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In construction, R-value is the measurement of a material’s capacity to resist heat fl ow from one side to the other. In simple terms, R-value measures the effectiveness of insulation and a higher number represents more effective insulation.

In order to determine the R-value, the thermal conductivity of the material must be measured in a laboratory – the “k-value” is then determined. The k-value or thermal conductivity is the time rate of steady state heat fl ow through a unit area of homogeneous material induced by a unit temperature gradient in a direction perpendicular to that area, W/m.K. As the mean temperature increases, the thermal conductivity will increase.

Therefore the R-value of thermal insulation depends on the type of material, its thickness and its operating temperature.


The highest total thermal resistance for a roof assembly in order to comply with SANS 10400-XA Energy usage in buildings is 3.7 m2K/W. The minimum added R-value for thermal insulation in this roof assembly is 3.37 m2K/W.

The following table and chart represent typical values and are provided for general comparison.



The heat flow through a building construction depends on the temperature difference across it, the conductivity of the materials used and the thickness of the materials. The temperature difference is an external factor. The thickness and the conductivity are properties of the material. A greater thickness means less heat fl ow and so does a lower conductivity. Together these parameters form the thermal resistance of the construction.

The thermal resistance is proportional to the thickness of a layer of the construction and inversely proportional to its conductivity. However, when a fibre blanket is compressed its values are compromised irrespective of what type of fibre it is.

Ensure compliance - don’t be misled.

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