Thermal Mass and Title 24
Thermal mass consists of exposed tile floors over concrete, mass walls such as stone or brick, and other heavy elements within the building envelope that serve to stabilize indoor temperatures. Thermal mass acts for temperature much like a flywheel – it tends to keep things warmer when it is cold outside and keep things cooler when it is hot outside. In California’s central valley and desert climates, the summer temperature range between night and day can be 30°F or more and thermal mass can be an effective strategy to reduce daytime cooling loads.
When thermal mass exists in exterior walls, it works to stabilize temperatures in two ways. First, there is a time delay between when the outside temperature of the wall reaches its peak and when the inside of the wall reaches its peak. For an 8-inch to 12-inch concrete wall, this time delay is on the order of 6 to 10 hours. Second, there is a dampening effect whereby the temperature range on the inside of the house is less than the temperature range on the outside of the house.
Interior thermal mass is especially important in passive solar buildings. Passive solar buildings have large areas of south-facing fenestration. The large window area means that solar gains are quite high on winter days when the south sun is low in the sky (passive solar buildings should have south overhangs to block the sun in the summer). Large window areas also contribute to increased heat loss in the evening and at night. Without thermal mass, the south glass would create uncomfortably warm temperatures in the day and uncomfortably cold temperatures at night. Thermal mass in passive solar buildings works best if it ispositioned so that the sun strikes it during the day. It can then better absorb the solar radiation for release later in the day when the space begins to cool. The Title 24 energy standards offer a generous compliance credit when thermal mass is modeled.
(Excerpted from the CEC Title 24 Residential Compliance Manual)