 | Thermal mass in buildings
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Energy efficiency and thermal comfort are an integral part of sustainable development. Selecting which construction products to use or which construction products to combine can affect the performance of a building’s components, including the floors. We wish to thank the Canadian Institute of Steel Construction (CISC) for allowing us to reproduce this article, published in Advantage Steel, Number 20, a special issue on sustainability and steel.
An increasing desire to address the issue of man's impact on the environment is leading to a fundamental review of how we construct and operate our buildings. The impact of the LEED® (Leadership in Energy and Environmental Design) green building rating system in North America and the adoption of the Climate Change Plan for Canada, which has set targets for improved energy use in commercial buildings, have both led to a greater interest in reducing energy use in buildings.
It is increasingly recognized that the use of thermal mass in office buildings can offer significant energy efficiency and thermal comfort advantages, and this has led to more interest from architects and engineers when designing low-energy, sustainable office buildings. However, a lack of understanding of the detailed mechanisms that affect thermal mass and how it can be used in modern office buildings has led to a tendency to equate high levels of physical mass with good passive thermal performance.
Designers assume that physically massive buildings are required to provide sufficient thermal mass. In reality, there are many other factors that need to be considered, and increasing physical mass above certain thresholds does not necessarily improve thermal performance.
Many commercial buildings today are structurally heavy but thermally lightweight. This is due to the use of finishes such as false floors, drop ceilings, gypsum wall linings, carpets, and other insulating finishes that effectively insulate the heavy structure from the internal environment. Generally, it is more important to ensure that the design makes the best use of available thermal mass than to add additional mass. It is also relevant to consider the likely increases in heating load in winter which may result from additional thermal mass.
Figure 1: Wessex Water headquarters achieved one of the best environmental ratings for an office building in the UK, using a steel frame with precast concrete floors.
Why use thermal mass
Thermal mass (or thermal capacity) is the ability of a material to absorb, store and release heat. It is measured in the number of Joules of thermal energy stored per unit of mass (J/kg•K) or per cubic metre of material (J/m3•K).
Generally, heavy materials such as concrete and masonry can store more heat per unit of volume than lightweight materials such as timber or insulation (Table 1). Using heavy structural elements like masonry walls as sinks to absorb heat during the occupied period of the day is an age-old strategy used for vernacular designs in warm countries such as in the Mediterranean.
Such buildings use high mass, variable air change rates, large surface areas, shading and low internal heat loads to control overheating. However, often there are penalties in the form of poorer comfort in winter or an increased need for heating.
Material | Material density
(kg/m3) | Specific heat capacity
(J/kg•K) | Volumetric heat capacity
(kJ/m3•K) | Thermal conductivity
(W/m•K) |
| Steel | 7800 | 480 | 3744 | 55 |
| Concrete (normal) | 2400 | 910 | 2184 | 1.83 |
| Concrete (light) | 1850 | 850 | 1573 | 1.0 |
| Limestone | 2350 | 810 | 1904 | 2.0 |
| Common brick | 1920 | 835 | 1603 | 0.72 |
| Softwood | 610 | 1420 | 866 | 0.13 |
Table 1: Properties of common building materials
The conditions and servicing strategies for modern office buildings are very different from those of Mediterranean vernacular buildings. A typical office building has:
- Deep plan
- High internal heat gains
- Large areas of glazing, leading to high solar gains
- A sealed facade
- A high density of occupation and internal finishes that insulate the mass from the internal space
Such buildings require a very different strategy for environmental control and the use of thermal mass. Typically, in multi-storey office buildings, the floor and ceiling slabs have the largest area of useful thermal mass. External walls and internal partitions are often lightweight and have little useful thermal mass. Exposing the surfaces of floor slabs allows the structural mass to interact thermally with the internal environment, thereby increasing the thermal inertia of the occupied spaces.
These components act as heat sinks during the day absorbing excess heat, thus avoiding or reducing overheating. At night, the cooler ambient air is used to ventilate the internal spaces and cool the slabs, removing the heat stored during the previous day and preparing the slabs for absorbing further thermal energy the following day. This can reduce or eliminate the mechanical cooling load in many buildings in summer and is particularly useful in office buildings, which tend to make high thermal gains from occupants, computers and other equipment, lighting and glazed facades.
Benefits of thermal mass
The appropriate use of thermal mass can contribute significantly to both energy efficiency and comfort within buildings, offering the following benefits:
- Less reliance on mechanical services to achieve comfort
- More stable daily temperatures in both summer and winter, giving greater comfort to the occupants
- Lower peak loading on the HVAC plant for both the heating and cooling systems
- Increased potential for passive cooling in summer
- Reduced cooling loads in air-conditioned buildings
- The potential for reduced running costs resulting from lower energy use
- Potentially reduced capital costs resulting from the lower capacity cooling unit
- A possible reduction in the building interior volume taken up by building services
But how much mass is appropriate in an office building, and how can the designer ensure that it is usefully integrated?
Heat transfer mechanisms
The ability of a building element to absorb and store heat is dependent on two key factors:
- The thermal characteristics of the element itself, particularly its capacity to conduct and store the thermal energy (thermal conductivity measured in W/m•K and thermal capacity measured in J/m3•K).
- The rate of heat transfer between the element and the air/space to which it is exposed (the surface heat transfer in W/m2•K), also defined as the Admittance (i.e. the rate at which a square metre of surface can absorb heat from the air at a temperature difference of 1oC).
Figure 2: The mechanism of heat transfer into the thermal mass
Detailed computer thermal modelling carried out to analyze the performance of alternative constructions suggests that, for most construction types used in office buildings, it is the surface heat transfer characteristics that determine or limit the thermal storage performance of a typical concrete floor slab, not the depth or volume of the slab1.
