Thermal Mass Calculator

Thermal mass is a material’s ability to absorb, store and release heat. Dense materials such as concrete, brick and stone have high thermal mass. They absorb excess heat during warm periods and release it when temperatures drop, naturally stabilising indoor temperatures.

Thermal admittance (Y-value, W/m²K) is the heat flow leaving the internal surface into the room per 1 K of internal temperature swing over a 24-hour cycle (BS EN ISO 13786). It is calculated including the internal surface film (Rsi = 0.13 m²K/W, horizontal heat flow per BS EN ISO 6946), not on the bare material, so it depends on thickness and the room-side finish. Examples from BS EN ISO 13786 / GreenSpec: a 100 mm dense-aggregate-block wall with wet plaster is about 5 W/m²K, the same wall dry-lined with plasterboard about 2.7 W/m²K, and a lightweight timber-frame wall about 1 W/m²K. A single exposed slab of dense concrete or stone sits at roughly 5–6 W/m²K — admittance rises with thickness towards a constant thick-slab limit of about 5.5–6 W/m²K and never reaches the much higher bare-material √(k·ρ·c·ω) figure, because the internal surface film caps it. Higher admittance means the surface soaks up more heat over the day.

The decrement factor (0 to 1) is the ratio of the peak heat flow out of the external surface, per 1 K of external temperature swing, to the steady-state heat flow through the element (BS EN ISO 13786). A value of 0.3 means the peak swing is attenuated to 30% by the time it reaches the far side. Lower values mean better peak attenuation. A thin, bare, conductive slab attenuates little (decrement near 1); a thick or well-insulated build-up attenuates strongly. This calculator models a single homogeneous layer, so adding insulation in a real wall will lower the decrement well below the single-layer figure.

Time lag (decrement delay) is the delay, in hours, between the external temperature peak and the resulting peak heat flow at the far surface (BS EN ISO 13786). Thick dense masonry can give time lags of around 8–12 hours, so afternoon heat arrives during the cooler evening when it can be ventilated away. Thin or lightweight elements give only a 1–3 hour lag.

The bare-material expression √(k·ρ·c·ω) is the thermally-thick limit with no surface film and ignores the internal surface resistance. By definition admittance is measured at the internal surface, so the BS EN ISO 13786 method applies the internal surface film (Rsi = 0.13 m²K/W) in series via the complex heat-transfer matrix. For 100 mm dense concrete the bare value is about 17 W/m²K but the correct internal ISO 13786 admittance is about 5.3 W/m²K; for a 200 mm slab the ISO 13786 reference value is 5.7 W/m²K (QuickField ISO 13786 case D.1). Admittance therefore rises gently with thickness towards the thick-slab limit and stays in the 5–6 W/m²K band for a dense single leaf — it does not climb to 10–11, which is the external-face admittance, not the room-side value. This calculator uses the ISO 13786 method, matching the CIBSE Simple Dynamic Model and The Concrete Centre / Arup dynamic-properties tool.

In the UK, thermal mass mainly helps prevent summer overheating by absorbing solar and internal gains during the day and releasing them at night when windows can be opened (BS EN ISO 13786 / CIBSE TM52 overheating assessment). In winter, exposed thermal mass absorbs heat from the heating system and re-radiates it slowly, reducing temperature swings and improving comfort.