Impact on indoor climate depending on the moisture buffering of building materials

It is well-established that the thermal mass of a building's envelope buffers large variations in temperature. The fact that there is also a moisture buffering effect is much less known and appreciated by the construction industry. Hygric mass is defined as the vapour absorption capacity of a material capable of buffering moisture variations inside a room. This is especially relevant for rooms in which the generation of moisture (due to human activity) shows big time dependant moisture loads but no corresponding increase of ventilation. A hygroscopic material has the ability to absorb and store moisture from the surrounding air. When the relative humidity (RH) changes, the changing vapour partial pressure results in absorption or desorption of moisture within the material in order to reach equilibrium. RH levels between 30 - 65% (DIN EN 15251, 2012) are beneficial to the health of building occupants, reducing risks from common pollutants such as bacteria, viruses, chemicals, allergies and respiratory infections. Most materials have a hygroscopic potential which is too low to have any impact on indoor humidity, however this ability can vary to a great extent for different building materials and is related to the available surface area/porosity and the diffusion properties. To characterize the moisture buffering effect of a material, the so-called moisture buffer value of the NORDTEST (Rode et al., 2005) has been widely used by researchers. However, in recognition of concerns about the test-conditions of the Nordtest a new test setup is developed using more representative boundary conditions, because they are based on a typical diurnal course of a moisture load in dwellings.