Temperature Damping & Thermal Lag

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 response is complex. The walls absorb heat from solar radiation and hot air, radiate this heat to cold skies and conduct heat to cooler air. They also take heat conducted to the interior and store it within the walls to be released to cooler surfaces. 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 of the wall changes accordingly. In addition, the greater the walls mass, the greater the energy that is stored.

Temperature damping is a characteristic of high mass construction. This thermal mass effect describes the way variable exterior temperatures and dynamic heat flows impact 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 near midday to a low temperature in the middle of the night. This diurnal temperature cycle is not directly transferred to the interior surface of the wall. The interior wall surface will experience much smaller temperature fluctuations. The mass wall “damps,” or reduces, the amplitude of the interior wall temperature. The lower temperatures on the interior means that the cooling loads are lower, and the inside of the building is more comfortable. The degree of damping depends the thermal resistance of the wall and its heat capacity. For two walls with the same thermal resistance, 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. This phenomenon is called thermal lag. With concrete and masonry walls, the time of highest interior temperature will be later than the time of highest exterior temperatures (typically three to eight hours). This often results in peak cooling loads being delayed to times when the air conditioning equipment operates more efficiently (due to lower outside temperatures), or when the building is unoccupied and not air conditioned at all.

Next: Temperature Gradients