Define thermal mass and explain its effect on a building's energy performance.

• A. Thermal mass is a material's ability to absorb, store, and release heat; it stabilizes indoor temperatures by delaying heat transfer.
• B. Thermal mass is the surface color of materials affecting heat absorption.
• C. Thermal mass is the insulation value measured in R per inch.
• D. Thermal mass is the speed at which a material heats up in direct sunlight.

Answer: (A)

Thermal mass is a material's ability to absorb, store, and release heat, which comes from its high heat capacity and density. Because such materials take in heat when conditions are warm and then release it slowly when they cool, they stabilize indoor temperatures by delaying heat transfer between the outside and the interior. This delaying of heat flow reduces peak cooling loads in hot weather and can lower heating needs in cold weather when paired with good insulation. In practice, dense materials like concrete, brick, or stone placed where they can absorb solar gains or interior heat act as a thermal reservoir, smoothing temperature swings over the day and night and lowering overall energy use for space conditioning. The other descriptions touch on surface color, insulation value, or the speed of heat rise, which relate to different aspects of heat transfer but do not define the capacity to store and release heat.