Tipping point for condensation water drainage on surfaces and interfaces of insulated wall assemblies - experimental method to define water content limits for hygrothermal simulation models

If condensation occurs on non-hygroscopic surfaces of insulated wall constructions, droplet runoff may happen if the amount of condensate exceeds certain limits. Depending on the situation this phenomenon may help to dry the wall, but it may also result in material degradation by water accumulating at the bottom where drainage is not intended. The limits for interstitial condensation amounts on non-hygroscopic materials calculated by the dew-point method in European standards differ with values up to 500 g/m². However, it is questionable whether these limits are based on rigorous experiments and whether they are suitable to evaluate hygrothermal simulation results. Therefore, a laboratory test method has been developed to determine the amount of condensate required for water to run off from vertical surfaces or interfaces of insulated assemblies. For this test 14 fibrous insulation materials (9 x mineral wool, 3 x wood fibre, 2 x cellulose) and 4 types of condensation planes (hydrophilic, hydrophobic, smooth, or rough surface) were examined. The results proved to be much lower than in the above-mentioned standards. Mostly, they ranged between 100 and 200 g/m². Furthermore, by correlating the acceptable amount of condensate with the hygrothermal properties of the insulation materials, a simple formula was derived to estimate the material specific limit value, using its moisture equilibrium at 80 % RH. Finally, by comparing the test results with hygrothermal simulation results, it can be concluded that the water content in the critical one-centimetre-thick layer of the assembly, referred to in DIN4108-3 (2018), is appropriate to assess the probability of condensate runoff.