Thermal decrement factor is inversely proportional to thermal lag. In other words, the higher the thermal lag, the lower the decrement factor. The reduction in cyclical temperature on the inside surface compared to the outside surface is know as thermal decrement. It is the attenuation of a wave traveling through an element of the building structure. For low thermal mass capacity building, the decrement factor will be 1.0 (100%). It will decrease as the thermal mass increases.
In the case of a 20 degree diurnal variation in external surface temperature, a material with a decrement factor of 0.5 would experiences only a 10 degree variation in internal surface temperature, which means only half the heat wave amplitude will pass through the wall.
Units and Measures
Thermal decrement factor is a fractional measure of the relative reduction in cyclical temperature on the inside surface compared to the outside surface of a material due to thermal lag.
Following are some typical thermal decrement values for a range of common building materials:
|Insulated Brick Veneer||0.2|
|Double Brick 250mm||0.14|
|Autoclaved Aerated Concrete (AAC) 250mm||0.14|
|Compressed Earth Blocks 250mm||0.09|
|Sandy Loam 1000mm||0.03|
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