Thermal conductivity at different humidity conditions: Part of technical heat and moisture investigation of alternative insulation materials

Finn Harken Kristiansen, Carsten Rode

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    The thermal conductivity (the l-value) of several alternative insulation products and a traditional product is determined under different humidity conditions in a specially constructed hot plate apparatus.The hot plate apparatus is constructed with an air gap on each side of the test specimen where humidified air can pass. Thus, it is possible to build up different degrees of moisture on each side of the test specimen.The thermal conductivity is determined for the following types of alternative insulation: sheep's wool, flax, paper insulation, perlite and mineral wool. The insulation products were Ekofiber Vind, Herawool (without support fibres), Heraflax, Isodan with and without salts, Miljø Isolering with and without salts, Perlite (water-repellent), and Rockwool A-batts for comparison.All measurements of the materials started with no affection of moisture. Nevertheless, results were achieved, with and without affection of moisture, which were 0-20% higher than expected for the materials used. The measurements of the existing investigation should therefore not be considered giving real absolute values of the thermal conductivity. They can, however, indicate the relative significance of the moisture level.For all the materials the thermal conductivity shows only a small dependence of the moisture level until this reaches a certain high level (average RH above appr. 75%). Over this level the thermal conductivity rises considerably - by up to 50%. However, this increase is presumably caused by an accumulation of moisture as condensation in the parts of the insulation that lie immediately close to the cold side of the apparatus. The high l-values found are therefore of no practical importance in structures where no condensation occurs. Disregarding these condensation situations, the maximum increase of the thermal conductivity is 3-8% because of the hygroscopic uptake of humidity from the ambient air.
    Original languageDanish
    Number of pages24
    Publication statusPublished - 1999

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