The R-value – Thermal Resistance is a direct indication of the performance of the product.  The higher the R-value, the better the insulating effect of the insulation will be.  R-value is determined in the following way:

R-value = Thickness (of the material) / Its Thermal Conductivity (or K-value)

The thickness is determined by the manufacturer – and will normally form part of the product range.  The K-value is determined in an accredited laboratory.  The manufacturer will produce a sample that is manufactured at a stated density (weight) and will submit to the laboratory for testing.  The idea is to achieve an as low as possible K-value, in other words, a low thermal conductivity – put plainly, the product must be a bad conductor of heat flow. 

Once the K-value is determined, the R-value can be calculated, for example:

For a 50mm thick blanket with tested K-value of 0.046:

0.05 (thickness) / 0.046 (k-value)  = 1.08 (R-value)

As long as the manufacturer manufactures the other thicknesses in his range at the same density than the submitted sample, he can use the same formula and k-value to determine the R-values, eg:

For a 100mm thick blanket:

0.1 (thickness) / 0.046 (tested k-value)= 2.17 (R-value)

This is where the consumer must be vigilant – some dubious manufacturers may reduce the density or weight of their products to save on raw material.  If they do this, they cannot use the tested k-value and thus cannot claim a good R-value. Always demand to see the test report and stated density that the manufacturer uses to determine R-values.

The R-value determined in the examples above is the so-called Product R-Value.  The fact is however that the roof tiles and ceiling (as well as other roofing materials) also contribute to the insulating effect.  In practice, the Total R-value of both roofing material and insulation material must thus be considered.  So, if the insulation material’s R-value is 2.17, and the roof and ceiling materials have a R-value of 0.53, then the Total R-value will be 2.7.  Refer to The Thermal Insulation Association of South Africa (TIASA) website for guidelines on Total R-values.