Rigid PU foams are valuable in many construction applications. Kenaf is a bast fiber plant where the surface stem skin provides bast fibers whose strength-to-weight ratio competes with glass fiber. The higher volume product of the kenaf core is an under-investigated area in composite applications. The naturally porous structure of kenaf-core provides a novel reinforcement particle. In this work, foams of rigid polyurethane with 5, 10, and 15% kenaf-core were formed. To date efforts at using it as a reinforcement have proven largely unsuccessful. This was mirrored in this effort when free expansion of the foam was utilized. However, introducing constraint during foaming resulted in reinforcement. The environmental scanning electron microscopy is used in conjunction with in-situ microCT compression to capture the change in void fractions before and after deformation. The results show that free foaming resulted in poor reinforcement while a constrained expansion on the foam increased the reinforcement potential of the kenaf core. POLYM. ENG. SCI., 55:132-144, 2015. (c) 2014 Society of Plastics Engineers