Abstract
Three different insulating scenarios were investigated using Energy-plus software, to study the effect of using expanded polystyrene boards on building energy consumption at the Libyan Center for Solar Energy Research and Studies. The three insulation scenarios with different polystyrene insulation thicknesses and densities were investigated. Polystyrene total cost, Payback period, total and net money saving using insulation, as well as the percentage gain were calculated for each case in the three investigated scenarios for 50 years. The most money saving scenarios after 50 years is by insulating the building roof and external walls with an average insulation thickness of 7 cm, reaching about 78000 L.D and a tariff of 0.9261 L.D /kWh. While the least money saving is the roof scenario with an insulation thickness of 2.5 cm, reaching more than 29000 L.D and the same tariff. The highest percentage gain values are 76 %, 103 % and1090 % at the roof scenario, 2.5 cm insulation thickness and energy tariffs of 0.15 L.D /kWh, 0.1732 L.D/kWh and 0.9261 L.D/kWh, respectively. While the lowest percentage gain values are about 17.5 %, 35.7 % and 725 % at the wall scenario 10 cm insulation thickness and energy tariffs of 0.15 L.D/kWh, 0.1732 L.D/kWh and 0.9261 L.D/kWh, respectively. the payback period of total insulation cost is significantly dependent on insulation thickness and energy tariff. These results show that low energy tariff (current energy tariff) values economically are not cost effective regarding the payback period. Consequently, prices do not motivate residents to save energy with the use of thermal insulation unless actual energy prices are considered.

Key words: Energy Plus, Polystyrene, insulation thickness, insulation density.