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

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 3.12.1.4 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.

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]