Lightweight as well as heavyweight frames can achieve optimal thermal mass in multi-storey buildings. Steel expert Michael Sansom outlines five key thermal mass issues to consider Illustration: Jamie Cullen
Thermal mass is a controversial and complex subject that often leads to argument over the merits of lightweight vs heavyweight frame construction. However, effective thermal mass solutions can be achieved with any well-designed structural solution as part of an integrated Fabric Energy Storage (FES) solution, and all steel framed buildings incorporate the optimum 100mm concrete needed for this in their floor slabs. In multi-storey buildings, the upper floors provide the most thermal mass potential.
Three common floor forms can be used to achieve this in steel framed buildings: precast concrete units, composite slabs and shallow floors. Solid or hollow core precast units can be supported on the top flanges of beams or on the bottom flanges of Asymmetric Slimflor Beams (ASBs). Composite slabs comprise reinforced concrete cast on profiled steel decking and are usually supported on the top flange of downstand beams. Shallow floor systems such as Slimdek comprise composite ASBs supporting deep profiled metal decking.
Thermal mass works by using the ability of materials within the fabric of a building to absorb, store and release heat energy and in doing so reduce variation in internal temperature. This helps maintain thermal comfort and prevent daytime overheating by absorbing heat from solar gains, human activity and electrical equipment. Heat stored during the day is purged overnight by using either natural ventilation and/or mechanical means so that the building is then able to repeat the process the next day.
There is general consensus among the BRE, Concrete Centre and CIBSE that when considering a diurnal 24 hour cycle in a UK-type climate, a thickness of 75mm-100mm of concrete in the floor slab is optimal since heat energy cannot easily be extracted from anything any deeper than that. There is therefore nothing to be gained in thermal mass performance by a greater thickness of slab. Anything greater than the optimal level is potentially wasteful and raises extra cost and embodied carbon issues.
One of the biggest misconceptions about thermal mass is that having sufficient mass is enough in itself. What really counts is not just that the optimum level of thermal mass is present, but that it is exposed so the heat energy can get in and out. This means that it remains accessible and is not masked or isolated by raised floors, suspended ceilings, carpets or, in the case of walls, dry lining. However, accessibility can bring acoustic, aesthetic and cost considerations to achieve a pleasing finish to a floor slab. Exposed soffits aren’t appropriate for every situation but there is a greater acceptance of them in, for example, more informal office environments. There is also scope for the use of permeable suspended ceilings that still allow a large proportion of the heat to be absorbed by the floor slab.
Thermal mass is not an isolated design concern but should be considered as part of an integrated, holistic solution – simply exposing the upper floor soffits is not enough. Building orientation, fenestration, shading and servicing should be considered to optimise natural ventilation and limit heat gains. A natural or mixed-mode ventilation system is also needed so the building can be ventilated and cooled at night.
Both passive and active FES systems can be used in steel framed buildings. Passive systems that use natural ventilation to disperse the heat absorbed by the upper floor slabs depend on wind direction and speed and are only suitable for buildings with relatively low cooling loads. Active systems use mechanical ventilation to enhance the heat exchange with the structure. They may use underfloor ventilation with exposed soffits, exposed hollow core slabs with mechanical ventilation, or water cooled slabs using pipes embedded within the floor slabs.
Building location and type
Location is an important factor – thermal mass with passive FES is not always appropriate in city centres where air quality, external noise and security may preclude a natural ventilation strategy. Generally it is easier to ventilate naturally in a rural or suburban environment. It is also much easier to do so in buildings with intermittent occupancy like offices and schools than in hotels and restaurants that don’t allow for the night time cooling phase.
Building form is also an issue. In deep plan offices it is much harder to get the cross ventilation needed although atria can be used to provide and enhance natural ventilation. The difficulties of ventilating highly compartmented buildings naturally however may preclude effective FES strategies in building types such as hospitals and hotels.
Thermal mass is no panacea, but with enlightened designers and the right type of building it can work well. It’s independent of frame material, so the design team can use a steel frame and take advantage of other benefits.
Michael Sansom is associate director of the Steel Construction Institute and lead author of the publication Steel Construction: Thermal Mass which can be downloaded from www.steelconstruction.info/Steel_construction_news
HOW WAS IT FOR YOU?
Engineering experts discuss the issues they encounter when incorporating thermal mass in steel buildings.
Fergal Kelly, director of structures, development consultancy Peter Brett Associates
There isn’t much formal education on thermal mass at degree level – it’s something you pick up from industry literature – and the general misconception among clients, architects and even M&E engineers is that concrete buildings have inherent thermal mass, and steel buildings don’t. Steel construction can achieve the same performance– although an expanse of exposed composite decking isn’t always what people want to see. A good compromise is a precast concrete slab with a flush soffit within a steel frame. Systems that use water pipes embedded within a concrete slab seem encouraging from the steel frame point of view since this gets around the potentially difficult issue of exposed soffits.
Angus Palmer, technical director, Buro Happold
People have got the message now that thermal mass doesn’t need 400mm of concrete. As a structural engineer I have no problem mixing steel and concrete. Using concrete planks on a steel frame is a good solution – you get the best of both worlds. One of the biggest challenges is the aesthetics of exposed ceilings. You have to manage expectations so that everyone understands what they’re getting and careful attention to detail is needed as everything remains visible.
Edward Murphy, technical director, Mott MacDonald
There is a lot of interest in using thermal mass to meet the new CIBSE TM52 and Education Funding Agency guidance on thermal comfort limits in schools. Contractors with lightweight standardised solutions are asking how they can add mass to steel framed structures, perhaps not realising there is already enough mass in the concrete floor slabs. What’s important is a good connection between the room air and the available concrete, so we maximise the amount of cooling from the available soffit area.