There are different ways to model heat and moisture transfer in building materials. This section gives an overview of the most commonly used techniques.
Uncoupled approach
When the full coupling between heat and moisture transfer is not taken into ac- count, the complexity of the heat and moisture transport equations significantly reduces at the expense of accuracy. This technique is called ‘the uncoupled ap- proach’ in this work and is used for models which feature no coupling at all as for models which feature a simplified coupling mechanism. The uncoupled approach is mostly used in BES and airflow models. As these two categories of models are most of the time used for problems where there is either no interest in the hygric behaviour (e.g. when only ventilation is studied) or where a simplified moisture model provides sufficient accuracy (e.g. moisture buffering in walls and its effect on indoor RH on a yearly basis) the use of the uncoupled approach is justified.
The uncoupled heat transfer in the material can be solved using response fac- tors as in the BES model TRNSYS, using control volumes as in CFD, with finite differences as in the zonal model by Inard [38], . . . In all these different solution techniques the entire thickness of the construction element is considered.
The moisture transfer in the material is usually described in a very simplified way in the uncoupled approach. In the airflow models it is even rarely taken into account: standard CFD models can model air flow through porous materials but are not capable of modelling moisture transfer in those materials as they do not consider hygroscopic or capillary effects; also most zonal models do not consider hygric effects, yet in a recent model by Wurtz [29] hygric buffering in walls was added to the zonal model. In BES models like TRNSYS and EnergyPlus the Effec- tive Moisture Penetration Depth (EMPD) model [39] is included to take the effects of moisture buffering in the construction elements on the indoor relative humidity into account. In the EMPD model only the moisture transfer to a thin material layer near the interface with the air is considered. The thickness of this material layer depends on the period of the relative humidity variation in the air and the associated moisture penetration depth in the material. The model is based on the observation that the response of the material to a RH fluctuation in the air is al- ready reduced to 5% at a depth of three times the penetration depth. In EnergyPlus a separate material layer is considered for each wall which buffers moisture and the influence of the wall temperature on the relative humidity in the wall is taken into account. In TRNSYS one virtual material layer is considered that represents all the different walls and the influence of variations in the wall temperature on
INTRODUCTION 17
the relative humidity in the material is not taken into account. The EMPD model has its use in predicting the effect of moisture buffering in the porous materials on the relative humidity in the indoor air, but cannot be used when the interest of the study lies in accurately predicting the effect of fluctuations in the air on the hygrothermal behaviour of the material.
Coupled approach
Coupled models take the full interaction between the heat and moisture transfer processes into account and solve the coupled heat and moisture transport equations for the material of interest. This approach is used in the HAM models. Besides the fact that not all HAM models consider air transport in the material (the ‘A’ in ‘HAM’), the main distinction that can be made between different HAM mod- els concerns the way they model moisture transfer in the material. Some models consider water vapour transport as the only moisture transport mechanism, while others consider both water vapour transport and liquid transport.
A second criterion to distinguish the different HAM models is the way they simulate multi-dimensional problems. Some HAM models like for instance HAM- tools [40] are one dimensional, while other models are two dimensional (e.g. [22]) or even three dimensional [41].
Discussion
To accurately predict the hygrothermal response of individual objects in real build- ings, the complete interaction between heat and moisture transfer in the object has to be modelled. This is the consequence of the important temperature fluctuations which can occur in the indoor climate of the building. Janssens showed that the interaction between such temperature changes and the moisture balance have a major impact on the humidity predictions and should be well described [42]. The uncoupled approach is thus clearly not suited for the applications aimed at in this thesis.
The driving force for the moisture transport in the objects is the fluctuation in the indoor relative humidity. As this study focuses on individual objects, located indoors and not being part of the building envelope, it can be safely assumed that the objects are not in contact with sources of liquid moisture such as driving rain and moisture rising from the soil. The relative humidity inside the objects will thus be limited by the maximum relative humidity of the indoor air. This however does not rule out the possibility that the object is exposed to liquid moisture by surface condensation (e.g. a painting hung against a cold wall). In such a case it is obvious that the object will be damaged and it is hence not necessary to model the moisture transport inside the material. For this reason the case of surface condensation will not be considered in this work.
18 INTRODUCTION
In building physics textbooks (e.g. [43]) the concept of critical moisture con- tent is used as a criterion to distinguish between vapour dominated transport and liquid dominated transport. As the critical moisture content is located at a higher relative humidity than the hygroscopic moisture content, which lies at 98% RH, it is an acceptable assumption that moisture transport in objects located in the in- terior is driven by water vapour transport. Hence, the HAM code used to model the response of the object, does not have to include the effects of liquid moisture transport.
The objects which are to be modelled can have an irregular shape. Sharp cor- ners and complex surfaces can result in multi-dimensional effects in the hygrother- mal response, which cannot always be modelled as one or two dimensional effects. The simulation of the hygrothermal response of an individual object thus re- quires the use of 3D coupled heat and moisture transport model, capable of mod- elling water vapour transport.