What is thermal mass?

Thermal mass is a material's ability to absorb, store, and release heat. Dense materials like 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.

What is thermal admittance?

Thermal admittance (Y-value, W/m2K) measures how much heat a surface absorbs from a room when temperature cycles over 24 hours. High admittance materials (concrete: ~6 W/m2K) absorb more heat than low admittance materials (plasterboard: ~1.5 W/m2K).

What is the decrement factor?

The decrement factor (0 to 1) indicates how much the peak external temperature is reduced as it passes through a wall. A value of 0.3 means only 30% of the external temperature swing reaches the inside. Lower values mean better peak temperature attenuation.

Time lag is the delay (in hours) between the peak external temperature and when it reaches the internal surface. Dense masonry walls can have time lags of 8-12 hours, meaning afternoon heat arrives inside during cooler evening hours when it can be ventilated away.

How does thermal mass help in the UK climate?

In the UK, thermal mass primarily helps prevent overheating in summer by absorbing solar gains during the day. In winter, exposed thermal mass absorbs heat from heating systems and radiates it back over time, reducing temperature fluctuations and improving comfort.

TradeCalculator

Thermal Mass Calculator

Calculate thermal admittance, decrement factor, and time lag

Material

Thickness

Area

How We Calculate This

Select a material to load its density, specific heat capacity, and thermal conductivity. Enter the thickness and area. The calculator computes thermal diffusivity, then derives the thermal admittance (Y-value), decrement factor (peak temperature reduction), time lag (hours delay), and total heat storage capacity in kWh for a given temperature swing. The SAP kappa value is also provided.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.