The term ‘decrement delay’ refers to the time it takes for heat to pass through an element of a building (such as an external wall or roof). Typically this is taken to be the delay in hours between the peak temperature of the outer surface of the element on a summer day and the resulting peak temperature of the internal surface.
This concept reflects increasing awareness that the thermal behaviour of buildings is dynamic, rather than static, and that thermal mass, as well as thermal insulation, has a significant impact on the energy efficiency of a building. For example, two buildings with identical U-values may perform very differently depending on their decrement delay, with a longer delay likely to reduce peak loads on building services systems.
The term ‘decrement factor’ refers to the amount by which conditions are moderated by an element of a building. So in the case of the peak temperature on the outer surface of a building on a summer day, this would be the amount by which the peak is reduced by the time it reaches the inner surface. It is expressed as the ratio between the internal surface cyclic temperature variation compared to the external surface.
To reduce summer overheating, a low decrement factor is required, and a decrement delay of 6 to 12 hours.
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