Beitrag zur Lösung des Problems der Algenbildung auf Außenwänden mit Wärmedämmverbundsystemen (WDVS)

Contribution to the solution of the algae problem on façades insulated with external thermal insulation compound systems (ETICS).

Heat and moisture propagation in building envelopes is determined both by quasi-periodic climatic conditions from external environment and by thermal and moisture insulation and accumulation properties of particular material layers. A typical phenomenon of reconstruction of dwelling structures is the presence of algae and the consequent surface degradation of new insulation systems. The same can be observed in case of new building objects, even of those with insulation layers made from special advanced materials. Several research works try to analyze this problem to detect its sources and possible solutions. The present study shows that the population of algae is conditioned by the elevated moisture content. This phenomenon can be explained by the night condensation of vapour from porous material structures, driven by the thermal radiation from building envelopes to the clear sky. To detect such potential difficulties just at the projection stage, to be able to search for alternative remedies, the reliable computational modelling of heat transfer should include both time-variable heat conduction and convection through particular constructive and insulation layers and short-wave (from sun through atmosphere to earth surface) and long-wave (back to atmosphere) radiation supplements, These heat transfer processes have to be coupled with the input, propagation and output of moisture in various phases in pores, although some of above sketched processes cannot be easily characterized by deterministic mathematical relations. The quantitative analysis of radiation components is significant for all producers of outer surface coatings because the decrease of emissivity is able to avoid condensation of vapour at all. The paper presents a relatively simple and physically transparent approach to the simulation of all sketched processes, validated by extensive measurements. The models based on the finite difference method applied to a non-stationary differential equation of heat conduction, coupled with certain finite volume scheme for the evaluation of moisture content, and supplied with boundary conditions involving convection and radiation.