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Modélisation des transferts d'air et leur impact sur le comportement hygrothermique de l'enveloppe des bâtiments

Abstract : Within the context of more stringent buildings codes, mastering airtightness is of importance to achieve energy efficient buildings. Unintended air leakage through the building envelope, which is due to bad design and poor workmanship, not only increases energy consumption, but also leads to moisture disorders, affecting building durability and occupants health. This moisture risk is present in particular for lightweight structures such as timber frame buildings, which are sensitive to air leakage. It is therefore necessary to better understand and to assess the impact of unintended air transfers on the hygrothermal field and the heat flux in the vicinity of an airtightness defect. To this end, two numerical models are developed, dealing with Heat-Air (HA) and Heat-Air-Moisture (HAM) transfer respectively. The HAM model is firstly validated in 1D using numerical benchmarks from literature. Then, temperature measurements in a cellulose insulation layer subjected to moist air flow are compared with the models outputs, and good agreement is obtained. The HAM model provides a better prediction of the temperature field compared to the HA model. Following this 2D experimental validation of the HAM model, it is applied to a complex defect geometry, including porous insulation materials and thin air gaps. This defect is meant to be realistic, as it is drawn from a measurement campaign aiming to identify typical envelope leakage points encountered in timber frame buildings. Long term simulations are performed under transient temperature and humidity conditions, in case of air exfiltration and air infiltration. This study helps identifying tendencies towards moisture risk: infiltrating air flow dries the assembly whereas exfiltrating air flow humidifies it. A methodology to assess heat fluxes through the defect is presented. Finally, a simplified approach is derived from the detailed HAM-model, to take into account the contribution of airtightness defects on the total heat loss on the building scale. It is shown that the additional heat loss induced by an airtightness defect may be described by a specific heat loss coefficient. In addition, the coupling between air flow and envelope has a significant impact on total heat flux calculations. The influence of moisture transfers on observed tendencies is also discussed.
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Clément Belleudy. Modélisation des transferts d'air et leur impact sur le comportement hygrothermique de l'enveloppe des bâtiments. Génie civil. Université Grenoble Alpes, 2016. Français. ⟨NNT : 2016GREA0002⟩. ⟨tel-02429503⟩

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