when dealing with porous structures, effective thermal conductivity is usually helpful specially in numerical simulations, resulting in fewer mesh cells and mitigated calculation time. however, the relation between porosity and thermal conductivity of each phase is quite structural dependent (for example unit cell of desired material or the way particles of each phase are in contact with each other). you may find quite fruitful information in this regard in the book: ''principals of heat transfer in porous media'' by Massoud Kaviany or his other researches.