Master the AIR transfers and their impact on the hygrothermal behavior of Wood Houses – MOB-AIR

Air leaks analysed come from professional feedbacks. Their analysis shows that leaks deal with different scales : from material by itself as for example wooden plate which isn't totally leak-tight to assembly defects due to ineffective taping. It is therefore necessary to develop modelling/experiments at different scales to undestand the effects of these leaks. 3 scales are committed: material scale, wall scale and finally whole building scale. Material scale goal is to understand air leak of a material by itself (most annoying materials are aimed such as glass and wooden fiber). Lattice Boltzmann method is selected for this. At the wall scale, the aim is to undestand path through hole and between layer of materials. The solution is Stokes simulations (feeded by materiel modelling). Finally, at the whole buiding scale, the simulations will be conducted by modified building simulations codes feeded by the preceeding results.

Expected results are mainly the leaks classification (especially their relative importance) and proposal of changes designed to avoid these leaks. Another ANR project named HAM is expected to collaborate with MOB-AIR. The aim of HAM is the detailed air/moisture simulation in buildings. Combining these two approaches will possibly lead to prediction of water condensation in walls.

At the moment, the professional feedback is acieved, the leaks list is build. The number of different leaks appears limited, even if some common defects aren't reproducible such as roller blind. A analysable leaks list has been defined. Expiremnts based on these leaks are in progress. By the same time, new experimental method have been validated like air path tracks through walls using flourescein tracer.

Defects list is important. It is included in project's deliverable PRE-ETUDE DES SOLUTIONS CONSTRUCTIVES EXISTANTES A OSSATURE BOIS. It is the first step of the project and in the same time the final step because professional are waiting for the analysis/solution for these defects.

The growth in timber frame construction, displayed in economic studies, will largely depend on the ability of the sector to respond to the evolution of construction techniques and timing of new thermal regulations, RT2012 and future RT2020.
Light envelopes of timber frame, are particularly sensitive to air transfer by structural defects and joints, very present in this type of structure. Air transfers have an impact on the total sealing of a building, but also on the hygrothermal local fields and can lead to performance degradation.
The guarantee of good permeability then goes through an implementation of an airtight membrane. Today, with a careful treatment of the sealing during the construction phase, we know that the timber frame houses can claim to possess good levels of air tightness. However, this excellence needs a very heavy treatment, too restrictive in terms of time and cost.
Scientifically, this is an anisothermal moist air flow problem in complex confined geometries dealing with hygroscopic porous media. Current simulation tools do not take into account these elements to optimize the design of the envelope.
In the MOB-AIR project we propose to address the air tightness of building timber frame at the scale of the wall and the building, developing appropriate numerical tools and full-scale experiments. The experiments will use an existing test building already instrumented and characterized, on the site of CSTB Grenoble, and wall scale facilities at CSTB, FCBA in Bordeaux and LOCIE in Chambery. For wall scale simulations, we propose to develop an original approach based on the Lattice Boltzmann method. The originality is to work in 3D and multiphysics, representing the coupled transfers of air, heat and water vapor in the confined cavities and permeable media. The building performance simulation tool will take into account heat-air-moisture balances in walls and air zones. The project will go above the standard pressure models in and will provide models coupling the hygro-thermal fields in the wall, with the paths of air infiltration. Then, the developed numerical models and the experimental results will be used for practice-related issues. This ambitious research program, joining research laboratories and industry aims to answer practical questions:
-How to precisely evaluate the impact of hazardous air infiltrations on the performance of timber frame construction?
- Which innovations can be proposed to timber frame constructions, in order to preserve their thermal and environmental qualities and simplify the cost and the construction process?
The consortium is composed of two public research laboratories, experts in transfer simulations and building behavior: CETHIL and LOCIE,, two technical centers CSTB (construction) et FCBA (timber) and industrials: consulting companies Albedo Energie and Wigwam, SDCC (timber frame construction company), EDF R&D (modelling) and Aldes (ventilation).