Based on results from a one-year intensive monitoring project of a Northern New York commercial building with energy-conserving design features, a thermal storage project was undertaken to optimize the design of a thermal mass storage system for a moderately sized commercial building and transfer the technology to the commercial building sector. A generic commercial building design of 27,000 square feet (2508 m2) was selected for the optimization project. Several different types of thermal mass designs were considered as potentially practical for a commercial building. These included a “sandmass” design such as the mass incorporated in the previously monitored commercial building mentioned above, a foundation slab of sufficient thickness to serve as a significant building thermal mass, and the use of poured cement in interior wall and floor construction. Five different office building thermal designs were selected which represented various thermal storage features and two different building insulation levels (R10 and R20). Energy performance of the five thermal designs was modeled in building energy simulations using DOE 2.1C (Department of Energy 2.1C) energy simulation code. Results of the simulations showed a reduction in peak heating and cooling loads would be experienced by the HVAC equipment. The reduction in peak heating and cooling loads was anticipated because thermal mass within a building serves to average peak heating and cooling loads due to the capacity of the thermal mass to store and release heat from all building heat sources over a period of time. Peak heating loads varied from 1972 kBtuh (578 kW) for the R-10 light construction base case to a minimum of 980 kBtuh (287 kW) for the R-20 heavy construction sandmass storage case. Peak cooling loads dropped from 772 kBtuh (226 kW) for the R-20 light construction case to 588 kBtuh (172 kW) for the R-20 heavy construction sandmass storage case. Results of the simulations also showed annual energy savings for the high thermal mass designs. Energy savings varied from 20 percent [16.0 kBtu/ft2 (50 kWh/m2)] for the R-10 high thermal mass design in comparison to its base case to 18 percent [12.2 kBtu/ft2 (39 kWh/m2)] for the R-20 high thermal mass design in comparison to its base case. The annual energy savings are due to the ability of the thermal mass to absorb heat from all sources of heat generation (lights, occupancy, solar, and auxiliary) during occupied periods and release the heat during unoccupied periods. An optimized thermal design was developed based on results from the DOE 2.1C simulations. The initial cost for the optimized thermal storage design is lower than the initial costs for light construction office buildings, since the lower initial cost of the down-sized HVAC system for the optimized thermal storage design more than offsets the increased cost of wall and floor systems incorporated in the optimized design. Annual energy savings are realized from the high thermal mass system in both cooling and heating modes due to the interaction of building HVAC systems operation in the simulated 27000 ft2 (2508 m2) office building. Annual operating savings of $3781 to $4465 per year are estimated based on simulation results.