Building physics problems in early modernism and how to solve - A school built by Baumann/Prachenzky (1929/31) as case study (CS5) in 3ENCULT M. Bianchi Janetti 1 , R. Pfluger 1 , K. Längle 1 , F. Ochs 1 , F. Nesi 1 , W. Feist 1,2 ABSTRACT A 3D model for the hygrothermal simulation of the ceiling-wall connection in a listed school building located in Innsbruck is presented. The model is implemented with the FE program COMSOL Multiphysics in order to estimate the risk of condensation and mold growth in case of application of internal insulation. The investigated construction is part of the case study CS5 within the EU-Project (seventh framework program) 3ENCULT, a detailed description of the problem and critical evaluation of the simulation results is reported. Valuable recommendations for planners and decision makers in terms of minimal invasive measures will be derived. Keywords Hygrothermal simulation, internal insulation, ceiling-wall connection, condensation risk, 3ENCULT 1. Introduction The renovation of existing buildings according to high energy efficiency standards will represent a significant contribution to the reduction of CO 2 -emissions (EPBD Recast 2010). In particular for the renovation of historical buildings, specific solutions with internal insulation have to be developed. However, such solutions require careful planning in order to avoid condensation risk and/or degradation of the construction. Hence, the study of heat and moisture transfer in construction materials has recently become even more relevant. Within the EU-project 3ENCULT, focused on developing technologies for the retrofitting of listed building, a 3D model for the simulation of transient heat and moisture transfer in building elements has been developed and implemented with COMSOL Multiphysics. This model has been used to evaluate the condensation and mold growth risk deriving from the application of internal insulation in a listed school building (Innsbruck, 1929/31, Baumann/Prachenzky, the Austrian 3ENCULT case study). 2. State of the art In the last twenty years several authors developed models describing heat and moisture transfer in porous materials with applications also in the building physics. Numerical solutions have been proposed for calculating temperature and moisture content inside constructions as function of time and position and commercial programs specific for hygrothermal simulations have been developed. In particular, the programs Delphin and WUFI ([1] [2]) are widely used supporting architects and engineers in designing the envelope of buildings. However, these programs are restricted to two dimensional geometries at present. COMSOL Multiphysics represent a powerful tool for solving a large number of physical problems described by systems of partial differential equations. In particular, as recently shown ([3] [4]), it can be useful also for solving heat and moisture transfer problems inside constructions, although applications to real cases in this area are still rare. One advantage of COMSOL is that also 3D problems can be modeled. The simulation of the ceiling- wall connection presented in this paper is an example of solving practical building physics problems using this program. 3. Theory of Heat and Moisture Transfer in Porous Materials In this section the model for heat and moisture transfer implemented in COMSOL is described. 3.1 Governing Equations and use of COMSOL Multiphysics As suggested by other authors ([5], [6], [7]), a macroscopic approach has been chosen since it allows porous construction materials to be treated as homogeneous media. Under this assumption, heat and moisture transfer processes can be described by a system of two partial differential equations derived by imposing the equilibrium balance of mass and energy within an infinitesimal element of volume. For the one-dimensional case the governing equations system assumes the following form:  ݑ    ݐ ൅   ݔ ൬െ ܦ ௠,ఝ   ݔ െ ܦ ௠,   ݔ ൰ൌ0 (1)     ݐ ൅     ݐ ൅   ݔ ൬െ ܦ ௘,   ݔ െ ܦ ௘,ఝ   ݔ ൰ൌ0 (2) The system of equations (1) and (2) can be solved with COMSOL Multiphysics using the PDE mode in the coefficients form. Temperature T and relative humidity φ are the dependent variables whereas t and x represent time and position. u is the moisture content and h the enthalpy. D m,φ , D m,T , D e,T and D e,φ are material specific diffusion coefficients which assume the following form: 1. University of Innsbruck, Unit for Energy Efficient Buildings, Austria, michele.janetti@uibk.ac.at 2. Passiv Haus Institut, Germany