Thermal mass

What is thermal mass? | Thermal mass in summer | Thermal mass in winter | How is thermal mass measured? | Does thermal mass have any disadvantages? | How long is the CO2 payback for thermal mass?

What is thermal mass?

Thermal mass is a term that describes the ability of a material to store heat; something many construction materials can do to a greater or lesser extent. But, to be useful in the built environment, they must also be able to absorb and release heat at a rate roughly in step with a building’s daily heating and cooling cycle. Concrete and masonry products do this well and, being dense materials, can also store a lot of heat. Timber absorbs heat too slowly to offer much effective thermal mass, and steel conducts heat too rapidly to be in synch with a building’s natural heat flows over the day.

Thermal mass in summer

On warm summer days, walls and floors with thermal mass will steadily absorb heat at their surface, conducting it inwardly, and storing it until exposed to the cooler air of the evening/night. At this point, heat will begin to migrate back to the surface and be released. In this way, heat moves in a wave-like motion alternately being absorbed and released in response to the change in day and night-time conditions. This ability to respond naturally to changing conditions helps stabilise the internal temperature and provides a largely self-regulating environment, reducing the risk of overheating and the need for mechanical cooling.

Thermal mass in winter

The ability of heavyweight buildings to stay cooler during the summer by soaking up heat on warm days is fairly well understood. Perhaps less well understood is that this daily cycle of absorbing and releasing heat continues on a year-round basis and can reduce the energy needed to keep a heavyweight building warm during the heating season. This works through the ability of thermal mass to capture and recycle heat gains from south facing windows, along with those produced by lighting, people and appliances. As the temperature drops overnight, this is slowly released back into the building, helping keep it warm and reducing the need for supplementary heating. Whilst lightweight buildings are also capable of doing this, the extent to which the heat gains can be utilised increases with the level of thermal mass; something that is now recognised in SAP which is compliance tool for Part L1A of the Building Regulations which deals with new dwellings. For a more detailed overview of thermal mass and how it can be used, see: Thermal Mass Explained, and for an example of how it can enhance SAP /Part L1A compliance see: Zero Carbon Performance - cost-effective concrete and masonry homes.

How is thermal mass measured?

Part L of the Building Regulations and its associated compliance tools (SAP & SBEM) account for thermal mass using k-values (kJ/m2K), which provide a gauge of thermal capacity per square meter of floor or wall. A lightweight wall might have a k-value of around 10 kJ/m2K, whilst for a heavyweight wall it can be up to 230 kJ/m2K. Some generic k-values for various types of construction are published in Table 1e of SAP 2009, whilst more comprehensive values for concrete and masonry constructions can be found in: Thermal Performance: Part L1A. Alternatively, bespoke k-values and other thermal mass related information can also be calculated using a free Thermal Properties Calculator

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-values. But, in order to know how effective it will be in practice, there are a couple of other important factors that need to be taken into account. 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 (i.e. heating during the day and cooling at night), and secondly, the rate of heat flow at its surface i.e. through carpet, plasterboard, tiles etc. These factors are both accounted for in admittance values, which provide a simple means of assessing the approximate in-use heat absorption performance of walls and floors etc. For more information see: Thermal Mass Explained

Admittance and k-values relate to the absorption of heat at the inner surface of a wall and floor, which is the most significant function of thermal mass in building design. There is however another thermal mass related property called decrement which has a bearing on summertime performance. 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 one side to the other (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 construction see: Thermal Performance: Part L1A

Does thermal mass have any disadvantages?

In summer, thermal mass is only beneficial if nighttime ventilation (or some other means) can be used to cool it down. Local issues such as, pollution and security concerns can sometimes make this impracticable, although there are often ways to overcome these problems. In winter, older heavyweight buildings with comparatively low levels of insulation and poor airtightness often required a relatively long pre-heat period to warm up the fabric, resulting in a slightly more fuel being used than in a similar lightweight building. However, the greatly improved standard of fabric performance in new build means this is no longer the problem it once was, as the fabric retains much more of its warmth during periods when the heating is off. In practice, the ability of thermal mass to reduce the cooling load in many building types, particularly offices, is far more significant than the preheat issue.

 

Nevertheless, in some types of intermittently occupied buildings, for example a weekend holiday cottage, thermally lightweight construction may still be the best option where heating is concerned, as it will enable a more rapid warm up period. For new mainstream housing, the preheat issue is negligible, and as we move towards the 2016 target for new homes to be zero carbon, the increasing standard of insulation and airtightness ensures the passive benefits of thermal mass during the heating season are of more significance.

How long is the CO2 payback for thermal mass?

This very much depends on the building type and the way in which its thermal mass is used. For commercial buildings such as offices, it should first be noted that the embodied CO2 in a typical office design is more or less the same regardless of whether it has a steel or concrete frame; a conclusion reached by studies undertaken by both the steel and concrete sectors. So, for many buildings it’s a moot question. In the case of a very heavyweight building, the additional concrete can result in a slightly higher level of embodied CO2, but the operational savings afforded by the thermal mass will typically offset this in a matter of months rather than years.

For housing the situation is slightly different: The embodied CO2 in a typical masonry home is about 4% higher than an equivalent timber frame home. Studies by Arup and the NHBC Foundation both arrived at this figure. The Arup study went on to look at operational impacts and found that the passive benefits of thermal mass during the heating seasons resulted in CO2 savings that offset the figure of 4% in around 11 years.

In respect of summertime performance, the CO2 payback for housing is less tangible as domestic air conditioning is not particularly common, although this may change as the effects of our warming climate are felt in the future. The Arup study considered the impact of climate change and found that lightweight homes are at greater risk of frequent overheating in the coming years, even when enhanced shading and ventilation are used to mitigate higher temperatures. At this point, the use of air conditioning may become necessary, resulting in increased annual CO2 emissions compared to heavyweight home which was shown to delay the occurrence of frequent overheating by a further 20 to 60 years depending on the level of thermal mass.

For further details of the Arup study please download the full report by clicking here.

For more information on the embodied CO2 in steel and concrete frame building see: Concrete Structures 10