Performance against water and water vapour transmission properties.
Thermal insulation products must be non-absorbent since any small increases in moisture content will reduce their thermal resistance. Therefore, the moisture content they can absorb when work is carried out on site and when they are in place is limited. This can be checked by water absorption tests through partial and complete, short and long-term immersion, and the water diffusion absorption test.
On the other hand, water vapour can spread through thermal insulators when there is a gradient of relative humidity between both sides of the material. Likewise, such diffusion can be interrupted by using certain coatings such as Kraft paper which acts as a vapour barrier. Therefore, in applications where a condensation study is required, e.g. on vertical enclosures, the resistance to water vapour diffusion (MU, µ factor) of the material or resistance to water vapour Z of the coating needs to be determined.