Static vs dynamic hygrothermal simulation for cellulose-based insulation in existing walls: A case study comparison

When dealing with energy-saving topics, it is increasingly common to focus on the efficiency of existing systems, rather than adopting new ones. In the specific case of the building envelope this practice is supported by the difficulty in completely replacing opaque components of the envelope, such as external walls or roofs. This work involves the renovation of a cavity wall, with the aim of improving its energy performance. A traditional cavity wall has been modified by blowing a bio-based insulating material obtained from cellulose flakes inside the air cavity gap. Although an operation of this type leads to a significant increase in thermal performance of the wall, it is not equally obvious that it is effective in terms of humidity and vapor condensation. The purpose of this work is to evaluate the effect of the blowing process on the hygrometric performance of the opaque component to ensure correct compliance with the performance parameters established by Italian legislation in terms of vapor transmission and condensation phenomena. In order to study the hygrometric behaviour, a numerical model of the construction was developed and simulated. The simulation involved two different regulatory approaches, which were compared: a first calculation was carried out in steady-state conditions, according to the UNI EN ISO 13788 standard (ISO, 2012). Afterwards, a dynamic simulation following the UNI EN 15026 standard was performed (CEN, 2007). The results obtained by both the methods were analysed and compared. The results demonstrate that by adopting the calculation procedure in steady-state conditions, the phenomenon of interstitial condensation occurs. A different result is obtained by applying the calculation method in dynamic regime, according to which the vapor would not condense inside the structure.