The article presents the results of ISOBIO, a European project that ran from 2015 to 2019, financed under the Horizon2020 program. The project developed new insulating materials and plasters from plant fibres, agricultural residues, and bio-based binders, with the aim of reducing both the embodied energy in the materials as well as operational energy consumption. The materials were incorporated into a prefabricated structural insulated panel, and installed and monitored in 2 demonstrators in the UK and Spain. The panel's hygrothermal performance was monitored and validated dynamically with a WUFI Pro model (to EN 15026 [1]), showing very reliable hygrothermal performance and a close correlation between modelled and measured values.