What is decrement delay? – Can I Just Ask? | Ep.31

Decrement delay describes the way a building’s fabric slows and reduces the transfer of heat from the outside to the inside — and it’s becoming an increasingly important concept as summer overheating replaces winter heat loss as the more pressing comfort challenge. It has two components: the decrement factor and the delay. The decrement factor is a number between zero and one that expresses how much of an external temperature swing is transmitted to the inner face of a wall or roof. If the outside temperature varies by 20 degrees across a day but the inside of the wall only varies by one or two degrees, the decrement factor is around 0.9 — meaning the fabric is absorbing and damping almost all of that variation rather than passing it through.

The delay is the time shift between the peak external temperature and the corresponding peak on the internal face — essentially how many hours the fabric holds onto that heat before it arrives inside. This is determined by the material’s thermal conductivity, its density, and its specific heat capacity: the energy required to raise the temperature of the material itself. A standard wood fibre build-up achieves a delay of around 15–16 hours, meaning the heat from a hot afternoon doesn’t reach the interior until the early hours of the following morning, by which point external temperatures have already dropped and the building can be ventilated to purge it. By comparison, mineral wool achieves around three hours, PIR around six, and polystyrene — the worst performer — as little as two to two and a half hours.

The practical implication is significant. Two build-ups with identical U-values can perform completely differently in summer: a lightweight, low-conductivity insulation transmits the thermal wave quickly and with relatively little attenuation, while a dense, high-mass insulation like wood fibre slows and flattens it dramatically. In a climate where sharp heat events are increasingly frequent, and in buildings where air conditioning is neither desirable nor affordable, decrement delay is one of the most effective passive tools available for maintaining a stable, comfortable internal environment year-round.