How thermal mass reduces heating demand?

Building materials have a unique ability to absorb, store and release heat; This is called thermal mass and is one of the major contributing factors to the artificial energy demand of a house. In other terms, the thermal mass of a building, if designed and located properly, could save energy bills and increase the thermal comfort of occupants.
Here, we try to investigate the effect of thermal mass on a 6-star reference building. Find out what you need here:

NCC 2022 and thermal mass
How to utilize thermal mass in our design effectively?
Investigating the effect of thermal mass on a Simple Reference Building

Key points of this article:

  • Thermal mass is the ability of a material to absorb and store heat and then release it later when it’s needed. It has a potential to add 1 star to the building.

  • Thermal mass, if designed and located correctly, reduces heating bills, especially in mild or cold climates.

  • The external wall containing the thermal mass must be faced towards the north and is better to be used as an interior finish.

  • In cold climates and for diurnal ranges of more than 10 degrees Celsius, high mass walls are preferred over lightweight construction.

  • The area of north-facing windows must be between 15%-25% of the area of an exposed high mass wall for the highest heating efficiency.

  • In a case study for a 50m2 single-zoned building, replacing lightweight walls with reversed brick veneer walls resulted in a 10.9% saving in heating demand.

Thermal mass energy rating

How thermal mass contributes to building heating and cooling loads?

Thermal mass is a material’s ability to:

  • Absorb heat: which depends on the external face colour and location of this high thermal mass wall or floor; and
  • Store heat: which depends on the thermal conductivity, density and specific heat of the material; and
  • Release heat: this is often called “thermal lag” and depends on the volumetric heat capacity of the material.

As a general rule of thumb, you would expect a north-facing dark wall with a high density and specific heat (and therefore high volumetric heat capacity) and low thermal conductivity to have a high thermal mass and high thermal lag effect. High thermal lag is associated with high thermal mass and means the delay in heat transfer from the exterior to the interior side of a wall. For example, you would expect your wall to be warm at noon, however having a high thermal mass delays the heat transfer and the warmth of the interior face of the wall could be felt in the afternoon and even at nighttime. The effect of thermal mass could be good or bad:

  • In hot climates, you would prefer lightweight walls as absorption and release of heat in later times (i.e. the thermal lag), could extend the cooling period and decrease thermal comfort. In mild climates, effect of thermal mass must be assessed by an accredited assessor using energy modelling.
  • In cold climates, the use of a north-facing wall with high thermal mass is often preferred. Due to the thermal lag effect, the wall absorbs and stores heat at noon (when heating is often not needed) and releases heat later in the afternoon or at sunset (where heating is often needed).

NCC 2022 and thermal mass

In NCC 2019, Part of Deemed to Satisfy (DTS) provisions, external walls with a surface density of more than 220 kg/m2 located in Climate Zone 6, are encouraged with an additional buffer or increase in thermal performance of windows; meaning that if you have 190mm dense weight concrete block walls instead of weatherboard walls, you are allowed to have cheaper windows with less overall U-values. This is due to the fact that the saving in heating imposed by this high thermal mass wall could be offset with cheaper windows with less U-values. However, there are no provisions for the account of thermal mass in commercial buildings in Section J DTS provisions.

In NCC 2022, the effect of thermal mass is more thoroughly accounted for and is explained in terms of additional R-value. As an example, in Climate Zone 6, an external lightweight wall must have an addition of R0.3 insulation added to it, compared with a brick veneer wall. This additional insulation mitigates the need for more artificial heating for buildings associated with low thermal mass walls in cold climates.

It is worth noting that the effect of thermal mass is accounted for in NatHERS rating tools, but in the same complex manner as energy modelling. Therefore, in cold climates, especially while you are required to obtain BESS Energy points for an ESD initiative in Victoria, higher star rating and therefore, higher BESS score could be achieved having a high thermal mass construction. On top of that, effectiveness of Phase Change Materials (PCMs), Trombe walls and other innovative designs could be investigated through a CFD analysis.

What materials have the highest thermal mass?

Building materials with higher density and specific heat have higher thermal mass and often, higher thermal lag. The term to better compare different materials here is called Volumetric Heat Capacity which is essentially the density of a material multiplied by its specific heat.

According to the table on the right, Concrete has the highest thermal mass and Weatherboard has the lowest thermal mass.

Material Density
Specific heat
Volumetric Heat Capacity
Brick 1690 0.92 1555
Concrete 2400 0.92 2208
140mm  block 1250 0.84 1050
Rammed earth 2000 0.84 1680
Weatherboard 506 1.20 607

Although thermal lag is associated with high thermal mass, but its also dependent on the ability of a material to transfer heat from one end to the other (i.e. thermal conductivity). The higher the thermal conductivity, the lower the thermal lag. According to Baggs et al. (2009), 250mm concrete wall has thermal lag of 6.9 hours; 3.4hours short of a 250mm rammed earth wall with the same thickness.

Did you know?

At Energy Compliance Consultants, we use one of the most advanced tools of energy modelling, DesignBuilder, capable of calculating an hour-by-hour effect of heat transfer to/from the building thermal mass?
We use this feature in our Green Star Rating, JV3 modelling and VURB projects.

How to utilize thermal mass in our design effectively?

