Utilising thermal mass within a steel frame can reduce a structure’s CO2 emissions while contributing to overall cost savings.
Many designers and engineers are looking to achieve the optimum thermal mass of a building to help minimise the energy required for cooling. This can ultimately save the client a lot of money otherwise spent on powering air conditioning units.
Rising energy costs and a possible increase in temperature over the next 100 years, due to climate change, have both prioritised the need to construct buildings in the most energy efficient way. Designers need to make use of thermal mass in buildings to address this issue.
Structural steelwork offers a number of benefits, such as cost efficiency and speed of construction, but using a structural steel frame can also offer the design team the perfect solution for an economic and cost effective method for achieving peak thermal mass.
“It’s a common misconception that a building needs to have large volumes of concrete to achieve thermal mass”
It has been thought that large, heavy buildings can mobilise greater amounts of thermal mass than lightweight alternatives, sometimes resulting in specification of reinforced concrete frames. Thermal mass is, however, independent of frame material.
“It’s a common misconception that a building needs to have large volumes of concrete to achieve thermal mass. In fact the fi rst 50mm to 75mm of an exposed concrete soffit is the bit that does all the work - the thickness of material exceeding this is thermally neutral in its beneficial eff ects on the space below,” explains Mott MacDonald technical director Edward Murphy.
Thermal mass is the ability of a structure to absorb excess heat. The element with the most thermal mass is the concrete in the floor slab and for this to work efficiently it requires the floor soffit to be exposed.
During the day, solar gain, electrical equipment and human activity generate heat, which warms the air in a building. Most structures would use air conditioning to artificially cool the internal environment, but if the concrete slab is exposed it can absorb and store heat during times of peak temperature, then release it later as internal temperatures fall at night.
Designing to eff ectively use thermal mass can remove the need for air conditioning. As mechanical cooling is energy intensive, this can have a huge impact on running costs, and will significantly reduce CO2 emissions.
St Johns Square
St Johns Square is a multi-use, four-storey council office block that formed part of a £19M redevelopment of Seaham town centre. The building houses a public library and cafe as well as offices for Durham County Council and Seaham Town Council.
The project team’s aim from the outset was to reduce running costs and strengthen the scheme’s sustainable credentials. The building needed to be kept cool in summer and warm in winter, sustainably.
A steel framed solution, comprising steel decking and composite concrete floor slabs was decided on as the best solution, to incorporate natural ventilation and thermal mass to control building temperatures.
Durham County Council design engineer Alasdair Cameron, says: “We wanted a naturally ventilated building with a design that would help cut down running costs and lower emissions. We also wanted to increase the thermal mass by exposing the floors to allow them to absorb heat during the day and dissipate it at night.”
The building has achieved a BREEAM “Very good” rating.
Cheshire Police HQ
Mott MacDonald and Fairhurst were commissioned by Cheshire Police to design its new 2,800m2 headquarters in Blacon, Cheshire. Sustainability and energy efficiency were key areas for this project along with cost and the need for a quick delivery.
The team opted for a steel framed solution, rather than concrete, due to the relative cost and time savings the material offered.
The steel framed building was designed to exploit thermal mass, providing “inertia” against temperature fluctuations.
Mott MacDonald technical director Edward Murphy, says: “Thermal mass can, if designed correctly, be just as eff ective within an exposed hollow floor deck steel frame building as it can within a concrete frame.
“In this instance we chose to use steel with hollow floor deck because of the cost and time effi ciencies that it offered.”
As a result of using a steel frame, approximately 5% was saved on the cost of the frame and the job was completed four weeks earlier than would otherwise have been expected.
How thermal mass works
During the day, solar gain, equipment use and human activity generate heat, which warms the air in a building. The warmed air rises, flows across exposed surfaces and is absorbed into the building’s floor slab.
At night, cool air is allowed into the building and flows across surfaces which have been used to absorb heat during the day. These surfaces are then ready to absorb heat again the following day.