An important matter in the field of building physics is the questioning of how well buildings sustain ageing, and how their overall efficiency evolves over their lifetime. Many causes for degradation are carried by moisture transfer through these porous materials. Indeed, infiltrated water may transport chemicals, alter mechanical properties, and cause freeze thaw damage or mould development. It may also affect thermal properties and energetic efficiency, as well as the health and comfort of the occupants. The understanding of how moisture transfer properties evolve during the lifespan of building materials is however far from complete. The pore structure of a material itself may change over time, or be altered by cracks and defects caused by mechanical loading and aggravated by moisture-induced degradation. All sizes of fractures may have a strong impact on heat and moisture flow in the building envelope, and their influence is to be accounted for in any long-term performance assessment, not only of building and building components, but of any built structure in general. A considerable amount of work has already been performed in order to allow predicting the hygrothermal behaviour of buildings over longer periods of time. However, an accurate prediction of all ranges of damage in a building component, from microscopic to macroscopic cracks, supposes an extensive knowledge of all damage-inducing, time-varying boundary conditions of the problem during the simulation time. This also implies high computational costs, as well as important needs for material characterisation. As a complement to these predictive methods, a new approach was undertaken, combining experimental characterisation of crack patterns and numerical simulations of coupled heat and moisture transfer. First, a preliminary study was conducted, consisting of measurements of the water vapour permeability of diffusely damaged construction materials. This allowed identifying the experimental and numerical requirements of the remainder of the work, which aimed at providing measurements of fracture network geometries for their explicit modelling in heat and moisture transfer simulations. Digital image correlation and acoustic emission monitoring were then performed during the degradation of cementitious materials, in order to obtain quantitative data on crack pattern geometries, and to assess the possibilities for damage monitoring at the building scale. The optical technique, along with an appropriate image processing procedure, was found suitable for providing precise measurements of fracture networks. A method was also proposed for the interpretation of acoustic emission recordings in terms of damage quantification, localisation and identification. Then, a new model for coupled heat and moisture modelling in cracked porous media was developed, that allows including such measurements of fracture patterns into a finite element mesh, and simulating flow accordingly. This model was validated on the basis of experimental measurements: digital image correlation was performed during the fracturing of concrete samples, in which moisture uptake was then monitored using X-ray radiography. A good accordance was found between experimental and numerical results in terms of 2-dimensional moisture concentration distributions. The validated code was then used for the simulation of test cases, in order to assess the hygrothermal performance of damaged multi-layered building components subjected to real climatic conditions. The consequences of fractures on the moisture accumulation in walls, on the amplitude of sorption/desorption cycles and on the thermal performance, were observed.
