intimate contacts were formed between the fine particles, which improved the mechanical properties of the board. However, higher amounts of fine particles with larger surface area need more adhesive, which could decrease the resin distribution of unit area of the straw particles, thus reducing the mechanical properties of the boards [50]. Decreasing the particle size from L- to N-type boards resulted in the IB value being reduced by 30% approximately. In addition, because intimate contacts were formed between the particles, which caused the water to penetrate into the boards more difficultly, the TS decreased with increasing particle size (Table 6). As a result, the L-type particle size of rice straw is appropriate for manufacturing thick insulation material.