There is a need to consider insulation materials which have a low to zero embodied carbon to meet climate change mitigation targets of the built environment. Considering circular principles recycled materials must be used where possible to reduce the use of virgin raw materials and landfill. This paper discusses the evaluation of agricultural crop waste from farms in Wales, UK for use as substrates for fabricating bio-based insulation board prototypes, following circular principles. Several waste crops used for animal substrate compositions have been evaluated. The production process at lab scale has grown and fabricated ten biomaterial sample slabs measuring 300 mm (width) by 380 mm (length) by 30 mm (depth). The processes for cultivation, fabrication and testing the thermal resistance of the biomaterial slabs are discussed. Fabricating and using an affordable hotbox to test the thermal performance of one biomaterial slab has recorded a U-value of 0.525 W/m2K, significantly better than 40 mm (depth) of wood fibre insulation U-Value of 1.05 W/m2K. The next steps in the research are discussed including scaling up the growing and fabrication of the biomaterials for use as insulation materials in timber-frame panel sizes, typically used in UK dwellings, and evaluating the acoustic performance potential.