How a building responds to changing temperatures is influenced by more than its heating and cooling systems. The materials specified within the building envelope can play an important role too.
Clay brick has high thermal mass, meaning it can absorb and store heat before gradually releasing it as surrounding conditions change. When thermal mass is effectively integrated into building design, it can help moderate internal temperature fluctuations and contribute to more stable, comfortable spaces.
This blog explores how thermal mass works, its relationship with passive design, and how material choices can influence occupant comfort and building performance over time.
Understanding Thermal Mass and Thermal Lag
Thermal mass describes a material’s ability to absorb, store and release heat energy. Dense materials such as clay brick can absorb heat as their surroundings become warmer and release it again as conditions cool.
Thermal capacity can also create a time delay between temperature peaks at the outside and inside surfaces of the building envelope, often referred to as thermal lag or decrement delay. The extent of this effect depends on the wall build-up and the position of heavyweight materials relative to the insulation.
Together, thermal mass and thermal lag can help reduce and delay temperature peaks, depending on the construction and wider building design.
However, thermal mass is not a standalone solution. It works most effectively as part of a considered building design alongside insulation, ventilation, glazing, orientation and shading.
The Thermal Cycle
The principle behind thermal mass can be understood as a simple cycle:
- Day: As internal temperatures and heat gains increase, exposed clay brick can absorb and store heat, helping to slow the rate of indoor temperature rise.
- Night: As conditions cool, stored heat can gradually be released. Where an appropriate night-ventilation strategy is provided, this heat can be removed from the building, allowing the thermal mass to cool for the following day.
This passive process can be summarised in four steps:
Absorb. Store. Release. Repeat.
Designing for Comfort and Climate Resilience
Building performance directly influences the experience of the people using a space, from homeowners and office workers to pupils, students and visitors to public buildings.
Spaces prone to rapid temperature fluctuations can affect occupant comfort and increase demand on heating and cooling systems. By helping to buffer these fluctuations, appropriately utilised thermal mass can contribute to more stable internal temperatures throughout the day.
As warmer weather and overheating become increasingly important design considerations, thermal mass can form part of a passive approach to managing internal temperatures when combined with an appropriate overall design strategy.
Key considerations include:
- Solar Gain & Glazing: Managing solar heat gains through appropriate glazing ratios, specification and positioning.
- Shading: Using measures such as overhangs, louvres or other shading strategies to limit unwanted solar gains.
- Ventilation: Providing appropriate ventilation strategies, including night-purge ventilation where suitable, to help remove stored heat.
- Thermal Mass: Considering the location and exposure of heavyweight materials so their thermal capacity can be effectively utilised.
No single material or design measure solves overheating on its own. Considering these elements together can help reduce overheating risk and create more comfortable internal environments during warmer periods.
Thermal Mass and Operational Energy
Energy performance starts with understanding the building as a whole.
Heating and cooling can represent a significant part of a building’s operational energy demand, although this varies considerably according to building type, use, occupancy and services strategy.
By helping to moderate internal temperature fluctuations, appropriately designed thermal mass can form part of a wider strategy for managing heating and cooling requirements and, in some buildings, reducing peak demand on mechanical systems.
Thermal mass is an inherent property of materials such as clay brick. Unlike an active building service, it does not require a separate system to provide its heat-storage capacity.
Clay brickwork can also provide a service life in excess of 150 years when appropriately specified, installed and maintained. Considering thermal performance alongside durability at the fabric-design stage can therefore form part of a long-term approach to building performance.
Designing as a Whole
Thermal performance is not determined by one material, product or system.
Orientation, insulation, airtightness, glazing, shading, ventilation, thermal mass, building services and patterns of occupation all influence how a building responds to changing conditions.
For architects and specifiers, understanding how these elements interact allows thermal performance to be considered from the earliest stages of design.
Clay brick’s thermal mass can form one part of this wider strategy, combining an established material characteristic with the design flexibility and durability expected from modern masonry construction.
Further Reading
To explore thermal mass, thermal shielding and decrement delay in greater detail, including BDA case studies of masonry construction, read the Brick Skin Thermal Performance Study: Brick Skin Thermal Performance Study
Useful Sections
- Introduction – page 3: Thermal mass and thermal shielding
- Britten Pears Archive – pages 4–5: Passive environmental control, thermal shielding and location of thermal mass
- Decrement Delay – page 6: How heavyweight construction delays and moderates temperature peaks
- Hanson EcoHouse – page 7: Thermal mass combined with natural ventilation
- Conclusion – page 8: Internal temperature moderation, occupant comfort and energy use