Know your climate

Thermal mass is not a good choice for hot climates and a good choice for cold climates. As for warm or mild climates (i.e. Climate Zones 5 and 6), you need to decide whether having a thermal mass is a good choice. For this matter, you need to take the diurnal range into account. The diurnal range is the maximum temperature difference experienced in a day. For example, if the temperature of the ambient is about 20 degrees (max) at noon and about 5 degrees (min) at dawn, then the diurnal range for that day is 15 degrees Celcius.

  • For diurnal ranges below 6 degrees Celcius, suspended timber floor and timber stud walls are more cost-effective and energy-efficient options than the high thermal mass choice (e.g. concrete slab and brick veneer wall).
  • For diurnal ranges between 6-10 degrees Celcius, the effect of thermal mass must be assessed by an accredited assessor; however, as a general rule of thumb, a concrete slab on ground and moderate thermal mass like brick veneer walls on the north side are preferred.
  • For diurnal ranges of more than 10 degrees Celcius, concrete slab on ground and high mass walls are certainly preferred. The higher the diurnal range, the higher the thermal lag you would want to have on the north. Therefore, a brick cavity wall or rammed earth wall is preferred over a brick veneer wall. [reference]
Wall type and location

A high mass wall (where needed) must face towards the north to absorb solar heat as much as possible. Note that the wall doesn’t have to face the north directly; an internal wall with a high mass that is located between a glazed corridor or living and south bedrooms could also be effective in decreasing the heating bills.

It is also worth mentioning that a brick veneer is not as effective as a reversed brick veneer. It is better for the high mass material of an external wall to be faced internally. The reason is that the high access to solar heat is achieved in the day and transfers (though slowly) through insulation and is absorbed and stored by the internal high thermal mass material. At night, there is no obstacle internally (i.e. insulation) for the high mass material to transfer the stored heat to occupants. Therefore, it is always preferable to install the high mass material internally rather than externally.

Glass to mass ratio

You may think that an internal rammed earth wall facing north to a fully-glazed wall is a good option for a cold climate as it introduces as much solar heat as possible to the internal rammed earth wall to absorb and store. But this is not the case at all. Always know this:

The thermal resistance (R-value) of even a good window with high thermal performance is lower compared to a brick veneer wall with no insulation at all.

Therefore, you are in for a trade-off here: is the poor thermal performance of my windows mitigate the positive solar heat gain or not? The answer really depends on the number of cloudy days and the degree of access to solar irradiation; this is a good subject for a cost-benefit assessment. Although, as a rule of thumb, the area of north-facing windows must be between 15%-25% of the area of an exposed high mass wall. [reference]

Investigating the effect of thermal mass on a Simple Reference Building

In this section, we investigate the effect of the thermal mass of a Simple Reference Building (SRB), against three alternative cases:

  1. Case A: where the north-facing façade is a lightweight wall with the same R-value and solar absorptance.
  2. Case B: where east and west-facing façades are lightweight walls with the same R-value and solar absorptance.
  3. Case C: where all façades are lightweight walls with the same R-value and solar absorptance.

Characteristics of the Simple Reference Building (SRB) are described below:

Star rating
Building and location

SRB is 6-star 10x5x2.4(m3) house with a daytime conditioned zone located in Melbourne (Climate Zone 6).
The large façade of the building faces north. Heating set point: 20 degrees and cooling set point: 25 degrees.
Heating demand: 62 MJ/m2Cooling demand: 78 MJ/m2Total thermal demand: 140 MJ/m2

Roof/ceiling construction

The reference building roof is a pitched metal roof with a 23-degree pitch having a solar absorptance of 0.6.
The ceiling is flat at 2.4m high with a 10mm plasterboard interior finish.
Total roof/ceiling R-value: R5.1

Star rating
Wall-glazing and floor construction

External walls are 110mm brick veneer walls with a Total R-value of R2.8 and solar absorptance value of 0.6.
The thermal performance of windows (H:1.5m/W:4m): U-value 2.25 and SHGC 0.35.
The floor construction
is a 100mm concrete slab on ground with no added insulation.


For results to be better captured and better comparable, the thermal zone inside the building is assumed a night-time conditioned zone where artificial heating is needed from 5 PM to 9 AM when the room temperature drops below 20 degrees Celsius. During this time, the house has little to no access to solar irradiation, however, the effect of heat storage inside the high mass material could be fully investigated. Therefore. This SRB* case heating demand is calculated to be 88.4 MJ/m2.

All external walls have the same Total R-value and solar absorptance values in all cases. The difference is therefore only limited to the external wall layers and the thermal properties of those layers.

As expected, the SRB* case requires the least thermal energy compared to the other three cases and of the three cases, Case C shows the worst performance compared to the other two. Comparing cases A and B, it is worth noting that having low mass wall on east and west facades slightly decreases heating demand of the building. This is partly due to the coincidence of early and late hours of AC operation (5 PM and 9 AM) and the lower thermal lag that facilitates the flow of heat (i.e. solar heat gain) inside the building.

According to this case study, comparing SRB* to Case C shows that having thermal mass, saves 2.7MJ per m2 area of the proposed building (3% reduction).

Thermal mass effect comparison

In another run, we changed the external walls of the SRB* from a typical brick veneer to a reversed brick veneer (where the high mass brick is installed internally). This resulted in 81.2 MJ/m2 heating demand (8.1% reduction compared to SRB* with typical brick veneer walls and 10.9% reduction compared to the Case C with lightweight walls).

Therefore, major saving through thermal mass could be gained in cases where the thermal mass is placed at the interior side of the wall.