Measuring closed cell / open cell content
The qualitative differences between closed and open cell systems can be measured accurately using a pycnometer, which measures the impenetrable volume of the sample by gas. Test methods should conform to EN ISO 4590 standards (or equivalent), and the product tested should be easily identified on the test report by its product name, description and any other definitive characteristics reported, such as the foam density.
What does this mean for phenolic foam and its impact on thermal performance?
In closed cell phenolic foams, the conductivity value, also known as the lambda value, of the final foam is directly related to the lambda value of the blowing agent that is present in the final product as the cell gas. The retention of this blowing agent in the closed cell structure is what delivers its high thermal performance. High quality closed cell phenolic foam has at least 90% closed cell content which can produce approximately double the R-value of open cell foams. Poorer quality foam insulation may have a high percentage of closed cells but the cell walls may be of inferior quality and have ‘windows’, or holes, that allow the gas to escape over time.
In open cell phenolic foams, blowing agents leak out of the porous material, being replaced by air. The degree of movement through the body of the material is dependent on a range of factors, including the size and density of the cells, moisture ingress, as well as the chemical composition of the material itself. This can have the effect of reducing the thermal performance of the product by around 50%.
The salient point is that porous, sponge-like characteristics are not ideal features of insulation. Quite simply, any flow through an insulating material is defeating its purpose as it facilitates unwanted energy transference. This is why only an aged R-value is a true representation of the performance of a foam and should be used for declaration and not the R-value achieved immediately after manufacture. Aging standards recognise the effect of leakage and stipulate that declared R-values should take this into account to gain a more accurate measure of long term performance of the insulation.