What is thermal mass?

Thermal mass describes the ability of heavyweight construction materials (such as concrete) to provide inertia against temperature fluctuations in buildings. This is achieved through their ability to absorb and release heat in response to internal conditions. During warm weather, much of the unwanted heat gains will be absorbed by the thermal mass in exposed floors and walls, helping prevent an excessive temperature rise and reducing the risk of overheating. In air-conditioned buildings, thermal mass also reduces and delays the peak cooling load, and can allow it to be switched off completely during more benign periods of weather.

The key reason that heavyweight floors and walls continue to provide a benefit to occupant comfort across the day is because they can absorb a significant amount of heat with only a small increase in the temperature. This ensures a relatively low surface temperature is maintained that provides a beneficial radiant cooling effect for the occupants across the day. At night, heavyweight buildings can be ventilated using the comparatively cool night air to purge heat from the fabric so it is ready to repeat the heating and cooling cycle the following day.

Thermal mass can also be used during the winter months to improve fabric energy efficiency through passive solar design. For more information on this application and thermal mass in general, see: Thermal Mass Explained  

What properties are needed to provide thermal mass?

For a material to provide a useful level of thermal mass a combination of three basic characteristics is required:

  1. A high specific heat capacity; so the heat squeezed into every kilogram is maximised.
  2. A high density; the heavier the material, the more heat it can store by volume.
  3. Moderate thermal conductivity; so the rate heat flows in and out of the material is roughly in step with the daily heating and cooling cycle of the building.

Heavyweight construction materials such as masonry and concrete have these characteristics. They combine a high storage capacity with moderate thermal conductivity. This means that heat moves between the material’s surface and its interior at a rate that roughly matches the building’s daily heating and cooling cycle. Some materials, like wood, have a high heat capacity, but their thermal conductivity is relatively low, limiting the rate at which heat can be absorbed during the day and released at night. Steel can store a lot of heat, but conducts it too rapidly to be practically useful, plus comparatively little is used in buildings. However, a modest amount of thermal mass may still be provided if concrete floors are used in steel frame construction, although these are usually limited to a depth of only 100mm and are usually covered by a false ceiling, limiting their ability to absorb and release heat. 

How is thermal mass measured?

k-values

Part L of the Building Regulations and its associated compliance tools (SAP & SBEM) account for thermal mass using k-values (kJ/m2K), which provides a basic indication of the thermal capacity per square metre of floor or wall. Lightweight walls have a low k-value of around 10 kJ/m2K, whilst for heavyweight walls it can be up to 230 kJ/m2K. Some generic k-values for various types of construction are published in Table 1e of SAP 10.2, whilst more comprehensive values for concrete and masonry constructions can be found in: Thermal Performance: Part L1A 2013. Alternatively, bespoke k-values and other thermal mass related information can be calculated using a free Thermal Properties Calculator produced by Arup in partnership with The Concrete Centre.

Admittance values

Describing a material or construction as having high, medium or low thermal mass gives a useful indication of its ability to store heat, as does its k-value. But, in order to get a better idea of how effective it is likely to be in practice, there are a couple of other important factors that need to be considered. These are firstly the length of time available to get heat in and out of the material, which is typically assumed to be 24 hours, and secondly, the rate of heat flow to and from the material. These factors are accounted for in admittance values, which provide a more detailed  means of assessing the approximate in-use thermal mass performance of walls and floors, making it a more sophisticated metric than the k-value. For more information on admittance values see: Thermal Mass Explained.

Decrement

Admittance values and k-values relate to the absorption of heat inside buildings, which is the most important use of thermal mass. There is however another thermal mass related property called decrement, which can influence summertime performance to some extent. Decrement describes the way in which the density, heat capacity and thermal conductivity of an external wall (for example), can slow the passage of heat from the sun as it passes from the outside to the inner surface of the wall (decrement delay), and also reduce those gains as they pass through it (decrement factor). For more information see: Thermal Mass Explained and for a comprehensive range of decrement values for concrete and masonry walls see: Thermal Performance: Part L1A 2013.

Does thermal mass have any disadvantages?

In summer, thermal mass is only beneficial if night-time ventilation (or some other means of cooling) can be used to remove the heat absorbed by the building fabric during the day. The provision of adequate ventilation can be challenging in some environments, particularly urban locations. However, the recently introduced Part O of the Building Regulations includes provisions to ensure measures that tackle overheating in new homes are practical and take adequate account of any related noise, pollution, security and safety issues.

In winter, older heavyweight buildings with comparatively low levels of insulation and poor airtightness will require a longer pre-heat period to warm up the fabric, resulting in more energy being used than in a similar lightweight building. However, for newer buildings the greatly improved standard of fabric performance means this is no longer the problem it once was, as the fabric retains most more of its warmth during periods when the heating is off. In practice, the ability of thermal mass to enhance summertime performance in many building types is of much greater significance.