Large thermal mass in buildings refers to using heavyweight building construction such as concrete for the building structure (wall, flooring and roof). Incorporating large thermal mass into the building design is a strategy that can be used for improving indoor thermal comfort and reducing building heating and cooling load and annual energy use. Thermal mass acts like a thermal battery to reduce the indoor temperature swings resulting from changes in outside air temperature. While the outdoor temperature peaks around midday to early afternoon (coinciding with peak occupancy and internal equipment loads), the heat transfer through a high thermal mass external wall will peak a few hours later and into the night (time lag). This time lag shifts the peak external heat load from coinciding with the peak internal heat load, which reduces the design cooling load of the mechanical equipment. In climates where the temperature drops overnight, the time lag heat released into the space can reduce the required heating energy overnight or be removed via ventilated air.

Technical Description

What is thermal mass? 

Thermal mass is an inherent property of a material that refers to the ability to absorb and store thermal energy for extended periods. For a material to have a high thermal mass, it must have three specific properties:

• High specific heat capacity (high heat storage per unit of mass) 
• High density (high mass per volume)
• Moderate Thermal Conductivity (the rate at which heat travels through material). 

Table 1. Examples of typical materials and their properties. Heavy-weight materials, such as concrete, are examples of commonly used high-thermal mass materials.

Thermal lag is another term used when describing thermal mass. It is the rate at which heat is absorbed and re-released. It indicates the time delay (in hours) for heat to be transferred through the construction materials. Thermal lag is increased by higher thermal mass and material thickness.

Figure 1. Thermal flux throughout the day [1]

Architects can select materials with high thermal mass to be used to construct the floor and/or walls of a building. This material, when exposed to external heat such as sunlight, or internal heat (people, lighting, equipment), can store heat during the day and be released later during the afternoon or night.

How does it work?

In summer, the thermal mass absorbs heat during the day from both the external air temperature and solar loads. This absorption slows the rate at which heat enters the space (thermal lag) and helps ‘smooth out’ the effects of extreme external temperature fluctuations. This creates a more stable internal temperature and subsequently more comfortable conditions for the occupants.

In most commercial and healthcare applications, the internal loads (people, computers, and lights) are highest in the midday-afternoon period which coincides with the highest external temperatures and solar load. The thermal lag created by the thermal mass delays the peak external heat from entering the space until later in the afternoon/night which can substantially reduce the peak thermal load of the space. This results in reduced HVAC equipment sizing. This shifting and reduction in peak cooling allows for opportunities to reduce operating costs by reducing demand costs and using lower-cost off-peak electricity.
If the building is located in a climate zone where the nighttime temperatures drop, the stored heat released during the night can help maintain the space in comfortable conditions or can be flushed out using ventilation strategies such as air-side economizers on the air handling units (rather than mechanical cooling). This can reduce the cooling energy of the building by reducing the mechanical cooling required.

Figure 2. Nighttime flushing

In winter, the thermal mass absorbs both the solar heat and internal heat (occupants, lighting, and equipment) during the day, and then slowly releases the heat into the room during the late afternoon and late evening hours when heating loads are highest. This can help reduce heating loads and HVAC equipment sizing.

Figure 3. Evening Heating

Design considerations and best practices 

The optimization of thermal mass depends not just on the properties of the materials, but also on the climate zone, glazing layout and shading, and insulation.

Climate

Thermal mass is most valuable in regions where the average daily temperature swings are high (ASHRAE climate zones 3B, 3C, 4B, and 5B). Large temperature drops at night enable the heat absorbed during the day to provide heating to the building or can be flushed out using ventilated air (as opposed to mechanical cooling).

Glazing Layout and Shading

Maximizing solar gains in the winter seasons, and minimizing in the summer months helps optimize the thermal mass benefits. This can be achieved by maximizing south-facing windows (in the northern hemisphere) and providing adequate shading. Passive shading or blinds can be designed to block the solar load when the sun is high in the sky in the summer months. This will help reduce the overall heat load in summer.

In winter, the south-facing windows will maximize the solar absorption during the day, which will be released back into the space during the night when heating is required.

Insulation

The location of the external wall insulation is an important factor when designing for thermal mass. Insulation should be placed on the outside of the external wall allowing for the thermal mass wall to have direct contact with the internal space. This positioning helps limit the external heat absorbed by the wall in summer, and in winter, allows the solar heat through the windows and internally generated heat to be directly absorbed by the thermal mass and released back into the space at night without interference from the insulation.

How does this decarbonize?

