Despite belief to the contrary, thermal mass can be achieved in steel-framed as well as concrete buildings

Want to achieve ther-mal mass? You’ll need a concrete-framed building then, won’t you? Not necessarily, say building en-gineering experts and the steel industry — which is under-standably keen to show that thermal mass can be used successfully within steel-framed buildings too, despite common perceptions to the contrary.

Indeed, in recent research by the British Constructional Steelwork Association and Tata Steel only 9% of respondents associated thermal mass with steel.

The capacity of buildings of all construction types to provide a comfortable and stable internal environment in the most energy efficient way possible is a growing priority, given rising energy prices and the projected increase in average temperatures over the next century due to climate change.

“It is a common misconception that a building needs lots of concrete or masonry to achieve thermal mass. In fact we only require a thin skin of concrete or masonry, and this can be constructed on a steel frame every bit as easily as on a concrete frame, provided the concrete or masonry surface is exposed directly to the internal environment,” says Edward Murphy, technical director of engineering and development consultant Mott MacDonald.

Doug King, visiting professor of building engineering physics at Bath University and founder of consulting engineer King Shaw Associates, believes that building designers need to make more use of thermal mass in buildings of all construction types, including lightweight steel and timber as well as concrete.

“I’d estimate that 40% of the energy used to control temperature in buildings is just wasted,” says King.

Of course there is nothing new about thermal mass — the Romans after all were experts in exploiting it in hypocaust heating systems. But King says its application today is hampered by a lack of skills and understanding within the industry and education, although he does acknowledge that this is improving.

Fabric energy storage

Thermal mass — also known as fabric energy storage — is the ability of the building fabric, in particular the exposed concrete slab, to absorb excess heat, thus improving thermal comfort and reducing the need for mechanical cooling.

Over a 24-hour cycle, the concrete can absorb and store heat during times of peak temperature, then release it later as internal temperatures fall at night time. After the concrete has cooled sufficiently, it can re-absorb heat when temperatures rise again. Such an approach works best in buildings, such as schools and offices, which do not have round- the-clock occupancy, rather than those that do, such as hospitals.

Concrete is effective for this use because it readily absorbs heat by radiation, but its relatively low conductivity means the heat remains isolated within the material.

“You can warm concrete and it will absorb an awful lot without its temperature changing a great deal. If we can have that exposed in a point of direct contact with the space, we can use it to absorb the heat of sun, people, computers and other electrical devices,” says Mott MacDonald’s Murphy.

The thickness of the concrete slab required is a moot point.

A thickness of 100mm of concrete, which can be accommodated in both steel and concrete buildings, is generally considered the optimum amount as long as the concrete is exposed directly to the internal environment, with the first 25mm playing the greatest role.

‘We only require a thin skin of concrete or masonry, and this can be constructed every bit as easily on a steel frame’

Beyond 100mm, there is little gain in thermal performance. King says it is a myth that the thicker the concrete, the more the thermal mass benefits over a typical 24- hour cycle. In thicker constructions of 1.5m-2m, the response is seasonal. According to King however, only a very thin skin — as little as 30-50mm — is needed to moderate temperatures over a 24 hour cycle for general purposes.

For thermal mass to work, the concrete soffits must be exposed to the air and not covered up — for example with dry lining or a suspended ceiling that would thermally isolate the concrete and limit its capacity to absorb excess heat. Instead, casting in provision for service ducts allows the soffits to be exposed, but this must be planned in upfront to be an integral part of the design.

Although high quality commercial buildings, such as AHMM’s Angel Building in north London, demonstrate the appeal of an exposed concrete aesthetic, there are many precast flooring systems available with a finely finished surface. And King says that plastering the soffit doesn’t reduce thermal performance greatly. Acoustic mineral fibre baffles that hang down from the ceiling are another solution where an exposed concrete soffit is felt to be aesthetically undesirable.

No-one is claiming that thermal mass in itself is a panacea — its full potential can only be exploited within a considered whole building environment strategy alongside factors such as building orientation, glazing and solar gain — but the building fabric can be used to regulate internal temperatures. However, this is achieved most successfully only if it is considered early in the project by the whole design team, says Murphy.

“It’s important that architects talk to building performance engineers when they start because we can have a very beneficial effect on the carbon performance of the building without curtailing innovation in the design process.”

Postscript:

For More information www.tatasteelconstruction.com/en/sustainability/thermal_mass/

In association with The British Constructional Steelwork Association and Tata Steel