Thermal mass (heavyweight materials like concrete, masonry, or water) absorbs and releases heat moderating temperature swings. During the day, thermal mass absorbs solar heat. At night, stored heat is released, reducing heating load. In climates with significant day-night temperature difference, thermal mass reduces HVAC load. Thermal mass effectiveness depends on exposure to heat sources and sinks. Internal walls and floors benefit from air circulation distributing stored heat. Insulated thermal mass may not exchange heat effectively with the space. Direct solar exposure through windows is the most effective heat source for thermal mass. Dark surfaces absorb more solar energy than light surfaces. Thermal mass material properties include specific heat capacity and conductivity. High specific heat capacity stores more energy per degree temperature change. High conductivity allows rapid heat transfer through the material. Concrete and masonry provide good thermal mass properties. Water in pools or thermal tanks provides even better thermal storage. Phase-change materials provide storage at specific temperatures. Appropriate thermal mass sizing for the climate improves performance. Excessive thermal mass adds cost without benefit. Insufficient thermal mass limits passive heating/cooling benefit. Design optimization balances thermal mass size with cost and space constraints.