Concerning rock wool insulation, which is a typical example for fibrous insulations, an empirical equation which relates the bulk density ρ and the effective thermal conductivity λ is proposed. The equation is derived based on the thermal conductivity data of only four kinds of specimens with different bulk densities that are obtained by the Cyclic Heat Method under evacuated conditions. The predicted values are shown to agree well with the measured results obtained by the Guarded Hot Plate Method over a wide range of the bulk density (ρ=30～250kg/m^3). Also, a fairly close agreement between the predicted and literature values was obtained. Furthermore, it is shown that the conventional approximation method, in which coefficients included in an empirical equation are simply determined by using measured values, can not predict the effective thermal conductivities within practical accuracy over a wider range of bulk densities.