how effective an insulation material is. In this article we will take a look at the main values and explain how they can be used to measure performance.

What is Lambda value?

The lambda value, also portrayed as ‘K-value’ or ‘λ-value’, measures a product’s thermal conductivity in units of W/mK.

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Good insulation will have as low a lambda value as possible to reduce heat loss. For example, our lower lambda products each have a lambda value of 0.019 W/mK across all thicknesses.

What about an R-value?

A product’s R-value measures its thermal resistance in units of m²K/W.

By dividing a material’s thickness (in metres) by its lambda value, you can discover how well it resists heat transfer at a specific thickness.

The best insulation will have a high R-value at a low thickness, indicating that it is just as good at reducing heat loss as its thicker counterparts.

What is a U-value?

A U-value is a sum of the thermal resistances of the layers that make up an entire building element – for example, a roof, wall or floor. It also includes adjustments for any fixings or air gaps. Our U-value calculator can make calculations easier, click here to access.

A U-value value shows, in units of W/m²K, the ability of an element to transmit heat from a warm space to a cold space in a building, and vice versa. The lower the U-value, the better insulated the building element.

A building element’s U-value is extremely important as there are certain standards that should be reached according to Building Regulations / Standards.

What is a Kappa value?

This relates to the thermal mass of a construction and is used within energy assessments. It is the measure of how much heat will be stored per metre squared of a building element and represents ‘κ’, measured in kJ/m²K. ‘κ’, or the heat capacity of a building element, can be calculated using the following equation:

κ = 10 ^{– 6} x Ʃ (d_j p_j c_j)

d_j = thickness of layer (mm)

p_j = density of layer (kg/m³)

c_j = specific heat capacity of layer (J/kgK)

The calculation is over all layers in the element, starting at the inside surface and stopping at whichever of the following conditions is encountered first (which may mean part way through a layer):