A study entrusted by the public authorities and timber companies to four partners – FCBA, Nobatek, Cerema and the CSTB – produced new methods for calculating moisture transfer in the interior walls of timber construction. This will improve assessments of summer thermal comfort in this type of building.
The low thermal conductivity of wood and its relatively low weight give it low thermal inertia. This limits the effects of thermal energy storage and thermal phase shift, and promotes summer thermal comfort. Also, wood allows water vapor to migrate, which can be beneficial. These findings suggest the possibility of adapting current rules for calculating thermal inertia to timber-framed structures.
To move forward on the subject, the Department for Housing, Urban Planning and Landscape (DHUP), the Professional Committee for the Development of French Furniture and Timber Industries (Codifab) and the association France Bois Forêt (FBF) commissioned a team consisting of the French Technology Institute for Forestry, Cellulose, Wood Construction and Furniture (FCBA), the private technology research center Nobatek, Cerema and the CSTB to conduct a research project.
The mission centered on several tasks, with the CSTB participating in two of them: assessment of hygroscopic properties and an analysis of the impact of timber-framed structures on the thermal inertia of the building.
It developed a method for factoring in a new phenomenon: hygroscopic inertia in interior walls. This involves determining whether the transfer of water vapor between a wall with timber-framed structure and the indoor air of the building results in heat transfers that affect interior summer thermal comfort. The CSTB created a simplified heat transfer model and integrated it into its thermal computing core. It is now possible to measure their impacts on indoor comfort, particularly on ambient temperature, with respect to regulatory requirements and building performance objectives.
At the same time, the CSTB studied the impact of the inertia of the studs of the timber-framed structure on the building. The simplified approaches developed as part of the project faithfully reproduce the results of the detailed models for the cases under study. The results concur with those obtained using the current regulatory model to measure the impact on thermal comfort.
Consideration of wood-related hygrothermal properties improves measurement of the impact of this construction material on summer thermal comfort. The effect on thermal performance of the envelope of other biosourced materials, such as hempcrete and mud brick, can also be quantified.
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