Building office building consumes large amount ofenergy to operate its air conditioning system. Part ofthis consumption results from the heat that transfersthrough the building envelope, especially the envelopewith glass as a major component (curtain wall). Thisresearch aims to find a means to improve energyconservation in office buildings by improving buildingsenvelope system. The research considers the possibilityof reducing the total air conditioning system of thebuilding, along with the economic feasibility ofvarious options. The appropriate object building forthe research is an old office building equipped withmonolithic glass walls. The Thai Farmers Bank Building,Phaholyothin Branch was selected as the building forthe case study. The research starts with the study of the factorsthe influence the heat transfer through the buildingenvelope. The properties of various types of glass thatare used for the building envelope are studied. Theresearch focuses on two major options for the study ofbuilding envelope improvement ,i.e., replacing with anew type of glass, or improving the existing glass byapplying heat-shield film or heat insulator. Theresearch is done by installing the sample glass on anobject building's wall facing to the South. The insidebuilding's temperature is controlled by the airconditioning system. The outside atmospherictemperature and the inside and outside glass surfacetemperatures are measured. The results are analyzed bycomparing the differential temperature of various typesof glass with the outside atmospheric temperature. Thecomparison of heat transfer of glass measurements andthe heat transfer calculated using the standard of theAmerican Society of Heating, Refrigerating and Air -Conditioning Engineers(ASHRAE) is also studied . The study shows that the temperature inside theobject building is as high as the outside atmospherictemperature. This implies that the building envelope ofthe object building allows a lot of heat to transfer orin other words, the glass has a high heat transfercoefficient. From the calculations, the energyconsumption of 5,241,743 kilowatt-hours per year isrequired by the air conditioning system to reduce theheat that enters the building. From the study, theoption of installing heat insulator inside the existingglass is not recommended because the temperature at thesurface of the glass will increase and the glass maybreak down. The heat-shield film can reduce the heattransfer level, but the temperature at the insidesurface of the glass is still high. A lot of energy isstill required to compensate for the high temperatureat the glass surface. For the option of installing theinsulating glass, the study has found that the glassinside surface temperature is low, but the outsidesurface temperature of the existing glass is highbecause the heat transfer coefficient is low. From thecalculations, the heat transfer through the insulatingglass into the building is less than the heat transferthrough the existing glass. After comparing all optionsby considering their ability to reduce heat transferinto the building and the economic feasibility, thestudy recommends the use of the Reflective Low-E typeof glass. The calculations show that this type of glasscan reduce the heat transfer though the buildingenvelope by 58.94% and the payoff period is only 5years.