Heating, Ventilation and Air-conditioning (HVAC) ducts are often lagged on the outside of the duct wall with a highly porous material, covered in turn with a thin impervious jacket. This arrangement is used to provide thermal insulation as well as the breakout noise reduction. In this paper, a prediction method based on the four-pole parameters is discussed to evaluate the lagged duct performance in terms of the breakout noise reduction. Transfer matrix of inner and outer duct wall in the transverse direction is calculated using the wall admittance as a function of the axial wave number. Assuming a common axial wave number ensures coupling between the acoustic waves and structural waves. It is a function of the wall admittance. Calculation of the common axial wave number requires an iterative procedure with the starting value of air acoustic wave number. Two to three iterations are required for convergence of the solution. Considerable difference between common axial wave number and the air acoustic wave number is observed near the coupling frequency region of the duct wall. The overall transfer matrix is developed in order to relate the inside plane wave pressure to the outer acoustic particle velocity. This is combined with the radiation impedance of the duct to predict the transverse transmission loss. Net Insertion loss of the lagged duct is calculated using the difference of transverse transmission loss of the lagged duct and the corresponding bare duct. Predicted values of the insertion loss are compared with the measured values from literature. Finally, results of parametric studies are presented.