Hygrothermal Simulation - Modelling Air Flow along Gap-like 3D Paths in Building Components with Darcy’s Law

Abstract

Unintended air flows have been recognised as a decisive factor when focusing on moisture damages and durability in building components (Hens 2017). Although efforts have been undertaken to better quantify convective moisture transport (Nusser, Bednar, and Teibinger 2010; Künzel, Zirkelbach, and Schafaczek 2010), IEA Annex 41 closed with a critical outlook on the”deeper exploration” of e.g. integrating air flow models (EBC 2013). Referring to this note, this paper presents an initial approach how to simulate air flows in constructions along 3D paths - to the difference of flows in porous media - such as in gaps and its implementation in hygrothermal simulation. An introductory overview of the established ways to model air flows in hygrothermal simulation tools and current developments in this field are given. Taking into account 3D air flows in the joints of the insulation as inviscid incompressible air flows proved to be a possibility to match measurement data with the results of a coupled heat, air and moisture (HAM) simulation. In this approach, space available to air flow was modelled as permeable cells. A leakage inducing a time dependant pressure difference was modelled as a mass flow sources. The data used for comparison consists of an experiment where the effect of leakages when exposed to pressure differences has been investigated. The comparison of the results show that with this approach the behaviour of the construction to the pressure differences could be qualitatively recreated. The refinement of the physical model and the experimental investigation of the hygrothermal properties of the gaps and joints are identified as future research topics. © 2022 ASHRAE.