Earth plasters have been used as a protective coating for buildings but, due to their low strength and low resistance to weather conditions, they have been abandoned for more resistant materials which in return lack vapour permeability. Earth plasters have usually a high moisture sorption rate, and their water vapour permeability is high, allowing the transfer of humidity through the material. These properties make them an interesting material for controlling vapour movement in humid rooms. Improving their strength can be done by adding aggregates and/or fibres, but the real impact of using one type or another of fibres or aggregate is unknown. This research aims to understand the consequence of the choice of fibre or sand in the improvement of strength of plasters and the conservation of the plaster hygro-thermal properties. Properties of plasters using alternative fibres or aggregates such as wool, cow hair, pine needles, sand aimed for concrete mixes, or not properly graded sand have been compared to plasters made more traditionally with chaff fibres and masonary sand.