Time Lag and Decrement Factor

The time delay due to the thermal mass is known as a time lag. The thicker and more resistive the material, the longer it will take for heat waves to pass through. The reduction in cyclical temperature on the inside surface compared to the outside surface is knows and the decrement. Thus, a material with a decrement value of 0.5 which experiences a 20 degree diurnal variation in external surface temperature would experience only a 10 degree variation in internal surface temperature.

timelag_decrement_factor thickness_decrement_factor

This effect is particularly important in the design of buildings in environments with a high diurnal range. In some deserts, for example, the daytime temperature can reach well over 40 degrees. The following night, however, temperatures can fall to below freezing. If materials with a thermal lag of 10-12 hours are carefully used, then the low night-time temperatures will reach the internal surfaces around the middle of the day, cooling the inside air down. Similarly, the high daytime temperatures will reach the internal surfaces late in the evening, heating the inside up.

In climates that are constantly hot or constantly cold, the thermal mass effect can actually be detrimental. This is because both surfaces will tend towards the average daily temperature which, if it is above or below the comfortable range, will result in even more occupant discomfort due to unwanted mean radiant gains or losses. Thus in warm tropical and equatorial climates, buildings tend to be very open and lightweight. In very cold and sub-polar regions, buildings are usually highly insulated with very little exposed thermal mass, even if it is used for structural reasons.

timelag_vs_thickness wall_surfac_temperature influence