The primary method for thermal mass approaches to contribute to decarbonization is by reducing the annual cooling and heating energy of the building. 

Thermal mass is most beneficial in reducing heating load and energy by releasing the stored daytime heat overnight when the heating load and energy can be highest. Heating is one of the more difficult aspects to decarbonize, as traditionally in healthcare, natural gas is used as the primary fuel source for heating. Therefore reducing heating load and energy where possible helps lead to a lower carbon-intensive building.

As healthcare buildings look to electrify and remove reliance on natural gas, designers are increasingly looking towards heat pumps (heat recovery chillers) to achieve the heating requirements of the building. Challenges with these systems usually occur in winter, when heating loads are high, and there is not enough cooling load to produce heat. Reducing the heating load via thermal mass design can provide a better balance for the system.

Implementation 

Barriers: Embodied Carbon

Having high thermal mass walls means using large amounts of dense material such as concrete. It is estimated that the “embodied energy” of office buildings can be 16-45% of the total building energy use. The production of materials such as concrete can lead to significant amounts of greenhouse gas emissions (embodied carbon). The production of Portland cement (one of the most commonly used materials in concrete) is an example of a material with high embodied carbon. There are currently more sustainable methods for the production of concrete, for example, using fly ash and ground granulated blast-furnace slag to replace large amounts of Portland cement, and finding manufacturers that use green energy in their production. All aspects of the materials used in thermal mass construction must be evaluated before construction.

Barriers: Culture and Cost

Currently, there is a trend to use lightweight curtain wall systems and maximize glazing in building design. This is perceived as both cost-effective and provides occupants with large views of the outdoors compared to a thermal mass design. However, designers should evaluate the cost of thermal mass design against both the downsizing of the HVAC system and operation savings, and integrating glazing that maximizes external views into the thermal mass envelope.

Strategy

Thermal mass should be studied early in the design phase to understand design implications if sustainable thermal mass materials can be sourced and a cost-benefit analysis completed.

Design Implications: A facade analysis should be completed including energy modeling. Glazing layouts should be configured such that south-facing windows can allow the winter solar loads into the space, but have adequate shading to block summer solar loads. An energy model should be created to study the effects of the thermal mass on reducing HVAC equipment sizing and annual energy savings.

Cost Analysis: Using the above analysis, a cost-benefit analysis should be completed to understand the life cycle costs of the thermal mass material. In many cases, the thermal mass material costs can be higher than other designs (for example curtain wall systems), but when the cost of reduced HVAC system and ongoing energy costs are analyzed, this can favor the thermal mass system.

Sustainable Materials: The availability of sustainable thermal mass materials should be addressed early in the design. A whole-of-life Carbon Assessment can be undertaken to understand whether the embodied carbon of the construction materials will offset the carbon savings through energy savings. Using sustainable materials (for example sustainably produced concrete) can dramatically favor the thermal mass design.

Financial analysis and business case

The cost of building materials and energy varies throughout the US, and each project will need to undergo a financial life cycle cost analysis to determine the cost-effectiveness of a thermal mass design. The initial cost of the thermal mass design can generally be higher than a typical lightweight design due to the increased materials costs, but this again varies based on the complexity of the design. The increased capital cost can, however, be offset by the reduction in mechanical equipment sizing and reduced annual energy savings. A study in 1990 looked at the optimization of thermal mass in commercial buildings in Northern New York and showed that the cost savings of the downsized mechanical equipment alone were able to offset the increased capital cost [3]. The design team should undertake a life cycle cost early in the project, including initial costs of construction materials and HVAC system, combined with an energy analysis to determine long-term energy savings.

The Simon Sainsbury Centre, part of the University of Cambridge Judge Business School in the UK, is a new building extension utilizing a concrete structure with high thermal mass. The thermal mass allowed for the building to absorb heat during the occupied hours and then use a night purge to cool the building overnight. Openable windows were also provided for occupants to control their environment. These features minimized the need for mechanical cooling (though high efficient local mechanical cooling was still installed for use in peak periods when windows were not opened). The thermal mass benefits combined with low energy consuming HVAC systems was able to reduce the building energy 25% below the UK Part L 2013 Energy Code requirements and achieve a BREEAM Excellent rating.

The “Solar NZEB Project” published in the ASHRAE Journal compared houses of different construction materials located in cold climates. The analysis showed that houses with high thermal mass can maintain comfort conditions overnight with limited HVAC compared to low mass buildings. This resulted in very limited use of HVAC heating overnight resulting in reduced heating energy requirements (energy savings were not quantified in the study).