It has been found that there is little benefit from increasing the slab thickness above approximately 100 mm, as it is the rate at which heat can be absorbed into the fabric that is the limiting factor for how much thermal energy can be stored2. For typical concrete floor construction types used in both steel and concrete frame office buildings, the capacity of the slab to store the thermal energy is superior to the rate of surface heat transfer over a 24-hour cycle.
Improvements in surface heat transfer can be achieved by increasing the surface area through the formation of coffers, troughs, or profiling the surface such as is the case for composite deck floor slabs. Typically, this can approximately double the exposed underside surface area and hence the heat transfer, and is likely to be more relevant than increasing the amount of mass. Current research into improving heat transfer3 includes blowing air through cores within the floor slab, or using water in embedded pipes to warm and cool the slab.
Figure 3: Increased surface area of a profiled composite slab, compared to a flat slab
People will feel comfortable at higher internal air temperatures when there are cool surfaces in a space. This is because the comfort temperature experienced within a space is dependent on both the air temperature and the radiant temperature of all the surfaces within a space.
Thermally massive components tend to have cooler surfaces, which in summer provide a cooling effect through radiative cooling to people within the space (the opposite of radiator heating in winter). This will further reduce the cooling load, but may lead to increases in heating load in winter.
Finishes
The widespread use of such features as false ceilings, carpets, floor voids and gypsum wall liners, all of which effectively insulate the thermal mass from the internal environment, considerably reduces the effect of thermal mass in the structure. These factors are more important than the framing material of the building, and the designers need to investigate options such as ceramic floor finishes and exposed concrete ceilings to ensure that the mass is connected to the space.
The use of solid drop ceilings obviously limits heat transfer by effectively insulating the slab from the space below. However, a significant level of heat transfer may still be achievable if the ceiling itself is made of a conducting rather than insulating material. Furthermore, partial thermal exposure of a slab surface can be achieved by using open-cell or perforated ceiling tiles, or covering only part of the slab with a drop ceiling.
This permits air to circulate between the ceiling void and space below, making direct use of convective heat transfer. Research in the UK suggests that as little as 15% open area is sufficient to allow significant air circulation and heat transfer4.
Other issues
It is important to note that there are several downsides to the exposure of thermal mass in office buildings. Computer thermal modelling suggests that, in the winter months, a 10 to 20% increase in heating energy demand may occur due to the additional heat required to warm the thermal mass. A thermally massive building will cool more slowly when heating is switched off on a winter evening, but will take longer to warm up in the morning before occupation. As a result of the higher temperatures at night in the building, there will be a higher rate of heat loss.
This is exacerbated if the building is poorly insulated and can lead to significantly increased heating costs. Conversely, during spring and autumn, lightweight buildings may require both heating and cooling over a 24-hour cycle, whereas thermally heavy buildings may maintain comfortable internal conditions without either supplementary heating or cooling. The balance between the reduction in cooling demand and the increase in heating demand is complex and will vary between buildings, being dependent upon the envelope design responsiveness of controls, occupancy period and the heating system design.
Exposing the underside of a concrete floor slab to take advantage of thermal mass also has implications on the acoustic environment of the space and the integration of services including lighting. The absence of a suspended ceiling can give rise to increased reverberation times and increased reflected sound across an open plan space. This can be addressed by using acoustically absorbent partitions, although these may affect day lighting levels, or by integrating acoustically absorbing panels in light fittings suspended below the slab. Alternatively, partial drop ceilings may be appropriate.
Conclusion
To exploit thermal mass, the choice of structural system need not be limited to any one particular material or system, and sufficient mass may readily be integrated into a steel frame structure. Since exposure of the mass is the most critical issue, the important aspect the designer must consider is how to ensure that the floor slabs are not insulated in such a way as to make the mass within them redundant. The design of the structure can be optimized for structural reasons, and there is little benefit in adding mass purely for thermal reasons.
Deeper slabs and more concrete are not necessary. Rather, it is the amount of exposure of the thermal mass to the internal spaces through increased surface area and the types of surface finishes (such as carpets, drop ceilings, etc.) that are more important to the efficiency with which the thermal mass is utilized. It may be appropriate to use different solutions depending on the location within a building. North facing spaces have less solar gains, so overheating may be less of an issue than in south facing locations.
Thanks to Daniel Pearl for his comments on this article.
References
- 1See Barnard, N. Making the most of thermal mass, Architects Journal, 21 October 1999
Or Cousins, F. & Lang, B. Aspects of structural and thermal mass, seminar paper available from the Steel Construction Institute, UK
- 2Barnard, Nick & Ogden, Ray, The thermal capacity of steel frame buildings, Seminar Paper, July 1996, available from the Steel Construction Institute, UK
- 3See for example Kendrick, Chris, Active fabric energy storage systems for steel framed non-domestic buildings. Chartered Institute of Building Services Engineers (CIBSE) National Conference, Harrogate, October, 1999
Or Kendrick, Chris & Ogden, Ray, Use of embedded water pipes to provide thermal comfort in steel frame buildings, Proceedings of the IISI conference, Steel in sustainable construction, Luxemburg, May 2002
- 4See Barnard, Nick et al, Modelling the performance of thermal mass, BRE Information Paper IP6/01, Building Research Establishment, UK
Or Amato, A. et al, Practical ceiling solutions for thermally efficient steel frame buildings, 1998 CIBSE National Conference
Information
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Canam Canada
Professor Mark Gorgolewski
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