What is thermal mass?

Parts of a building with high heat capacity, such as concrete or brick. They store daytime heat and release it at night, smoothing indoor temperature — both by delaying and damping outdoor swings reaching inside (time lag, decrement) and by absorbing indoor temperature swings themselves.

How much does a wall delay heat?

For a daily cycle, 200 mm of concrete delayed it about 5.7 h, 300 mm 8.3 h and 400 mm 11 h, scaling the amplitude by 0.51, 0.30 and 0.18. A lightweight gypsum-and-insulation wall delayed only about 1 h with almost no damping.

Insulate inside or outside?

With the same U-value, room-side behaviour differs greatly. 200 mm concrete with 100 mm insulation outside has a room-side admittance of 5.9 W/m²K, so the concrete absorbs indoor swings; with insulation inside it's 0.34 W/m²K, cutting the concrete off from the room. External insulation is better for using thermal mass.

Thermal Mass Effect

Category: Thermal – HVAC & Buildings | Revised 2026-10-01

CAE visualization for thermal mass theory - technical simulation diagram

Heat storage (thermal mass) effect

Theory: what the U-value doesn't tell

Overview

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Wall insulation is compared by U-value. Is thermal mass something separate?

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It's separate. U-value describes heat flow at a steady temperature difference and can't capture behaviour when temperatures vary in time. How a wall responds to the daily cycle depends on how much it delays outdoor changes (time lag), how much it shrinks them (decrement factor) and how much heat it can exchange on the room side (admittance). These depend strongly on heat capacity and layer order. Let's compare concrete and lightweight walls, and internal versus external insulation, with transfer matrices.

Transfer matrix

$$ \mathbf{Z}_{layer} = \begin{pmatrix}\cosh\xi(1+i) & -\dfrac{\delta}{k(1+i)}\sinh\xi(1+i)\\[4pt] -\dfrac{k(1+i)}{\delta}\sinh\xi(1+i) & \cosh\xi(1+i)\end{pmatrix},\quad \xi = \frac{d}{\delta},\ \delta = \sqrt{\frac{\alpha P}{\pi}} $$

Layer matrices are multiplied in order from the inside surface resistance (0.13 m²K/W) to the outside one (0.04 m²K/W). δ is the penetration depth for period P (24 h).

Time lag and decrement factor

$$ Y_{12} = -\frac{1}{Z_{12}},\qquad f = \frac{|Y_{12}|}{U},\qquad \Delta t = \frac{P}{2\pi}\arg\!\left(\frac{1}{Y_{12}}\right) $$

$Y_{12}$ is the periodic thermal transmittance — the room-side heat-flux amplitude per kelvin of outdoor swing; the decrement factor f divides it by the steady U-value.

Coffee Break Trivia

The wisdom of thick walls

Traditional houses in deserts and around the Mediterranean used thick stone or earth walls to delay daytime heat into the night and carry night coolness into the day. The admittance method, developed in Britain and elsewhere in the 1960s and 70s, put this empirical wisdom into equations and leads to today's transfer-matrix method in ISO 13786. Even in well-insulated modern buildings, the placement of thermal mass is being revisited to curb temperature rise from summer sun and internal gains.

Worked example: daily cycle

Concrete (1.8 W/mK, 2,400 kg/m³, 1,000 J/kgK), 100 mm EPS insulation (0.035 W/mK); lightweight wall of 12.5 mm gypsum + 100 mm mineral wool + 12.5 mm gypsum.

Comparison of walls

Wall	U-value	Decrement	Time lag	Room-side admittance
Concrete 200 mm	3.56	0.51	5.7 h	5.70
Concrete + external insulation	0.32	0.17	7.9 h	5.94
Internal insulation + concrete	0.32	0.33	7.0 h	0.34
Lightweight (gypsum + mineral wool)	0.36	0.99	1.0 h	0.90

U-value and admittance in W/m²K.

Concrete thickness

Thickness	Decrement	Time lag
100 mm	0.83	2.8 h
200 mm	0.51	5.7 h
300 mm	0.30	8.3 h
400 mm	0.18	11.0 h

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Inside and outside insulation share a U-value, yet room-side admittance differs seventeenfold.

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U-value ignores layer order, but the time-varying response depends on which layer faces the room. With external insulation the concrete faces the room, absorbing heat when the room warms and releasing it when it cools — captured by a room-side admittance of 5.9 W/m²K. With internal insulation the insulation separates the concrete from the room, leaving only 0.34 W/m²K; from the room it behaves like a lightweight wall, so sun and occupants raise temperature quickly. For outdoor heat, external insulation also damps better, with a decrement factor of 0.17, half that of internal insulation.

Design tips

Compare decrement, time lag and room-side admittance as well as U-value.
To use thermal mass, put heavy layers on the room side and insulation outside.
Combine with night ventilation to purge stored heat.
Coverings (carpet, ceiling panels) over thermal mass reduce its effect.
Evaluate annual loads with hourly dynamic simulation.

Coffee Break Trivia

“More insulation, hotter summers”

An office renovation added insulation inside its concrete walls plus a suspended ceiling. Winters grew warmer, but summer afternoon temperature rises became larger than before: the concrete mass was cut off from the room by insulation and ceiling panels and could no longer absorb sun and internal gains. Adding night ventilation and opening parts of the ceiling to expose the concrete brought summer rises back under control.

Common mistakes

Mistakes and fixes

Mistake	Effect	Fix
Comparing U only	Summer overheating missed	Check dynamic metrics
Ignoring layer order	Mass effect wrong	Use transfer matrices
Omitting finishes	Mass overrated	Actual build-up
Steady-state load calculation	Wrong peak loads	Dynamic simulation
No night ventilation	Heat accumulates	Provide a heat purge

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