Methodology of hygro-thermal design of energy efficient buildings – HYGRO-BAT

The idea is to develop a methodology for hygrothermal building design, based on reliable tools and methods to qualify and quantify innovative technical solutions exploiting the thermal dimension related to the mass transfer. To achieve and consolidate this approach, we propose to analyse the case of wood-based construction and more generally cellulose-based materials, as they are highly hygroscopic. By pooling the experience of major French players in the field and a specific experimental and numerical «benchmark« approach we aimed to understand the phenomena involved and propose adapted experimental (characterization methodology) and numerical (predictive transient simulations) tools. Our approach was gradual, starting with the basics of metrology and characterization of materials, in order to guarantee the reliability of the input data. Then measurements and simulations on simple assemblies under laboratory transient boundary conditions were performed. More complex scale of the real climate conditions was also investigated, using simulations and well-controlled PASSYS experimental cells. The process ended with simulations of real wall assemblies and a proposal of a guide for hygrothermal simulations.
The ambitious program of our project was supported by a consortium of seven recognized academic institutes (CETHIL, LOCIE, LASIE, LMDC, LERMAB, LGPM andI2M) three standard EPIC centres (CEA-INES, CRITT-Bois, CSTB) and three industrials (EDF, LIGNATEC , NR GAIA).

To meet the announced ambitious goals, we implemented complex experimental tools essential to the study of physical phenomena in our configurations. The complexity lies in the diversity of scenarios (homogeneous material, material assemblies ...), boundary conditions (constant conditions, step changes, real climate), but also by the level of reliability of experimental protocols and measurements. Consequently, the high-reliability experimental results were obtained. These have served to support detailed analyses conducted in our project and they will be available for future investigations.
The comparison of experimental results with numerical simulations allowed to progress in the understanding of physical phenomena. It also showed that the modelling tools we have, despite their diversity, are representative of the same physical reality. However, in some configurations, differences between the experimental measurements and numerical simulations were noted. Different explanations of these differences were investigated. In particular, a detailed analysis of generally accepted methods of hygrothermal characterization of building materials was initiated. It demonstrated the difficulty of measurement of some characteristics, due, among others, to very complex physical phenomena, such as non-Fickian diffusion and gas permeability.
At the end of the project we have tried to give answers to a number of recurring questions from building practitioners, such as: why wood seems to have a positive impact on energy consumption and comfort in buildings? Does water vapour transfer play or not an important role in this impact? These responses, materialized in the form of a guide, are not yet perfect; they still allow starting filling the gap between the expectations of building practitioners and research in hygrothermal transfers.

Our approach has been gradual, starting with the basics of metrology and material characterization, development of experimental tools for the analysis of hygrothermal behavior of highly hygroscopic envelopes in a controlled or real environment and simulation. The approach ended with a guide to develop the most efficient solutions. This has allowed to highlight the importance of the characterization of materials and difficulty in reliably measure some properties. Efforts must be provided to this end by proposing alternative methods using transient set-up and refining the description of hygrothermal transfers by integrating the multiscale dimension.

The scientific results were presented in national and international conferences and submitted in refereed journals. The results for the characterization of materials were published by the LaSIE, LMDC and LGPM, those concerning the behaviour of a wall assembly by LERMAB, CETHIL and LOCIE.
The project has also been presented at conferences and ANR different workshops; in addition one workshop was specifically dedicated to coupled transfers (during national conference SFT2015 in La Rochelle).
We should also mention the doctoral theses of Kamilia Abahri at LaSIE , Yannick Kêdowidé at LOCIE and Zakaria Slimani at CETHIL directly related to the project.
Finally, a «Good Practice Guide for the evaluation of hygrothermal performance of buildings« coordinated by CRITT Bois, was proposed.

There is a gain of interest for Coupled Heat-Air-Moisture (HAM) transfers processes in buildings. Researchers from building energy field foresee here an opportunity to reduce energy needs for buildings. Indeed, some recent works had shown the impact of mass transfers on energy behaviour of buildings, under varying external and internal loads.

However, even if a lot of work had been done, we are still not able to explain some observations. For example, differences between simulated and measured values have been remarked in a few national and international projects (Annex 24 and 41 of International Energy Agency, French ANR PREBAT OPTI-MOB). The reason of these differences is not really known: measuring system, properties of materials, HAM models themselves? Past and current projects showed the complexity of the problem, but, up to now, were not able to explain clearly reasons of divergence and elaborate adapted tools. However, this is a fundamental question, because energy savings are expected precisely in dynamic thermo-hygric behaviour. Indeed, few publications, where the impact of dynamic moisture conditions is analysed, show energy savings up to 30%.

Our project gathers the experience of leading French institutions in this field around experimental and numerical benchmarks. Our ambition is to understand and master main physical phenomena and to elaborate experimental (characterisation methods) and numerical (coupled HAM-BES simulation) tools .

Our main objective is to elaborate a methodology for hygro-thermal design of buildings, based on reliable tools and methods. The aimed methodology should be able to qualify and to quantify the novel solutions, representing energy impact of mass transfers. Proposed project uses wood-based construction, and more generally construction materials based on vegetal fibres, as application example. These materials have interesting hygroscopic properties and complex hygro-thermal behaviour. Moreover, the perspective of the further economic development of the corresponding industrial sector in France seems promising.

We propose a progressive working plan starting at precise measurements of material properties, as reliable inputs for numerical models. Then measurements and numerical simulations of dynamic behaviour of materials and wall assemblies will be performed, first under well known laboratory conditions. Then a more complex scale of building behaviour under real climate will be assessed, first in an idealised way (using well known PASSYS cells) then using real buildings. The possibilities of integration of project results in official standards will also be assessed.

HYGRO-BAT project is proposed by 13 institutions: 7 research laboratories form Universities (CETHIL, LEPTIAB, TREFLE, LERFOB, LERMAB, LOCIE, LMDC), 3 semi-public technical centres (CEA-INES, CRITT-Bois, CSTB) and 3 enterprises (EDF, LIGNATEC, NR GAÏA).