In addition to holding up the roof and keeping out the rain, walls act as thermal dividers between conditioned interior space and the outdoor environment. Their thermal function is complex, because heat is absorbed from solar radiation and hot air, radiated to cold skies and transferred to cool air, conducted to the interior, stored within the walls, and absorbed from and released to the interior. The directions and magnitudes of these heat flows are constantly changing in the environment, and the amount of heat stored and released within the mass wall changes accordingly.
Temperature damping is a characteristic of mass construction that describes the way exterior temperatures and heat flows affect the interior of a building. For example, in the summertime, the temperature on the outside surface of a wall fluctuates widely, from a high temperature during the sunny midday to a low temperature in the middle of the night. This can be thought of as a temperature "wave." The inside surface of the wall, however, will experience a much smaller temperature fluctuation or wave. The wall "damps," or reduces, the amplitude of the temperature wave. The narrower temperature fluctuation on the interior means that the cooling loads are lower, and the inside of the building is more comfortable. The damping depends on both the insulation and the heat capacity of the construction. For two walls with the same insulation, the more massive wall will display greater temperature damping characteristics.
Another result of thermal mass is that the time of peak temperatures and heat gains on the interior is delayed, compared to the peak times on the exterior. This phenomenon is called thermal lag. With concrete and masonry walls, the time of highest interior temperature will be three to eight or more hours later than the time of highest exterior temperatures. As a result, peak cooling loads are delayed to cooler times of the day when the air conditioning equipment operates more efficiently, or when the building is unoccupied and not air conditioned at all.