In this study, the economic and environmental impacts of insulation material are determined for different sizes of heating, ventilation and air conditioning (HVAC) duct. The optimum insulation thickness (OIT), energy-saving (ES) and payback period (PP) for HVAC duct are estimated using Life cycle cost (LCC) analysis. The analysis considers coal, natural gas (NG), liquefied petroleum gas (LPG), fuel oil (FO), bagasse, rice husk (RH) and geothermal as an energy source and the fiberglass as an insulation material. The results indicate the OIT and PP for an HVAC duct increase with the size of the duct while ES decreases. The maximum value of OIT, ES and minimum value of PP for different sizes and energy sources are determined as 48.27 mm in size A (300 mm) and NG, 84.91% in size E (500 mm) and LPG, and 0.2035 years in size A and NG, respectively. Additionally, the environmental analysis results indicate emission of CO2, CO and SO2 decreases with insulation thickness. The maximum value of CO2 and CO emission is determined for size E and NG i.e. 81.8% and SO2 emission for size E and FO i.e. 76.66%, respectively.