In this work, biodegradable foams were produced by extrusion of raw materials that are economically important in South America, such as cassava starch and sugarcane bagasse fibre. The addition of polyvinyl alcohol at high proportions (40%) increased the expansion index and led to a significant reduction in the water adsorption of starch foams, with a general improvement of the foam properties. The addition of high levels of fibres (40%) led to denser and more compact structures with a yellowish aspect. The mechanical properties were affected by both relative humidity (RH) of storage and foam formulation. In general, as the RH increased, the foam strength decreased for all formulations. At higher RH (90%), fibre addition in high proportions resulted in a reinforcing effect on starch foams, improving their compression strength. Between the foams produced with the ternary mixture of starch, PVA, and fibres, the sample formulated with 66.6% starch, 6.7% fibres and 26.7% PVA showed the best combination of mechanical and expansion properties. The use of these materials as loose-fill packaging requires further research, mainly to improve their expansion properties.