Is 100mm of concrete enough? (air-conditioned buildings)

A question often asked by architects and designers; "is 100mm of concrete enough to achieve thermal mass?".

In air-conditioned buildings the heating/cooling cycle is generally limited to 24 hours because the tighter control of temperature should ensure there is no build up of heat in the fabric over periods longer than a day. This might suggest that 100mm of concrete would provide the optimal level of thermal mass needed to reduce the air conditioning load. Whilst this is true for a concrete floor slab exposed on one side (i.e. the soffit), in practice slabs are often exposed on both sides or internally. This greatly increases the surface area for heat absorption and the effective depth of concrete that can be utilised in a 24 hour period, which in turn significantly increase the cooling capacity provided by the slab (see figure 2) This is commonly achieved by combining an exposed soffit with underfloor mechanical ventilation, enabling heat transfer from top and bottom slab surfaces.

Similarly, another option is to have an exposed soffit in conjunction with hollow cores through which air is channelled, which is another way of maximising the heat transfer area e.g. the Termodeck or Concrete Core Cooling systems. As these systems incorporate mechanical ventilation, they are also able to ensure air passing over the concrete is relatively turbulent, which significantly increases the rate of heat transfer at the surface and, as a consequence, the depth heat will penetrate in the limited time available; its worth noting that the 100mm benchmark often quoted assumes the use of natural ventilation i.e. smooth/laminar air flow at the surface, resulting in a lower rate of heat transfer.

A further active cooling option increasing in popularity is the use water rather than air to regulate slab temperature, made possible by embedding water pipes in the soffit.  Heat flow between the water and the concrete is by conduction, making it relatively rapid, enabling all the available thermal mass in the slab can be exploited. Hydrodeck and Climaspan are examples of this system.

So, in conclusion regardless of whether a passive or active approach to cooling is used, the thermal mass in precast or insitu floor slabs with depths well in excess of 100mm can be fully utilised to maximise cooling output and provide enhanced performance during hot conditions. The suggestion sometimes made that a 100mm of concrete provides sufficient thermal mass is too simplistic and fails to take account of how floors are actually used in practice and the range of conditions under which they must perform.

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