Bio-based insulation materials have the potential to make a significant contribution to the reduction in the global warming potential of the construction industry world-wide. They contribute in two ways. First they provide the opportunity to reduce the embodied energy in the fabric of buildings. They do this because they are renewable and recyclable. Plant-based insulation materials also sequester carbon dioxide through photosynthesis, sealing up atmospheric carbon dioxide for the life-time of the building. Second they are able to reduce the in-use energy consumption of buildings in more ways than by simply reducing energy transmission. They have the ability to buffer heat and moisture, which is most evident in dynamic situations. This paper discusses the hygrothermal performance of bio-based insulation materials, examining the hygrothermal effects associated with their vapour activity. The incremental performance offered by these materials is not allowed for in building regulations, nor is it readily accounted for in many commercially available building physics models. The paper discusses the reasons for this and identifies the need for the transient performance of bio-based insulation materials to be taken into account, because this will better reflect their actual contribution to the energy performance of a building.